Numa escavação de salvamento descobrimos uma necrópole, este indivíduo estava isolado a cerca de 50 metros do fosso que limitava a necrópole, sem grave goods e com as mãos numa posição diferente (possivelmente atadas).
Tuesday, 20 April 2010
Tuesday, 13 April 2010
‘Heroism’ in Warfare as a Functionally Specific Form of Altruism.
Oleg Smirnov
Political Science, University of Miami
Holly Arrow
Psychology, University of Oregon
Doug Kennett
Anthropology, University of Oregon
John Orbell
Institute of Cognitive and Decision Sciences
University of Oregon
Authors’ Note: We thank James Fowler, Warren Holmes, Timothy Johnson, David Osborn,
Frances White, members of the “Warfare and Killing” spring 2006 seminar and several
anonymous reviewers for helpful comments on earlier drafts. We are also grateful to members
of the Evolution Focus Group of the Institute for Cognitive and Decision Sciences for helpful
feedback after early presentations of this project.
“Heroism” in Warfare
As a Functionally Specific Form of Altruism.
“When two tribes of primeval man, living in the same country, came into competition, the tribe
including the greater number of courageous, sympathetic and faithful members…would without
doubt succeed best and conquer the others.” Darwin, The Descent of Man, chapter 5, p. 156.
Abstract
We use two simulations to explore the possibility that heroism (risking one’s life fighting for the
group) evolved as a specialized form of altruism, selected as a result of warfare through humans’
ancient past. In the first simulation, altruism as a general disposition promotes all forms of
altruistic behavior, including heroism. In the second, heroism and communitarianism (altruistic
actions beyond warfare) are separate, domain-specific dispositions. Warfare selects weakly for
altruism, more strongly for communitarianism, and substantially for heroism. Heroism evolves
more readily when groups are small and mortality in defeated groups is high—features plausibly
characteristic of humans’ ancestral past.
When Homer asks in the Iliad “What god drove them to fight with such a fury?” he was
posing a question that remains problematic three thousand years later: What makes soldiers
willing to fight at great personal risk for their polis, their tribe, their ethnic group, or their
country? There is no doubt that soldiers often do fight in this manner; the history of warfare is, in large part, the history of ordinary men (and, in recent times, women) who willingly confronted the risk of death when fighting for their tribe, polis, country, or other group. In the present paper we call this behavior “heroism.” Heroism is well recognized as a potential human behavior, but the existence of heroism is most commonly treated as a background assumption in examining the nature of war. In this paper we address what we see as the central puzzle of heroism: How could it have evolved?
Most studies that address heroic actions in warfare have focused on “proximate”
answers—emotional, cognitive or other mechanisms that prompt such behavior in the here and
now. Thus, for example, loyalty to small groups of fighting comrades (Gat 1999; Shills and
Janowitz 1948; Stern 1995) or, perhaps, a regiment (Holmes 1985), the rage and fear provoked
by an attack, or threat of an attack, against one’s own group (Horowitz 2001), and cognitive
errors such as overconfidence about the risks involved in heroism (D. D. P. Johnson 2004; D. D.
P. Johnson et al. 2006; Wrangham 1999b) have all been argued to support such behavior. By the same token, social psychologists have identified many mechanisms likely to be implicated in
humans’ frequent xenophobic willingness to engage in hostile action against other groups—most
famously, the ease with which we categorize others into “in-group” and “out-group,” favoring
the former and, often, demonizing the latter (Ackerman et al. 2006; Rothbart and Lewis 1988;
Rothbart and Taylor 1992; Tajfel and Turner 1979).
But this level of explanation, while certainly compelling, is only one level of explanation,
the other being the “ultimate” or “evolutionary” processes that selected for such proximate
mechanisms through evolutionary time. Most theorizing in the social sciences invokes only
proximate mechanisms, but such mechanisms must have evolved via some evolutionary process of selection on genes, on cultural patterns, or on gene-culture interactions (Boyd and Richerson
1985; Odling-Smee, Laland, and Feldman 2003; Richerson and Boyd 2005). A complete
explanation of the behavior in question should thus consider both proximate and ultimate
causation. Here we focus on the latter: How could mechanisms supporting “heroism” have
been positively selected during humans’ ancestral past? By definition, heroism involves a
fitness cost to the individual (via increased risk of death), hence we might expect it to be selected against, eventually disappearing from the human population. Yet heroism certainly does happen
with some frequency across a wide variety of cultures, suggesting that it is a species typical trait. How could this have come to be?
One standard answer is Hamilton’s (1964) famous explanation of how altruism in general
might have evolved. Hamilton turned attention to the selective fate of genes rather than of
individuals per se, pointing out how sacrificial action by one individual could result in genes
supporting that act spreading more rapidly than had the individual acted selfishly—as long as the beneficiaries of such action were appropriately close relatives.1 Inclusive fitness has been
invoked by Alexander (1979), Masters (1983), Shaw and Wong (1989), Thayer (2004), van der
Dennen and many others, in attempts to understand the genetic basis of humans’ propensity to
warfare and, using our term, therefore to heroism.2 The general point is that, during humans’
remote ancestral past groups were, most probably, comprised largely of quite close kin, making a disposition toward sacrificial action on behalf of the group an attribute that could evolve, despite adaptive costs to the individual per se.
Importantly, this account explains heroism as a product of the same ultimate, selective
processes that might produce any other altruistic behaviors. Sharing meat from a hunt, for
example, could be explained as readily as fighting for one’s group in a battle with other groups.
The account nicely combines a plausible reconstruction of family-based social organization in
our ancestral past with a foundational idea in modern evolutionary biology. It is also consistent
with the pervasiveness of kinship rhetoric—the “fatherland, “band of brothers” etc—in modern
warfare (G. Johnson 1987). It is quite plausible that the particular form of altruism that interests us, heroism in defense of one’s group, evolved via the combined impact of multiple processes
that have been invoked for explaining altruism in general—including kin altruism, reciprocity in
exchange relationships (Axelrod 1984; Trivers 1971) and multi-level selection (most notably,
Sober and Wilson 1998).3
Here we develop the possibility that, even without the impact of kin altruism and
reciprocity, heroism could have evolved as a “domain specific” form of altruism in response to
warfare in our ancestral past. The idea that human cognitive architecture consists, in substantial
part, of functionally specific information processing modules is widely accepted in evolutionary
psychology and in cognitive neuroscience more broadly, although scholars differ in arguing for a
strong version of modularity (notably Cosmides and Tooby 1994; Hirschfeld and Gelman 1994;
Sperber 1994) or for a combination of specialized modules and some more general functionality
(notably Buller 2005; Fodor 2000; Mithen 1996). The standard strong modularity argument is
given by Cosmides and Tooby (1994 p. 89) as follows:
…different adaptive problems require different solutions… Speed,
reliability, and efficiency can be engineered into specialized mechanisms
because there is no need to engineer a compromise between competing task
demands…As a rule, when two adaptive problems have solutions that are
incompatible or simply different, a single general solution will be inferior to
two specialized solutions.
However this argument plays itself out (for a constructive appraisal see Barrett
and Kurzban 2006), the extensive literature on human altruism has, to our knowledge,
focused exclusively on the problem of how a generalized disposition to altruistic
behavior might have evolved. The possibility that multiple, domain specific altruistic
dispositions might have evolved independently, each in response to a distinctive
adaptive problem and each producing, therefore, at least somewhat different proximate
mechanisms appears to have been overlooked. Since heroism is, by our definition, an
altruistic response on behalf of one’s group in the event of war with some other group,
its task demands would appear prima facie quite distinct from those of other altruistic
behaviors (e.g., providing food to others and caring for the sick). This makes it
plausible that heroism could have evolved on its own distinct trajectory, independent
of such other forms of altruism, and with warfare as the agent of selection.4
Warfare is a group-level phenomenon, and its broad outcomes (victory, defeat,
and standoff) are also group-level phenomena. Accordingly, all group members—those
who fight as well as those who do not—can suffer “genetic death” should the group be
defeated.5 However, the consequences of group-level outcomes can differ among the
individuals comprising such groups. Most obviously, those who fight in the group’s wars
run an increased risk of dying as a result, reducing the probability that their genes will
pass on to the next generation. Should the group be victorious, survivors stand to benefit
significantly, but heroes are less likely to be among those survivors. Along with males’
sexual access to females from the defeated group (viz: rape, captured concubines etc.),
the spoils of victory can include access to the defeated group’s territory and whatever
resources that territory contains. Clearly, frequent warfare during humans’ ancestral past
could have had major adaptive consequences—positive and negative—for individuals in
the warring groups.6 Since heroism as we have defined it is central to the outcome of
wars, the evolution of heroism is a good candidate for being among those consequences.
How frequent was warfare in our ancestral past? Attempts to answer this question
have provoked a good deal of controversy. Some accounts of the archeological record
suggest that war was quite frequent. LeBlanc (2003, pp. xii - xiii) comments:
Just how common was warfare in the past? I have finally concluded that
warfare was quite common… and that my findings on three continents and
within multiple time periods were not a fluke but the norm. This has led me
to reason that if conflict was common, then it must have been an important
occurrence in the course of human history. (Italics in the original.)broadly, “coalitional violence”—has been present, at least to some extent, throughout our
ancestral past, and our question is whether warfare, happening with any significant frequency,
could have selected for heroism. The simulations that we will now describe allow us to study the
relationship between frequency of warfare and such selection, and we will report findings to that
effect later in the paper.
Two Simulations
In the first simulation, altruism is domain general, promoting: (1) communitarianism—a
group-benefiting behavior that increases the mean fitness of group members but does not involve
fighting enemies; and (2) heroism a behavior that benefits the group by contributing to its
success in warfare. In the second simulation, these two behaviors are promoted by two domain
specific attributes that are free to evolve independently. A formal description of the two
simulations can be found in Appendix 1 which is available on the Journal’s website; here we
provide a verbal account of the simulation’s key features.
Model 1: Domain general altruism
Groups and resources. Four groups each occupy a fixed resource base on which its
members are critically dependent for nourishment. The size of the resource base for each group
is a parameter; it can vary across groups but not across time and sets a limit on sustainable group size.
Altruism. Each group member has a propensity to behave altruistically, which is modeled
as a continuous variable with values between 0 (never) and 1 (always). The starting mean
altruism of each group is a parameter of the model; the altruistic propensity of a group’s
members varies in a uniform distribution centered at the specified mean. An individual’s
altruism increases the reproductive success of all group members and contributes to the group’s
success in warfare as a function of that individual’s altruism score (the more altruistic, the more
benefit to others across both domains); the reproductive cost paid by the individual is similarly a function of that score (the more altruistic, the greater the cost to self).
Baseline Fertility and Individual Reproduction. Baseline fertility is the probability of
each group member’s having an offspring in a given generation—prior to any adjustments based
on the incidence of altruism in the group (the more altruism summed across all group members,
the greater each group member’s probability of reproducing), the individual’s own altruism score (the greater that score, the smaller the probability of that individual’s reproducing), or the
outcome of war (group members who do not survive wars do not reproduce). Should there be
more than one war in a generation, the cost of participating (reduced probability of surviving to
reproduce) is assessed separately for each.
Going to War. A group goes to war when its population exceeds the carrying capacity of
the resource base—that is, when the resource base per capita drops below a specified threshold.
At this point the only way a group can support its members is to capture the resources held by
another group. Target groups are chosen at random, with any group that is attacked by another
having to defend itself by fighting.
We recognize, of course, that competition for scarce economic resources is not the only
factor that increases the probability of war; this is merely a simplifying assumption for the
"generic" wars of our model. Chagnon (1988) and others, for example, point to the frequency
with which capturing females is the stated reason for hunter-gatherers going to war, and females are obviously a reproductively significant “resource” (Trivers 1972). It is also the case that
particular wars may have highly visible “triggering events” (the seduction of Helen, the
assassination of the Archduke Ferdinand). This is not, however, inconsistent with an ultimate
explanation in terms of population pressures on groups’ resources (Shaw and Wong 1989, p.11).
Helen’s seduction, for example, took place during a time of mounting economic competition
between the Trojans and the Greeks. Certainly, there must have been a complex relationship
between capturing land and capturing females motivating our ancestors to go to war—as
suggested by Betzig (2005, p. 335) in her analysis of the Biblical case as recorded, among other
places, in Deuteronomy 20:13-17:
God handed Moses two different laws about spoils. One was for wars with
close neighbors; the other was for more remote wars. When their enemies
lived far away, he said, ‘you shall put all the males to the sword, but the
women and the little ones, the cattle, and everything else in the city, all its
spoil, you shall take as booty for yourselves.’ But when their enemies lived
nearby, ‘you shall save alive nothing that breathes, but you shall utterly
destroy them.’ Land was the limiting factor.
The assumption on which our model is based—that resource scarcity leads to
war—is perfectly compatible with the capture of female reproductive capacities as one
among many proximate causes of war. The idea that land, thus somatic resources, was
a critical “limiting factor” is, nevertheless, widely assumed among anthropologists,
biologists, ethologists, political scientists, psychologists, sociologists and others
addressing the causes of ancient warfare (e.g. Alcock 1978; Carneiro 1970; Durham
1976; Eibl-Eibesfeldt 1979; Kennett et al. 2006; Kennett and Kennett 2000, 2006;
Lambert 1997; Lambert and Walker 1991; Shaw and Wong 1989; Thayer 2004; van
den Berghe 1978; Wilson and Wrangham 2003) and we adopt it here.
Victory and Defeat. The group with the highest summed altruism score wins. This
represents the total amount of heroic fighting available within each group. Consistent with the
“imbalance of power” hypothesis developed by biologists and primatologists (Alexander 1979;
Wrangham 1999a)—as well as with Napoleon’s comment that “God is on the side of large
armies”—the larger group will win when the mean propensity to altruism is equal across two
contending groups. However, a smaller population can defeat a larger one if its members are
substantially more altruistic (hence more heroic).
Death. Some proportion of the defeated group is killed. We will report our simulation
findings from the strong assumption that all are killed, so that defeated groups are eliminated
from the evolving population. While this extreme assumption is certainly consistent with some
examples from the historical record (Potidaea, Mytilene, and Melos in the Peloponnesian war,
for example), the proportion of a defeated group that is killed is an important parameter, to which we will return in a later analytic section (specifics are available on the Journal’s website). In the simulation, members of the victorious group migrate to the newly captured territory such that per-capita resources in the two areas are equalized. Once migration is complete, no “memory” of prior ties persists, so the two “daughter” groups created by the schism are just as likely to attack one another in subsequent rounds as they are to attack a different group.
Variation. Selection on altruism—as on any other attribute—can, of course, only happen
in the context of variation on that attribute. As noted, members of each group vary in altruism at the start of the simulation. During reproduction, variation is maintained by two mechanisms.
One models random mutation (defined as 1 M in the appendix on the Journal’s website); by
default, it occurs here in less than one percent of offspring, and can result in the offspring having
an altruism level quite different from the parent. The other mechanism (defined as 2 M in the
appendix) simulates (without directly modeling) sexual reproduction, such that offspring
resemble their parent within a range of variability.
Model 2: Domain specific with both “communitarianism” and “heroism”
The second simulation has the same general structure, except that each individual now
has two domain specific altruistic propensities, heroism and communitarianism, each similarly
modeled as varying between 0 and 1. Starting mean heroism and communitarianism for each
group are separate parameters of the model. Member levels for the two attributes are
independently drawn from a uniform distribution centered at the specified mean. Reproductive
costs from communitarianism are paid every generation; those from heroism are paid per war.
Method
We ran 10,000 simulations of each of the two models. To ensure that the data we report
reflect a wide variety of possible evolutionary environments, the following parameters were
drawn randomly7 for each run:
1. Each group’s resource endowment ( g R )—specified separately for each group;
2. Baseline fitness—the probability of reproductive success for all agents before the
consequences of altruism, communitarianism, heroism or warfare are incorporated ( b P );
3. The fitness advantage to group members from an individual’s communitarianism ( c P );
4. The cost to the individual agent of its own communitarianism ( C C );
5. The cost to the individual agent of its heroism ( H C );
6. (In Model 1) Mean starting altruism ( A S );
7. (In Model 2) Mean starting communitarianism and heroism ( C S , H S );
8. The two sources of variation (two types of mutation) in the attributes of offspring as
specified in the appendix ( 1 M , 2 M ).
Two parameters were fixed across all the simulations: the survival threshold, T =1 (one unit of
resource per capita), and the number of groups, which was four.
Each of the 10,000 simulations was run for 1100 generations, allowing a sufficient
number of generations for the system to evolve away from diverse starting values. For the last
100 generations we recorded the moving average of altruism in the domain general model, and
of communitarianism and heroism in the domain specific model. The simulation parameters
(which varied across runs as described above) and the frequency of wars across all generations
(an emergent variable) were also recorded for each simulation. In a smaller sample of runs to
check for the system’s sensitivity to some of our choices, we increased the number of
generations and varied both the number of groups and the length of the moving average across
which we recorded data. None of these modifications affected our results.
Findings
With the domain general model we observed a modest but significant selection on
altruism. This is shown in Figure 1, which is a frequency distribution for evolved altruism. The
horizontal axis specifies the altruism level averaged across the last 100 generations of the 10,000 simulations and the vertical axis records the density of cases across the randomized parameter space. The mean altruism value in this positively skewed distribution is 0.25 (the median is 0.17), interpretable as the mean individual being 25% altruistic—or, alternatively, that the mean individual would have a .25 probability of contributing to group members’ fitness via its communitarian activities and a .25 probability of fighting heroically in any war.
Despite the fact that both behavioral consequences of general purpose altruism are
reproductively costly to individuals, therefore, we do observe some selection on that attribute.
Agents pay a reproductive cost proportional to the strength of their generalized disposition to
altruism, but this cost is outweighed by the benefits that individuals reap from their group’s
success in warfare. Multiple regression of ending altruism values on parameters in the model
(see Appendix II on the Journal’s website) shows a negative association between evolved
altruism and (1) the cost of altruism; (2) the average size of group resource bases (hence, the
average size of groups); and (3) variation in resource base size (hence, variation in size among
the groups); it also shows a positive association between altruism and the frequency of war. This
suggests that higher (and less typical) evolved values of altruism occurred in simulation runs
where the personal cost of altruism was low, where groups were smaller and more equal in size,
and where wars were quite frequent.
With the domain specific model, in which communitarianism and heroism were free to
evolve as separate attributes, Figures 2a and 2b show that: (1) heroism evolved to substantially
higher levels than communitarianism with means of 66.9% and 30.5% respectively and medians
of 70.3 and 22.6 respectively; (2) both attributes evolved to significantly higher levels than did
altruism in the domain general model. There are, therefore, three findings to be explained—first, the fact that there was some positive selection on general purpose altruism, despite its costs to the individual; second, that there was stronger selection on heroism than on communitarianism
despite their respective costs; and third, that when communitarianism and heroism are free to
evolve separately, both evolve to higher levels than when they are combined as expressions of a
single, general purpose altruism attribute. We will address the evolution of general purpose
altruism shortly; first we discuss the relative evolution of the two domain specific forms.
Why does heroism evolve to higher levels than communitarianism?
In our model heroism is only costly in the event of war. Given that war only occurs
periodically—in the simulations any given group goes to war with a probability of about .5 per
generation—while communitarianism occurs in every generation, perhaps heroism evolves to a
higher level simply because the cost of heroism is paid less frequently than the cost of
communitarianism.
This does appear to be part of the story. Figure 3 reports the distribution of heroism
when its cost is paid every generation rather than per war. With the mean of this distribution
now at .475 (as opposed to .669 in the original domain specific model), clearly this cost
difference accounts for some of the relatively greater selection on heroism. Across the range of
parameters, it appears that selection on heroism is stronger when such behavior is not invoked
every generation, as is communitarianism. But that is only part of the story, since the mean of
heroism is still substantially higher when it is evoked every generation than is the mean of
communitarianism (.305) when it, too, is evoked every generation.
Another part of the story concerns the immediacy of the military benefits a group reaps
from communitarianism and heroism. Since reproduction happens at the end of each generation,
communitarianism increases the number of fighters a group will be able to field in the next
generation while heroism increases a group’s fighting capacity in the present one. Thus
communitarianism can only increase a group’s future fighting ability—assuming the group
survives any wars in the current generation, which will depend on group size and heroism now.
The delayed effect of communitarianism is, of course, consistent with real-life population
dynamics: since infants and young children confer no advantage in war, the war-fighting
benefits of increased fertility cannot be realized immediately.
Communitarianism is also a “double-edged sword” for a group’s fighting ability. While
larger groups are more likely to win wars, high population growth can also be responsible for
getting a group into war in the first place (faster population growth means that a group reaches
its carrying capacity more rapidly), when, in a war, there is some chance of its losing, thus of
selection against communitarianism. Heroism, on the other hand, is an unambiguous good for a
group. In the absence of war, and assuming its cost is only paid in the event of war, it does no
harm to the individual or the group, but in the event of war it can be decisive for victory.
Ultimately, selection on communitarianism and on heroism must be a function of whether
the behavior produced by those dispositions is “critical” to the group’s success in warfare—thus
to the individual’s survival and reproduction—and while both contributions might be critical for
the outcome of a given war, heroism is more likely to be critical than communitarianism.
Why, then, does altruism evolve?
The positive selection on general purpose altruism in the single attribute model, modest
as it is, can now be explained in the same way as selection on heroism and communitarianism in
the two attribute model. Altruists fight for their groups, and a given altruist’s fighting can be
critical for the group’s survival, thus for the altruist’s own survival. Similarly—although with a
lower probability—an altruist’s communitarian behaviors might also be critical for such survival
via the role that numbers as such can play in outcome of wars.
Why do communitarianism and heroism evolve further than general-purpose altruism?
An unexpected finding was that when heroism and communitarianism are free to evolve
independently, both evolve to higher levels than does the single, general purpose altruism.8 What accounts for this?
In retrospect, at least, the explanation is clear. When a single, general purpose mechanism
performs two distinct functions (A and B) both of which decrease individual fitness, selection
against that mechanism based on the costs of A will also select against B, and vice versa. The
general purpose capacity will, in other words, pay a cost whenever either of the two behaviors is
invoked. Conversely, when distinct, special purpose mechanisms are dedicated to A and B,
selection based on the individual costs of B has no implications for selection based on the costs
of A, and vice versa. Thus, breaking the general purpose mechanism down into two special
purpose mechanisms means that the evolution of one attribute is not handicapped by the cost of
the other. Notice that heroism, invoked only every second generation or so, benefits more from
such “de-linking of costs” than does communitarianism.
This finding points to the cost-benefit logic that could support a modular design for the
evolved “cognitive architecture of altruism.” The degree to which propensities for different
forms of altruistic behavior are, in modern humans, actually correlated (positively or negatively) is, of course, an empirical question awaiting investigation by other means than computational
modeling. Also, findings from the model do not resolve the general problem of how privately
costly altruistic dispositions (whether unitary or multiple) actually did evolve. Each disposition
must still be positively selected despite the costs that it generates for the acting individual.
Nevertheless, the finding does raise the possibility that thinking about altruism as a bundle of
special purpose forms of altruist behaviors—each evoked in some circumstances but not in
others—and not as a single general purpose disposition evoked regardless of the contextual
specifics, will prove a fruitful path to follow. Whatever they might be, selective pressures
favoring special purpose altruisms (plural) would not have to be as strong, ceteris paribus, to
overcome the associated costs as would selective pressures favoring general purpose altruism
(singular) if the behavior in question were to evolve.
The case for humans’ past being one of “constant battles” becomes increasingly difficult
to make the further back in time one looks and, as discussed above, this has fueled arguments
about just how “constant” warfare actually was in our ancestral past. While those arguments are
for archeologists to resolve, as shown on-line in Appendix II, in our simulation the frequency of
warfare does positively predict selection on altruism in the first model, and on communitarianism and heroism in the second one.9
One strong assumption of both simulations is that all members of defeated groups are
killed. To examine what happens when this is relaxed, we turned to an analytic model.
An Analytic Model of Heroism
Although our simulations make a range of simplifying assumptions, they are still more
complex than what can be readily modeled analytically. Accordingly, we employed a simplified
analytic model to explore more rigorously the impact of a few key parameters on the evolution of heroism.10 The cost of this precision was to reduce the number of groups from four to two
equally sized groups and to model heroism as a dichotomous variable with each agent being
either fully heroic or a complete coward. (Appendix III on the Journal’s webpage contains a
formal description of the model.)
As noted above, the simulation results are based on the strong assumption that a defeated
group is entirely wiped out. Although the archeological record does suggest that genocide was at
least sometimes practiced (Keeley 1997; LeBlanc and Register 2003) and the elimination of a
competing group has been documented in a chimpanzee population (Wilson and Wrangham
2003), we simply do not know just how pervasive this actual outcome was. What happens when
the simulation’s genocide assumption is relaxed?
In the analytic model, any individual’s probability of surviving a war is defined by 1 –
dG, when d is the probability of the group’s being defeated (a function of the relative summed
heroism scores in the two contending groups) and G is the proportion of the defeated group that
is killed (the genocide parameter). At one extreme, therefore, if the genocide parameter were 1.0 and the entire defeated group is to be killed, then an individual hero’s actions would be critical if the two warring groups were otherwise equal in their summed heroism scores—critical not only for the group’s survival, but also for the hero’s own survival. At the other extreme with the genocide parameter at zero, all individuals survive the war despite their defeat, meaning that a hero’s action, while costing him personally, does nothing to promote his own survival. It
follows, therefore, that as G increases, the probability of an individual’s heroism rebounding to
his own advantage also increases. Figure 4 shows the proportion of heroes q* as a function of
the G parameter across three different population sizes11 when, in equilibrium, it is equally
beneficial to be a hero or a coward. (See Proposition 3 and related proof in Appendix III on the
Journal’s website.) As population size increases from 50 through 100 to 200, the slopes of the
curves flatten out and the equilibrium proportion of heroes for a given level of the genocide
parameter is reduced.
In summary, selection on heroism in the context of warfare is governed by two
parameters, genocide and group size: (1) The larger the proportion of the defeated group that is
killed, the more critical a hero’s action likely will be for his own survival and reproduction, thus
for selection on heroism; (2) The smaller the group, the more likely it is that any given
individual’s heroic action will be critical to the group’s survival, thus again for his own survival
and for selection on heroism.
Summary and Conclusions
Using simulation, we have demonstrated that heroism—a willingness to fight for one’s
group even when it places oneself at a reproductive disadvantage relative to other group
members—can evolve based on the selective pressures of war within a population of groups that, t least sometimes, go to war with each other over scarce resources needed for individual and
group survival. In the first simulation agents are characterized by a general purpose altruism
(supporting both “communitarianism” and “heroism”) and there is modest positive selection on
that attribute. When agents are characterized by the two independent, special purpose attributes heroism and communitarian, however, both of those attributes evolve to higher levels than general purpose altruism, with heroism evolving to substantially higher levels than
communitarianism. All three attributes evolve insofar as the individual’s action, while
personally costly, can increase the possibility of the group surviving a war, therefore of the
individual’s also surviving—and thus of reproducing.
We have also shown that heroism is particularly likely to evolve when group size is small
and when the casualty rate for defeated groups is high. The ecological validity of the first of
these parameters is, of course, compatible with the accepted fact that our remote ancestors did
live in quite small groups (Dunbar 1993, 1996), but the second is more problematic. On the one
hand, capture of reproductively valuable females would have reduced the death rates of defeated groups, allowing some gene flow between defeated and winning groups. On the other, the probability of most or all members of a defeated group being wiped out would, surely, have been higher in small groups than in larger ones. This said, however, the analytic model does
demonstrate that complete genocide is not necessary for heroism to evolve. It can still do so—
albeit, to lower (and we expect more realistic) levels—with substantially less draconian
outcomes to warfare.
Our findings with respect to the possible impact of war on heroism are not incompatible
with heroism also evolving in response to Hamiltonian (1964) inclusive fitness. Heroism
certainly could have evolved among (small) kinship groups in which members fought in
response to threats to their kin. But our results suggest that heroism could also have evolved as a consequence of ancient warfare even absent close kinship among groups’ members. In fact,
attention to the proximate mechanisms we sketched at the outset suggests a complex interaction among the respective mechanisms. If kinship were all that mattered, we would expect that people drawn into modern, large-scale wars would be less easily “fooled” into risking their lives for a group of strangers by mere kinship rhetoric. On the other hand, if the processes we have identified were all that mattered, we would expect such kinship rhetoric to be irrelevant—which it is not. Clearly, further research is needed to investigate the relative contribution of these different selective forces.
Does our model “take the heroism out of heroism” (cf. Trivers 1971)? At the ultimate
level at which we are working, it does. This can be seen by considering the extreme case where
group size is reduced to one (the acting individual) and the genocide parameter is set at 1. Here
the individual is unambiguously fighting solely for his own life. While increasing group size
beyond that does produce other beneficiaries, selection still happens on heroism only to the
extent that an individual’s heroism is critical for his own survival—and thus can offset the
reproductive costs the hero must pay relative to other group members who benefit from his
action without having to pay them.
But, of course, most heroes are, no doubt, responding to particular emotions and
cognitive processes, not running such “private fitness accounting equations” in their heads.
Hector in front of Troy, for example, could have been motivated by knowing that only he had
any chance of defeating Achilles and, therefore, of saving his own life. But there is nothing of
that in The Iliad. On the contrary, the story makes it clear that he was responding not only to
fear for his own life—which he clearly does experience—but to his love of Troy, loyalty to his
comrades, honor and, perhaps in the end, to overconfidence that led him to think he did have a
chance of defeating Achilles. It is his response to those proximate emotions and beliefs that
justifies his status as a hero, not the evolutionary logic that, over thousands of previous
generations, produced those things in his particular, unfortunate circumstances.
Our finding that communitarianism and heroism, when modeled as separate, domain
specific attributes, both evolve to higher levels than domain general altruism has, we believe,
implications for research addressing the evolutionary roots of altruistic behavior in general. It
does not, of course, mean that the standard problem of “compensating” altruism for the private
costs individuals incur in performing altruistic acts is resolved. Models of how domain-specific,
special purpose altruistic dispositions might evolve must still respond to that problem—as,
indeed, our model has for heroism and communitarianism. Nevertheless, to the extent that
altruisms (plural) will evolve to higher levels when their costs are decoupled, we would expect
natural selection to favor such decouplings.
How far such decoupling might proceed would depend, of course, on the particular
design efficiencies to be gained by decoupling, and we might expect at least some limits in this
respect as well as complementaries at the proximate level (and, presumably, also at the neural
level). In general, however, our findings suggest that empirical research might profitably turn
attention toward identifying how different forms of altruism evolve differently across different
domains—a move that, once again, will surely be more successful if conducted with an explicit
sensitivity to both ultimate and proximate causation.
If the propensity for heroism in war is, indeed, a species typical attribute of humans as
our findings suggest is possible, does that condemn us to a future of “constant battles”? Our
model does not address that question. In that model, war is triggered by resource stress, but that is only an assumption (plausible, we believe, in light of the archeological studies cited earlier) not a finding. An answer to the question, then, depends on the particular proximate emotions and cognitive processes that provoke heroic action in the here-and-now, and whether those could, in themselves, also provoke warfare—not only among our ancestors, but also among
ourselves and our descendants.
Notes:
1 Hamilton’s rule is that altruism can evolve under the condition rb > c, that is when the benefit
to the recipient (b) multiplied by the relatedness of the recipient to the altruist (r) is greater than
the cost (c) to the altruist.
2 The list includes Sir R. A. Fisher who, in his foundational work The Genetical Theory of
Natural Selection (1930, p. 163), addressed the problem of “heroism” in tribal societies finding a
solution in terms that anticipated Hamilton’s relatedness-based theory of altruism.
3 Various authors have discussed the ways in which mechanisms that have been presented as
conceptually distinct might be subsumed by one another. For example: Sober and Wilson
(1998) see kin selection as being subsumed by multi-level selection; Reeve (2000) sees Sober
and Wilson’s model as a special case of Hamilton’s equations, and Humphrey (1997) sees kin
selection and reciprocity as generalizing into a single broad principle.
4 The term “Heroism” is commonly used more broadly. Becker and Eagly (2004), for example,
examine “heroism” exhibited by Carnegie medalists, by non-Jews who risked their lives to
rescue Jews during the Holocaust, by kidney donors, and by volunteers for the Peace Corps and
Doctors of the World. Here we restrict the term only to altruistic action in warfare.
5 We do recognize that female non-combatants who survive a war are historically less likely to
be slaughtered than are males. For simplicity here, however, we do not address possible
differences between males and females in the evolutionary consequences of warfare.
6 A growing literature has addressed multi-level selection on normative systems and
institutions, some of it paying particular attention to warfare as the agent of selection. Soltis,
Boyd and Richerson (1995), for example, used the extensive ethnographic literature on New
Guinea to assess whether warfare and associated group extinctions had been sufficiently frequent
to account for the evolution of group-benefiting normative and institutional patterns, concluding
that it had not; for such selective processes to have significantly influenced even one such group
attribute, between 500 and 1000 years would have been necessary. Other work assessing the
selective impact of warfare in multi-level terms includes Dawson (1999) and Richerson and
Boyd (1998); a foundational work on multi-level selection is Sober and Wilson (1998). For
critical assessments of multi-level selection, see Reeve (2000) and Maynard Smith (1998). In the
model to be developed here, groups can live or die as a consequence of warfare, but individual
attributes are what replicate, with the individual’s behavior potentially being critical for the
success and survival of the group, thus for replication of his own attributes.
7 From a multivariate uniform distribution with an identity correlation matrix with
~ U[50,100] g R , ~ U[0,0.5] b P , ~ U[0,0.5] C P , ~ U[0,0.1] C c , ~ U[0,0.1] H c , ~ U[0,1] A S ( C S ,
SH for the two attribute model), ~ [0,0.005] 1 M U , ~ [0,0.05] 2 M U . Notations specified here
are employed in the formal statement on the Journal’s website.
8 Mean evolved communitarianism and heroism values have a zero correlation. Hence there
was no tendency for something akin to generalized altruism to evolve in the second, domain
specific model; nor was there evidence for agents to “specialize” in a particular form of domainspecific
altruism.
9 Other key input parameters such as cost of heroism, the size of groups’ resource bases, and
variation in the size of groups’ resource base were all significant negative predictors of heroism.
10 In our simulation, genocide in the event of defeat is the only way that agents die. Reducing
the level of genocide in the simulation produced exponential population growth and constant
war—along with a vastly increased demand on computational resources. With G = 1, however,
population size remains more or less constant, cycling around the same mean. Accordingly, for
the analysis of genocide and population size, we have relied exclusively on the analytic results.
11 In the analytical model with the total population consisting of two groups, the curves in
Figure 4 correspond to group sizes of 25, 50, and 100. In the simulation runs reported above (in
which G =1), the average resource base size (which constrains group size) was 75 (and ranged
from 50 to 100), with mean evolved values of heroism of .475 and .669, depending,
respectively, on whether the cost of heroism was paid per generation or per war.
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Figure 1.
Frequency distribution of evolved altruism in the one-attribute model. Mean altruism =
0.250.
Figure 2a.
Frequency distribution of evolved heroism in the two-attribute model. Mean heroism =
0.669.
Figure 2b.
Frequency distribution of evolved communitarianism in the two-attribute model. Mean
communitarianism = 0.305.
Figure 3.
Frequency distribution of evolved heroism in when the cost of heroism is paid every
generation. Mean heroism = 0.475.
Figure 4.
Equilibrium proportion of heroes q * across two groups as a function of genocide G for
three different population sizes n, 0.5 b P = , c = 0.05 .
Political Science, University of Miami
Holly Arrow
Psychology, University of Oregon
Doug Kennett
Anthropology, University of Oregon
John Orbell
Institute of Cognitive and Decision Sciences
University of Oregon
Authors’ Note: We thank James Fowler, Warren Holmes, Timothy Johnson, David Osborn,
Frances White, members of the “Warfare and Killing” spring 2006 seminar and several
anonymous reviewers for helpful comments on earlier drafts. We are also grateful to members
of the Evolution Focus Group of the Institute for Cognitive and Decision Sciences for helpful
feedback after early presentations of this project.
“Heroism” in Warfare
As a Functionally Specific Form of Altruism.
“When two tribes of primeval man, living in the same country, came into competition, the tribe
including the greater number of courageous, sympathetic and faithful members…would without
doubt succeed best and conquer the others.” Darwin, The Descent of Man, chapter 5, p. 156.
Abstract
We use two simulations to explore the possibility that heroism (risking one’s life fighting for the
group) evolved as a specialized form of altruism, selected as a result of warfare through humans’
ancient past. In the first simulation, altruism as a general disposition promotes all forms of
altruistic behavior, including heroism. In the second, heroism and communitarianism (altruistic
actions beyond warfare) are separate, domain-specific dispositions. Warfare selects weakly for
altruism, more strongly for communitarianism, and substantially for heroism. Heroism evolves
more readily when groups are small and mortality in defeated groups is high—features plausibly
characteristic of humans’ ancestral past.
When Homer asks in the Iliad “What god drove them to fight with such a fury?” he was
posing a question that remains problematic three thousand years later: What makes soldiers
willing to fight at great personal risk for their polis, their tribe, their ethnic group, or their
country? There is no doubt that soldiers often do fight in this manner; the history of warfare is, in large part, the history of ordinary men (and, in recent times, women) who willingly confronted the risk of death when fighting for their tribe, polis, country, or other group. In the present paper we call this behavior “heroism.” Heroism is well recognized as a potential human behavior, but the existence of heroism is most commonly treated as a background assumption in examining the nature of war. In this paper we address what we see as the central puzzle of heroism: How could it have evolved?
Most studies that address heroic actions in warfare have focused on “proximate”
answers—emotional, cognitive or other mechanisms that prompt such behavior in the here and
now. Thus, for example, loyalty to small groups of fighting comrades (Gat 1999; Shills and
Janowitz 1948; Stern 1995) or, perhaps, a regiment (Holmes 1985), the rage and fear provoked
by an attack, or threat of an attack, against one’s own group (Horowitz 2001), and cognitive
errors such as overconfidence about the risks involved in heroism (D. D. P. Johnson 2004; D. D.
P. Johnson et al. 2006; Wrangham 1999b) have all been argued to support such behavior. By the same token, social psychologists have identified many mechanisms likely to be implicated in
humans’ frequent xenophobic willingness to engage in hostile action against other groups—most
famously, the ease with which we categorize others into “in-group” and “out-group,” favoring
the former and, often, demonizing the latter (Ackerman et al. 2006; Rothbart and Lewis 1988;
Rothbart and Taylor 1992; Tajfel and Turner 1979).
But this level of explanation, while certainly compelling, is only one level of explanation,
the other being the “ultimate” or “evolutionary” processes that selected for such proximate
mechanisms through evolutionary time. Most theorizing in the social sciences invokes only
proximate mechanisms, but such mechanisms must have evolved via some evolutionary process of selection on genes, on cultural patterns, or on gene-culture interactions (Boyd and Richerson
1985; Odling-Smee, Laland, and Feldman 2003; Richerson and Boyd 2005). A complete
explanation of the behavior in question should thus consider both proximate and ultimate
causation. Here we focus on the latter: How could mechanisms supporting “heroism” have
been positively selected during humans’ ancestral past? By definition, heroism involves a
fitness cost to the individual (via increased risk of death), hence we might expect it to be selected against, eventually disappearing from the human population. Yet heroism certainly does happen
with some frequency across a wide variety of cultures, suggesting that it is a species typical trait. How could this have come to be?
One standard answer is Hamilton’s (1964) famous explanation of how altruism in general
might have evolved. Hamilton turned attention to the selective fate of genes rather than of
individuals per se, pointing out how sacrificial action by one individual could result in genes
supporting that act spreading more rapidly than had the individual acted selfishly—as long as the beneficiaries of such action were appropriately close relatives.1 Inclusive fitness has been
invoked by Alexander (1979), Masters (1983), Shaw and Wong (1989), Thayer (2004), van der
Dennen and many others, in attempts to understand the genetic basis of humans’ propensity to
warfare and, using our term, therefore to heroism.2 The general point is that, during humans’
remote ancestral past groups were, most probably, comprised largely of quite close kin, making a disposition toward sacrificial action on behalf of the group an attribute that could evolve, despite adaptive costs to the individual per se.
Importantly, this account explains heroism as a product of the same ultimate, selective
processes that might produce any other altruistic behaviors. Sharing meat from a hunt, for
example, could be explained as readily as fighting for one’s group in a battle with other groups.
The account nicely combines a plausible reconstruction of family-based social organization in
our ancestral past with a foundational idea in modern evolutionary biology. It is also consistent
with the pervasiveness of kinship rhetoric—the “fatherland, “band of brothers” etc—in modern
warfare (G. Johnson 1987). It is quite plausible that the particular form of altruism that interests us, heroism in defense of one’s group, evolved via the combined impact of multiple processes
that have been invoked for explaining altruism in general—including kin altruism, reciprocity in
exchange relationships (Axelrod 1984; Trivers 1971) and multi-level selection (most notably,
Sober and Wilson 1998).3
Here we develop the possibility that, even without the impact of kin altruism and
reciprocity, heroism could have evolved as a “domain specific” form of altruism in response to
warfare in our ancestral past. The idea that human cognitive architecture consists, in substantial
part, of functionally specific information processing modules is widely accepted in evolutionary
psychology and in cognitive neuroscience more broadly, although scholars differ in arguing for a
strong version of modularity (notably Cosmides and Tooby 1994; Hirschfeld and Gelman 1994;
Sperber 1994) or for a combination of specialized modules and some more general functionality
(notably Buller 2005; Fodor 2000; Mithen 1996). The standard strong modularity argument is
given by Cosmides and Tooby (1994 p. 89) as follows:
…different adaptive problems require different solutions… Speed,
reliability, and efficiency can be engineered into specialized mechanisms
because there is no need to engineer a compromise between competing task
demands…As a rule, when two adaptive problems have solutions that are
incompatible or simply different, a single general solution will be inferior to
two specialized solutions.
However this argument plays itself out (for a constructive appraisal see Barrett
and Kurzban 2006), the extensive literature on human altruism has, to our knowledge,
focused exclusively on the problem of how a generalized disposition to altruistic
behavior might have evolved. The possibility that multiple, domain specific altruistic
dispositions might have evolved independently, each in response to a distinctive
adaptive problem and each producing, therefore, at least somewhat different proximate
mechanisms appears to have been overlooked. Since heroism is, by our definition, an
altruistic response on behalf of one’s group in the event of war with some other group,
its task demands would appear prima facie quite distinct from those of other altruistic
behaviors (e.g., providing food to others and caring for the sick). This makes it
plausible that heroism could have evolved on its own distinct trajectory, independent
of such other forms of altruism, and with warfare as the agent of selection.4
Warfare is a group-level phenomenon, and its broad outcomes (victory, defeat,
and standoff) are also group-level phenomena. Accordingly, all group members—those
who fight as well as those who do not—can suffer “genetic death” should the group be
defeated.5 However, the consequences of group-level outcomes can differ among the
individuals comprising such groups. Most obviously, those who fight in the group’s wars
run an increased risk of dying as a result, reducing the probability that their genes will
pass on to the next generation. Should the group be victorious, survivors stand to benefit
significantly, but heroes are less likely to be among those survivors. Along with males’
sexual access to females from the defeated group (viz: rape, captured concubines etc.),
the spoils of victory can include access to the defeated group’s territory and whatever
resources that territory contains. Clearly, frequent warfare during humans’ ancestral past
could have had major adaptive consequences—positive and negative—for individuals in
the warring groups.6 Since heroism as we have defined it is central to the outcome of
wars, the evolution of heroism is a good candidate for being among those consequences.
How frequent was warfare in our ancestral past? Attempts to answer this question
have provoked a good deal of controversy. Some accounts of the archeological record
suggest that war was quite frequent. LeBlanc (2003, pp. xii - xiii) comments:
Just how common was warfare in the past? I have finally concluded that
warfare was quite common… and that my findings on three continents and
within multiple time periods were not a fluke but the norm. This has led me
to reason that if conflict was common, then it must have been an important
occurrence in the course of human history. (Italics in the original.)broadly, “coalitional violence”—has been present, at least to some extent, throughout our
ancestral past, and our question is whether warfare, happening with any significant frequency,
could have selected for heroism. The simulations that we will now describe allow us to study the
relationship between frequency of warfare and such selection, and we will report findings to that
effect later in the paper.
Two Simulations
In the first simulation, altruism is domain general, promoting: (1) communitarianism—a
group-benefiting behavior that increases the mean fitness of group members but does not involve
fighting enemies; and (2) heroism a behavior that benefits the group by contributing to its
success in warfare. In the second simulation, these two behaviors are promoted by two domain
specific attributes that are free to evolve independently. A formal description of the two
simulations can be found in Appendix 1 which is available on the Journal’s website; here we
provide a verbal account of the simulation’s key features.
Model 1: Domain general altruism
Groups and resources. Four groups each occupy a fixed resource base on which its
members are critically dependent for nourishment. The size of the resource base for each group
is a parameter; it can vary across groups but not across time and sets a limit on sustainable group size.
Altruism. Each group member has a propensity to behave altruistically, which is modeled
as a continuous variable with values between 0 (never) and 1 (always). The starting mean
altruism of each group is a parameter of the model; the altruistic propensity of a group’s
members varies in a uniform distribution centered at the specified mean. An individual’s
altruism increases the reproductive success of all group members and contributes to the group’s
success in warfare as a function of that individual’s altruism score (the more altruistic, the more
benefit to others across both domains); the reproductive cost paid by the individual is similarly a function of that score (the more altruistic, the greater the cost to self).
Baseline Fertility and Individual Reproduction. Baseline fertility is the probability of
each group member’s having an offspring in a given generation—prior to any adjustments based
on the incidence of altruism in the group (the more altruism summed across all group members,
the greater each group member’s probability of reproducing), the individual’s own altruism score (the greater that score, the smaller the probability of that individual’s reproducing), or the
outcome of war (group members who do not survive wars do not reproduce). Should there be
more than one war in a generation, the cost of participating (reduced probability of surviving to
reproduce) is assessed separately for each.
Going to War. A group goes to war when its population exceeds the carrying capacity of
the resource base—that is, when the resource base per capita drops below a specified threshold.
At this point the only way a group can support its members is to capture the resources held by
another group. Target groups are chosen at random, with any group that is attacked by another
having to defend itself by fighting.
We recognize, of course, that competition for scarce economic resources is not the only
factor that increases the probability of war; this is merely a simplifying assumption for the
"generic" wars of our model. Chagnon (1988) and others, for example, point to the frequency
with which capturing females is the stated reason for hunter-gatherers going to war, and females are obviously a reproductively significant “resource” (Trivers 1972). It is also the case that
particular wars may have highly visible “triggering events” (the seduction of Helen, the
assassination of the Archduke Ferdinand). This is not, however, inconsistent with an ultimate
explanation in terms of population pressures on groups’ resources (Shaw and Wong 1989, p.11).
Helen’s seduction, for example, took place during a time of mounting economic competition
between the Trojans and the Greeks. Certainly, there must have been a complex relationship
between capturing land and capturing females motivating our ancestors to go to war—as
suggested by Betzig (2005, p. 335) in her analysis of the Biblical case as recorded, among other
places, in Deuteronomy 20:13-17:
God handed Moses two different laws about spoils. One was for wars with
close neighbors; the other was for more remote wars. When their enemies
lived far away, he said, ‘you shall put all the males to the sword, but the
women and the little ones, the cattle, and everything else in the city, all its
spoil, you shall take as booty for yourselves.’ But when their enemies lived
nearby, ‘you shall save alive nothing that breathes, but you shall utterly
destroy them.’ Land was the limiting factor.
The assumption on which our model is based—that resource scarcity leads to
war—is perfectly compatible with the capture of female reproductive capacities as one
among many proximate causes of war. The idea that land, thus somatic resources, was
a critical “limiting factor” is, nevertheless, widely assumed among anthropologists,
biologists, ethologists, political scientists, psychologists, sociologists and others
addressing the causes of ancient warfare (e.g. Alcock 1978; Carneiro 1970; Durham
1976; Eibl-Eibesfeldt 1979; Kennett et al. 2006; Kennett and Kennett 2000, 2006;
Lambert 1997; Lambert and Walker 1991; Shaw and Wong 1989; Thayer 2004; van
den Berghe 1978; Wilson and Wrangham 2003) and we adopt it here.
Victory and Defeat. The group with the highest summed altruism score wins. This
represents the total amount of heroic fighting available within each group. Consistent with the
“imbalance of power” hypothesis developed by biologists and primatologists (Alexander 1979;
Wrangham 1999a)—as well as with Napoleon’s comment that “God is on the side of large
armies”—the larger group will win when the mean propensity to altruism is equal across two
contending groups. However, a smaller population can defeat a larger one if its members are
substantially more altruistic (hence more heroic).
Death. Some proportion of the defeated group is killed. We will report our simulation
findings from the strong assumption that all are killed, so that defeated groups are eliminated
from the evolving population. While this extreme assumption is certainly consistent with some
examples from the historical record (Potidaea, Mytilene, and Melos in the Peloponnesian war,
for example), the proportion of a defeated group that is killed is an important parameter, to which we will return in a later analytic section (specifics are available on the Journal’s website). In the simulation, members of the victorious group migrate to the newly captured territory such that per-capita resources in the two areas are equalized. Once migration is complete, no “memory” of prior ties persists, so the two “daughter” groups created by the schism are just as likely to attack one another in subsequent rounds as they are to attack a different group.
Variation. Selection on altruism—as on any other attribute—can, of course, only happen
in the context of variation on that attribute. As noted, members of each group vary in altruism at the start of the simulation. During reproduction, variation is maintained by two mechanisms.
One models random mutation (defined as 1 M in the appendix on the Journal’s website); by
default, it occurs here in less than one percent of offspring, and can result in the offspring having
an altruism level quite different from the parent. The other mechanism (defined as 2 M in the
appendix) simulates (without directly modeling) sexual reproduction, such that offspring
resemble their parent within a range of variability.
Model 2: Domain specific with both “communitarianism” and “heroism”
The second simulation has the same general structure, except that each individual now
has two domain specific altruistic propensities, heroism and communitarianism, each similarly
modeled as varying between 0 and 1. Starting mean heroism and communitarianism for each
group are separate parameters of the model. Member levels for the two attributes are
independently drawn from a uniform distribution centered at the specified mean. Reproductive
costs from communitarianism are paid every generation; those from heroism are paid per war.
Method
We ran 10,000 simulations of each of the two models. To ensure that the data we report
reflect a wide variety of possible evolutionary environments, the following parameters were
drawn randomly7 for each run:
1. Each group’s resource endowment ( g R )—specified separately for each group;
2. Baseline fitness—the probability of reproductive success for all agents before the
consequences of altruism, communitarianism, heroism or warfare are incorporated ( b P );
3. The fitness advantage to group members from an individual’s communitarianism ( c P );
4. The cost to the individual agent of its own communitarianism ( C C );
5. The cost to the individual agent of its heroism ( H C );
6. (In Model 1) Mean starting altruism ( A S );
7. (In Model 2) Mean starting communitarianism and heroism ( C S , H S );
8. The two sources of variation (two types of mutation) in the attributes of offspring as
specified in the appendix ( 1 M , 2 M ).
Two parameters were fixed across all the simulations: the survival threshold, T =1 (one unit of
resource per capita), and the number of groups, which was four.
Each of the 10,000 simulations was run for 1100 generations, allowing a sufficient
number of generations for the system to evolve away from diverse starting values. For the last
100 generations we recorded the moving average of altruism in the domain general model, and
of communitarianism and heroism in the domain specific model. The simulation parameters
(which varied across runs as described above) and the frequency of wars across all generations
(an emergent variable) were also recorded for each simulation. In a smaller sample of runs to
check for the system’s sensitivity to some of our choices, we increased the number of
generations and varied both the number of groups and the length of the moving average across
which we recorded data. None of these modifications affected our results.
Findings
With the domain general model we observed a modest but significant selection on
altruism. This is shown in Figure 1, which is a frequency distribution for evolved altruism. The
horizontal axis specifies the altruism level averaged across the last 100 generations of the 10,000 simulations and the vertical axis records the density of cases across the randomized parameter space. The mean altruism value in this positively skewed distribution is 0.25 (the median is 0.17), interpretable as the mean individual being 25% altruistic—or, alternatively, that the mean individual would have a .25 probability of contributing to group members’ fitness via its communitarian activities and a .25 probability of fighting heroically in any war.
Despite the fact that both behavioral consequences of general purpose altruism are
reproductively costly to individuals, therefore, we do observe some selection on that attribute.
Agents pay a reproductive cost proportional to the strength of their generalized disposition to
altruism, but this cost is outweighed by the benefits that individuals reap from their group’s
success in warfare. Multiple regression of ending altruism values on parameters in the model
(see Appendix II on the Journal’s website) shows a negative association between evolved
altruism and (1) the cost of altruism; (2) the average size of group resource bases (hence, the
average size of groups); and (3) variation in resource base size (hence, variation in size among
the groups); it also shows a positive association between altruism and the frequency of war. This
suggests that higher (and less typical) evolved values of altruism occurred in simulation runs
where the personal cost of altruism was low, where groups were smaller and more equal in size,
and where wars were quite frequent.
With the domain specific model, in which communitarianism and heroism were free to
evolve as separate attributes, Figures 2a and 2b show that: (1) heroism evolved to substantially
higher levels than communitarianism with means of 66.9% and 30.5% respectively and medians
of 70.3 and 22.6 respectively; (2) both attributes evolved to significantly higher levels than did
altruism in the domain general model. There are, therefore, three findings to be explained—first, the fact that there was some positive selection on general purpose altruism, despite its costs to the individual; second, that there was stronger selection on heroism than on communitarianism
despite their respective costs; and third, that when communitarianism and heroism are free to
evolve separately, both evolve to higher levels than when they are combined as expressions of a
single, general purpose altruism attribute. We will address the evolution of general purpose
altruism shortly; first we discuss the relative evolution of the two domain specific forms.
Why does heroism evolve to higher levels than communitarianism?
In our model heroism is only costly in the event of war. Given that war only occurs
periodically—in the simulations any given group goes to war with a probability of about .5 per
generation—while communitarianism occurs in every generation, perhaps heroism evolves to a
higher level simply because the cost of heroism is paid less frequently than the cost of
communitarianism.
This does appear to be part of the story. Figure 3 reports the distribution of heroism
when its cost is paid every generation rather than per war. With the mean of this distribution
now at .475 (as opposed to .669 in the original domain specific model), clearly this cost
difference accounts for some of the relatively greater selection on heroism. Across the range of
parameters, it appears that selection on heroism is stronger when such behavior is not invoked
every generation, as is communitarianism. But that is only part of the story, since the mean of
heroism is still substantially higher when it is evoked every generation than is the mean of
communitarianism (.305) when it, too, is evoked every generation.
Another part of the story concerns the immediacy of the military benefits a group reaps
from communitarianism and heroism. Since reproduction happens at the end of each generation,
communitarianism increases the number of fighters a group will be able to field in the next
generation while heroism increases a group’s fighting capacity in the present one. Thus
communitarianism can only increase a group’s future fighting ability—assuming the group
survives any wars in the current generation, which will depend on group size and heroism now.
The delayed effect of communitarianism is, of course, consistent with real-life population
dynamics: since infants and young children confer no advantage in war, the war-fighting
benefits of increased fertility cannot be realized immediately.
Communitarianism is also a “double-edged sword” for a group’s fighting ability. While
larger groups are more likely to win wars, high population growth can also be responsible for
getting a group into war in the first place (faster population growth means that a group reaches
its carrying capacity more rapidly), when, in a war, there is some chance of its losing, thus of
selection against communitarianism. Heroism, on the other hand, is an unambiguous good for a
group. In the absence of war, and assuming its cost is only paid in the event of war, it does no
harm to the individual or the group, but in the event of war it can be decisive for victory.
Ultimately, selection on communitarianism and on heroism must be a function of whether
the behavior produced by those dispositions is “critical” to the group’s success in warfare—thus
to the individual’s survival and reproduction—and while both contributions might be critical for
the outcome of a given war, heroism is more likely to be critical than communitarianism.
Why, then, does altruism evolve?
The positive selection on general purpose altruism in the single attribute model, modest
as it is, can now be explained in the same way as selection on heroism and communitarianism in
the two attribute model. Altruists fight for their groups, and a given altruist’s fighting can be
critical for the group’s survival, thus for the altruist’s own survival. Similarly—although with a
lower probability—an altruist’s communitarian behaviors might also be critical for such survival
via the role that numbers as such can play in outcome of wars.
Why do communitarianism and heroism evolve further than general-purpose altruism?
An unexpected finding was that when heroism and communitarianism are free to evolve
independently, both evolve to higher levels than does the single, general purpose altruism.8 What accounts for this?
In retrospect, at least, the explanation is clear. When a single, general purpose mechanism
performs two distinct functions (A and B) both of which decrease individual fitness, selection
against that mechanism based on the costs of A will also select against B, and vice versa. The
general purpose capacity will, in other words, pay a cost whenever either of the two behaviors is
invoked. Conversely, when distinct, special purpose mechanisms are dedicated to A and B,
selection based on the individual costs of B has no implications for selection based on the costs
of A, and vice versa. Thus, breaking the general purpose mechanism down into two special
purpose mechanisms means that the evolution of one attribute is not handicapped by the cost of
the other. Notice that heroism, invoked only every second generation or so, benefits more from
such “de-linking of costs” than does communitarianism.
This finding points to the cost-benefit logic that could support a modular design for the
evolved “cognitive architecture of altruism.” The degree to which propensities for different
forms of altruistic behavior are, in modern humans, actually correlated (positively or negatively) is, of course, an empirical question awaiting investigation by other means than computational
modeling. Also, findings from the model do not resolve the general problem of how privately
costly altruistic dispositions (whether unitary or multiple) actually did evolve. Each disposition
must still be positively selected despite the costs that it generates for the acting individual.
Nevertheless, the finding does raise the possibility that thinking about altruism as a bundle of
special purpose forms of altruist behaviors—each evoked in some circumstances but not in
others—and not as a single general purpose disposition evoked regardless of the contextual
specifics, will prove a fruitful path to follow. Whatever they might be, selective pressures
favoring special purpose altruisms (plural) would not have to be as strong, ceteris paribus, to
overcome the associated costs as would selective pressures favoring general purpose altruism
(singular) if the behavior in question were to evolve.
The case for humans’ past being one of “constant battles” becomes increasingly difficult
to make the further back in time one looks and, as discussed above, this has fueled arguments
about just how “constant” warfare actually was in our ancestral past. While those arguments are
for archeologists to resolve, as shown on-line in Appendix II, in our simulation the frequency of
warfare does positively predict selection on altruism in the first model, and on communitarianism and heroism in the second one.9
One strong assumption of both simulations is that all members of defeated groups are
killed. To examine what happens when this is relaxed, we turned to an analytic model.
An Analytic Model of Heroism
Although our simulations make a range of simplifying assumptions, they are still more
complex than what can be readily modeled analytically. Accordingly, we employed a simplified
analytic model to explore more rigorously the impact of a few key parameters on the evolution of heroism.10 The cost of this precision was to reduce the number of groups from four to two
equally sized groups and to model heroism as a dichotomous variable with each agent being
either fully heroic or a complete coward. (Appendix III on the Journal’s webpage contains a
formal description of the model.)
As noted above, the simulation results are based on the strong assumption that a defeated
group is entirely wiped out. Although the archeological record does suggest that genocide was at
least sometimes practiced (Keeley 1997; LeBlanc and Register 2003) and the elimination of a
competing group has been documented in a chimpanzee population (Wilson and Wrangham
2003), we simply do not know just how pervasive this actual outcome was. What happens when
the simulation’s genocide assumption is relaxed?
In the analytic model, any individual’s probability of surviving a war is defined by 1 –
dG, when d is the probability of the group’s being defeated (a function of the relative summed
heroism scores in the two contending groups) and G is the proportion of the defeated group that
is killed (the genocide parameter). At one extreme, therefore, if the genocide parameter were 1.0 and the entire defeated group is to be killed, then an individual hero’s actions would be critical if the two warring groups were otherwise equal in their summed heroism scores—critical not only for the group’s survival, but also for the hero’s own survival. At the other extreme with the genocide parameter at zero, all individuals survive the war despite their defeat, meaning that a hero’s action, while costing him personally, does nothing to promote his own survival. It
follows, therefore, that as G increases, the probability of an individual’s heroism rebounding to
his own advantage also increases. Figure 4 shows the proportion of heroes q* as a function of
the G parameter across three different population sizes11 when, in equilibrium, it is equally
beneficial to be a hero or a coward. (See Proposition 3 and related proof in Appendix III on the
Journal’s website.) As population size increases from 50 through 100 to 200, the slopes of the
curves flatten out and the equilibrium proportion of heroes for a given level of the genocide
parameter is reduced.
In summary, selection on heroism in the context of warfare is governed by two
parameters, genocide and group size: (1) The larger the proportion of the defeated group that is
killed, the more critical a hero’s action likely will be for his own survival and reproduction, thus
for selection on heroism; (2) The smaller the group, the more likely it is that any given
individual’s heroic action will be critical to the group’s survival, thus again for his own survival
and for selection on heroism.
Summary and Conclusions
Using simulation, we have demonstrated that heroism—a willingness to fight for one’s
group even when it places oneself at a reproductive disadvantage relative to other group
members—can evolve based on the selective pressures of war within a population of groups that, t least sometimes, go to war with each other over scarce resources needed for individual and
group survival. In the first simulation agents are characterized by a general purpose altruism
(supporting both “communitarianism” and “heroism”) and there is modest positive selection on
that attribute. When agents are characterized by the two independent, special purpose attributes heroism and communitarian, however, both of those attributes evolve to higher levels than general purpose altruism, with heroism evolving to substantially higher levels than
communitarianism. All three attributes evolve insofar as the individual’s action, while
personally costly, can increase the possibility of the group surviving a war, therefore of the
individual’s also surviving—and thus of reproducing.
We have also shown that heroism is particularly likely to evolve when group size is small
and when the casualty rate for defeated groups is high. The ecological validity of the first of
these parameters is, of course, compatible with the accepted fact that our remote ancestors did
live in quite small groups (Dunbar 1993, 1996), but the second is more problematic. On the one
hand, capture of reproductively valuable females would have reduced the death rates of defeated groups, allowing some gene flow between defeated and winning groups. On the other, the probability of most or all members of a defeated group being wiped out would, surely, have been higher in small groups than in larger ones. This said, however, the analytic model does
demonstrate that complete genocide is not necessary for heroism to evolve. It can still do so—
albeit, to lower (and we expect more realistic) levels—with substantially less draconian
outcomes to warfare.
Our findings with respect to the possible impact of war on heroism are not incompatible
with heroism also evolving in response to Hamiltonian (1964) inclusive fitness. Heroism
certainly could have evolved among (small) kinship groups in which members fought in
response to threats to their kin. But our results suggest that heroism could also have evolved as a consequence of ancient warfare even absent close kinship among groups’ members. In fact,
attention to the proximate mechanisms we sketched at the outset suggests a complex interaction among the respective mechanisms. If kinship were all that mattered, we would expect that people drawn into modern, large-scale wars would be less easily “fooled” into risking their lives for a group of strangers by mere kinship rhetoric. On the other hand, if the processes we have identified were all that mattered, we would expect such kinship rhetoric to be irrelevant—which it is not. Clearly, further research is needed to investigate the relative contribution of these different selective forces.
Does our model “take the heroism out of heroism” (cf. Trivers 1971)? At the ultimate
level at which we are working, it does. This can be seen by considering the extreme case where
group size is reduced to one (the acting individual) and the genocide parameter is set at 1. Here
the individual is unambiguously fighting solely for his own life. While increasing group size
beyond that does produce other beneficiaries, selection still happens on heroism only to the
extent that an individual’s heroism is critical for his own survival—and thus can offset the
reproductive costs the hero must pay relative to other group members who benefit from his
action without having to pay them.
But, of course, most heroes are, no doubt, responding to particular emotions and
cognitive processes, not running such “private fitness accounting equations” in their heads.
Hector in front of Troy, for example, could have been motivated by knowing that only he had
any chance of defeating Achilles and, therefore, of saving his own life. But there is nothing of
that in The Iliad. On the contrary, the story makes it clear that he was responding not only to
fear for his own life—which he clearly does experience—but to his love of Troy, loyalty to his
comrades, honor and, perhaps in the end, to overconfidence that led him to think he did have a
chance of defeating Achilles. It is his response to those proximate emotions and beliefs that
justifies his status as a hero, not the evolutionary logic that, over thousands of previous
generations, produced those things in his particular, unfortunate circumstances.
Our finding that communitarianism and heroism, when modeled as separate, domain
specific attributes, both evolve to higher levels than domain general altruism has, we believe,
implications for research addressing the evolutionary roots of altruistic behavior in general. It
does not, of course, mean that the standard problem of “compensating” altruism for the private
costs individuals incur in performing altruistic acts is resolved. Models of how domain-specific,
special purpose altruistic dispositions might evolve must still respond to that problem—as,
indeed, our model has for heroism and communitarianism. Nevertheless, to the extent that
altruisms (plural) will evolve to higher levels when their costs are decoupled, we would expect
natural selection to favor such decouplings.
How far such decoupling might proceed would depend, of course, on the particular
design efficiencies to be gained by decoupling, and we might expect at least some limits in this
respect as well as complementaries at the proximate level (and, presumably, also at the neural
level). In general, however, our findings suggest that empirical research might profitably turn
attention toward identifying how different forms of altruism evolve differently across different
domains—a move that, once again, will surely be more successful if conducted with an explicit
sensitivity to both ultimate and proximate causation.
If the propensity for heroism in war is, indeed, a species typical attribute of humans as
our findings suggest is possible, does that condemn us to a future of “constant battles”? Our
model does not address that question. In that model, war is triggered by resource stress, but that is only an assumption (plausible, we believe, in light of the archeological studies cited earlier) not a finding. An answer to the question, then, depends on the particular proximate emotions and cognitive processes that provoke heroic action in the here-and-now, and whether those could, in themselves, also provoke warfare—not only among our ancestors, but also among
ourselves and our descendants.
Notes:
1 Hamilton’s rule is that altruism can evolve under the condition rb > c, that is when the benefit
to the recipient (b) multiplied by the relatedness of the recipient to the altruist (r) is greater than
the cost (c) to the altruist.
2 The list includes Sir R. A. Fisher who, in his foundational work The Genetical Theory of
Natural Selection (1930, p. 163), addressed the problem of “heroism” in tribal societies finding a
solution in terms that anticipated Hamilton’s relatedness-based theory of altruism.
3 Various authors have discussed the ways in which mechanisms that have been presented as
conceptually distinct might be subsumed by one another. For example: Sober and Wilson
(1998) see kin selection as being subsumed by multi-level selection; Reeve (2000) sees Sober
and Wilson’s model as a special case of Hamilton’s equations, and Humphrey (1997) sees kin
selection and reciprocity as generalizing into a single broad principle.
4 The term “Heroism” is commonly used more broadly. Becker and Eagly (2004), for example,
examine “heroism” exhibited by Carnegie medalists, by non-Jews who risked their lives to
rescue Jews during the Holocaust, by kidney donors, and by volunteers for the Peace Corps and
Doctors of the World. Here we restrict the term only to altruistic action in warfare.
5 We do recognize that female non-combatants who survive a war are historically less likely to
be slaughtered than are males. For simplicity here, however, we do not address possible
differences between males and females in the evolutionary consequences of warfare.
6 A growing literature has addressed multi-level selection on normative systems and
institutions, some of it paying particular attention to warfare as the agent of selection. Soltis,
Boyd and Richerson (1995), for example, used the extensive ethnographic literature on New
Guinea to assess whether warfare and associated group extinctions had been sufficiently frequent
to account for the evolution of group-benefiting normative and institutional patterns, concluding
that it had not; for such selective processes to have significantly influenced even one such group
attribute, between 500 and 1000 years would have been necessary. Other work assessing the
selective impact of warfare in multi-level terms includes Dawson (1999) and Richerson and
Boyd (1998); a foundational work on multi-level selection is Sober and Wilson (1998). For
critical assessments of multi-level selection, see Reeve (2000) and Maynard Smith (1998). In the
model to be developed here, groups can live or die as a consequence of warfare, but individual
attributes are what replicate, with the individual’s behavior potentially being critical for the
success and survival of the group, thus for replication of his own attributes.
7 From a multivariate uniform distribution with an identity correlation matrix with
~ U[50,100] g R , ~ U[0,0.5] b P , ~ U[0,0.5] C P , ~ U[0,0.1] C c , ~ U[0,0.1] H c , ~ U[0,1] A S ( C S ,
SH for the two attribute model), ~ [0,0.005] 1 M U , ~ [0,0.05] 2 M U . Notations specified here
are employed in the formal statement on the Journal’s website.
8 Mean evolved communitarianism and heroism values have a zero correlation. Hence there
was no tendency for something akin to generalized altruism to evolve in the second, domain
specific model; nor was there evidence for agents to “specialize” in a particular form of domainspecific
altruism.
9 Other key input parameters such as cost of heroism, the size of groups’ resource bases, and
variation in the size of groups’ resource base were all significant negative predictors of heroism.
10 In our simulation, genocide in the event of defeat is the only way that agents die. Reducing
the level of genocide in the simulation produced exponential population growth and constant
war—along with a vastly increased demand on computational resources. With G = 1, however,
population size remains more or less constant, cycling around the same mean. Accordingly, for
the analysis of genocide and population size, we have relied exclusively on the analytic results.
11 In the analytical model with the total population consisting of two groups, the curves in
Figure 4 correspond to group sizes of 25, 50, and 100. In the simulation runs reported above (in
which G =1), the average resource base size (which constrains group size) was 75 (and ranged
from 50 to 100), with mean evolved values of heroism of .475 and .669, depending,
respectively, on whether the cost of heroism was paid per generation or per war.
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Figure 1.
Frequency distribution of evolved altruism in the one-attribute model. Mean altruism =
0.250.
Figure 2a.
Frequency distribution of evolved heroism in the two-attribute model. Mean heroism =
0.669.
Figure 2b.
Frequency distribution of evolved communitarianism in the two-attribute model. Mean
communitarianism = 0.305.
Figure 3.
Frequency distribution of evolved heroism in when the cost of heroism is paid every
generation. Mean heroism = 0.475.
Figure 4.
Equilibrium proportion of heroes q * across two groups as a function of genocide G for
three different population sizes n, 0.5 b P = , c = 0.05 .
Beating ploughshares back into swords:warfare in the Linearbandkeramik - Mark Golitko & Lawrence H. Keeley∗
∗ Department of Anthropology, University of Illinois at Chicago, 1007 W. Harrison St. M/C 027 Chicago, IL
60607, USA (Email: mgolit1@uic.edu, lkeeley@uic.edu)
antiquity 81 (2007): 332–342
60607, USA (Email: mgolit1@uic.edu, lkeeley@uic.edu)
antiquity 81 (2007): 332–342
Armed with a number of powerful arguments, the authors invite us to face up to the evidence for violence in early Neolithic Europe. Linearbandkeramik (LBK) people first attacked the huntergatherers
they encountered and then entered a period of increasingly violent warfare against
each other, culminating in an intense struggle in the area of central and western Germany. The
building of fortifications, physical mutilation and cannibalism, while no doubt enacted with
ritual airs, nevertheless had their context and purpose in the slaughter of enemies.
Keywords: Europe, Neolithic, LBK, fortification, warfare, mutilation, cannibalism
Introduction
The prevalence of conflict or warfare between social groups in prehistory is itself a
hotly contested topic at present. Where many prehistorians note evidence for violence
in fortifications, skeletal trauma and weapons, others prefer to assign it to ritual or symbolic
practice. This intellectual thrust and parry is exemplified in the study of the earliest farming
culture of central Europe, the Linearbandkeramik (Linear Pottery or LBK) culture. Here,
evidence includes a large number of enclosed (and likely fortified) village sites, and an
abundance of burial trauma, which might suggest that violence was common and at times
abnormally intense among these early European agriculturalists. However, as in many other
regions of the world, there have been criticisms of the interpretation of this data as relating
to inter-group conflict. We broadly define warfare here as ‘armed conflict between any social
and political units’ (Keeley & Quick 2004: 110; see Keeley 1996: Chapter 1 for a more indepth
discussion).
It is our purpose to review the evidence for warfare found at LBK archaeological sites,
particularly burial trauma and the fortification of sites.We conclude that conflict was highly
prevalent, particularly at later period western sites, and, furthermore, that there is increasing
evidence to support the claim previously put forth by one of the authors (Keeley 1998) that
this conflict not only occurred between LBK communities, but also between LBK farmers
and indigenous hunter-gatherers. We wish to place this violence in its proper prehistoric
context, as to its frequency and social context at the time of the earliest appearance of
agriculture in Central Europe some 7500 years ago.
Evidence for conflict in the Linearbandkeramik
The Linearbandkeramik is perhaps the best-studied Neolithic culture in all of Europe, with
hundreds of sites having been subjected to excavation over the last century. It was initially
believed that the movement of agriculture into central Europe occurred via a process of
peaceful migration of peoples deriving from the Near East. Little was made of the fate of
indigenous hunting-gathering peoples that had previously occupied central Europe, and no
solid evidence existed to demonstrate the occurrence of violence of any kind. It has become
clear in recent years that the earlyNeolithic was in fact a much more complex and sometimes
very violent period.
While the idea of a large-scale migration into central Europe by farmers has been criticised
recently (see Whittle 1996 for instance), many researchers studying the LBK still hold that
physical migration of a substantial number of people offers the best explanation for the
sudden appearance of a radically new material culture and subsistence system between 5700
and 4900 calBC (Bogucki 2000; Gronenborn 1999; Keeley & Golitko 2004). A recent
review of radiocarbon dates shows that, in contrast to some other regions of Europe, the
Mesolithic/Neolithic transition in the LBK region was quite abrupt, with little overlap
between dates for the two traditions, though this does not rule out low levels of mixing
between populations (Gkiasta et al. 2003: 59). While regional chronologies exist, we here
accept a four-period division: the oldest phase (with expansion out of Hungary into Austria,
the Czech Republic and Slovakia, southern Poland, and eastern and central Germany), two
middle phases (expansion through the Rhine Valley into the low Countries, Alsace and the
Paris Basin, and in the east into Poland, Romania and the Ukraine), and the youngest phase
(regional diversification in already settled areas with significant population growth).
Evidence of traumatic injury
Perhaps the most obvious evidence for past conflicts is provided by the presence of certain
types of traumatic injury in burial populations. This is particularly the case if these involve
embedded projectile points or traumas indicating blunt instruments. When no healing is
evident, the trauma was the probable cause of death (Keeley 1999: 340; Keeley & Quick
2004: 110; Milner et al. 1991: 589). In addition, many other types of ‘culturally modified’
human remains that are found at archaeological sites are most plausibly explained as indirect
evidence of inter-group conflict.
There are a number of now well known LBK contexts that have demonstrated that
violence was often quite severe during the early Neolithic of Central Europe. This was first
demonstrated with the publication in 1987 of the mass grave at Talheim (in the middle
Rhine valley) (Wahl&K¨onig 1987). There, a pit containing LBK culturalmaterial (younger
to youngest phase) was found to contain the remains of 34 skeletons, comprising 16 children
and infants, 9 adult males, and 7 adult females, representing, in the opinion of the authors,
the whole population of a small LBK village. All were killed by blunt force trauma to the
head caused by LBK axes or adzes. One adult male skull had evidently been struck by
an arrow. There were no signs of resistance in the form of parry fractures (Vencl 1999:
60-61).
Figure 1. a) Enclosed LBK sites of theOldest Phase b)Older and Younger Phases c) Youngest Phase ( =Sites with V-sectioned
ditches and/or complex gates = Sites without V-sectioned ditches or complex gates, stippling indicates approximate limits
of LBK settlement).
A similar massacre occurred at the enclosed site of Schletz-Asparn, near Vienna. The
small section of ditch excavated there contained the remains of 66 individuals; almost all
had been killed with LBK axes or adzes, though an arrow had killed one individual. The
remains were fragmented and showed signs of gnawing, indicating exposure for some time
after death, and, again, the demography suggests that a full village was wiped out. The
excavators estimate that had the full ditch been excavated, upward of 300 individuals might
have been uncovered (Windl 1999a, 1999b; Teschler-Nicola et al. 1996).
At Herxheim, in the Rhine Valley, some 173 skulls and skull-pates were found within
two enclosure ditches and the interior settlement. In addition to the skulls, upwards of 334
individuals may be represented by scattered remains, while two articulated skeletons were
found sprawled out within the inner ditch. These remains contrast with a number of typical
semi-flexed burials found within the settlement area (H¨außer 2000: 82; Spatz 1998: 18).
The removal of skull-pate at the site seems to have followed a regular procedure involving
cutting and slight burning (Haidle & Orschiedt 2001: 147-153).
At Vaihingen, an enclosed LBK site near Stuttgart, a dozen individuals were deposited
within two large rubbish pits, while further scattered remains were found throughout the
site, contrasting with the typical burials dug into the enclosure ditch at a later time. Most
of these individuals appear to have been somewhat unceremoniously dumped into their
final burial context, and showed signs of various forms of injury and mutilation. Some of
the typical LBK burials in the ditch bore evidence of violence as well, with one individual
having suffered a parry fracture, and another killed by a crushing blow to the skull (Krause
et al. 1998: 93-95; 97-98).
Mutilated remains are known from many sites, for instance skull drinking cups found
in the enclosure ditch at Eilsleben, or cannibalised leg bones at the enclosed site of Ober-
H¨ogern (Kaufmann 1990: 21-22; Kneipp & B¨uttner 1988: 494-96; Spatz 1998: 13).
Remains bearing evidence of cannibalism are known from a number of sites, many of
which were enclosed (Peter-R¨ocher 1994: 104-108; V encl 1999: 64).While these instances
represent quite dramatic examples of violent death, not all are necessarily the direct result of
inter-group warfare, for instance the skull caches at Herxheim.
Were this the only evidence of skeletal trauma present at LBK sites, it would be easy
to dismiss violence as only an occasional or infrequent occurrence. However, evidence
from typical semi-flexed LBK burials points towards a more regular presence of conflict in
LBK life. In a comprehensive study, Petrasch has calculated the total percentages of LBK
individuals who suffered fromtraumatic injury at some point during their lives. Limiting his
study only to those burial populations that have been subjected to pathological examination,
he arrives at the staggering figure of almost 20 per cent. Removing the data from Talheim,
Schletz-Asparn, andHerxheim, 6.2 per cent of all known burials show evidence for traumatic
injury (Petrasch 1999: 508-509). Some of these injuries were survived; several individuals
at Talheim, for instance, bore traces of having survived traumatic head injuries, only to be
killed later in life (Wahl&K¨onig 1987: 177-78). Individuals must have engaged in repeated
violent engagements throughout their lives.
Violence was evidently more intense in the western LBK area (roughly from central
Germany westwards), the upper value being a staggering 32 per cent. This compares with
only 2 per cent of skeletons in the east having suffered injuries (Milisauskas 2002: 178).
While 2 per cent is representative of a society in which conflict is prevalent, the western
LBK is comparable to the most violent known societies, in which conflict is a constant
preoccupation (Keeley 1996: Tables 6.1 & 6.2; LeBlanc & Register 2003: 224). The arrow
wounds at Schletz-Asparn and Talheim suggest that this rate may have been even higher at
times, as upwards of 70 per cent of all arrow wounds produce no skeletal trauma (Milner
2005: 150).While Petrasch does not separate his data by gender, the under-representation of
women in the population at Schletz-Asparn (Windl 1999a: 43) may indicate that they were
taken alive, suggesting that men suffered the majority of injuries. While trauma seems to
have been most frequent in later western LBK contexts, including the material from Talheim
and Herxheim, the material recovered at Vaihingen (Flomborn phase) and Schletz-Asparn
(Notenkopf phase) indicates that violence was not unknown in earlier LBK contexts. It has
even been suggested that violence in the later western LBK was so extreme as to entail a
‘crisis’ period (Spatz 1998).
LBK enclosures: evidence for fortification
The existence of enclosed LBK settlements has been known since the early twentieth
century, with the first large-scale excavation being carried out by Buttler and Haberey at
K¨oln-Lindenthal between 1929 and 1934. While most are ditched enclosures, there are also
a number of sites that are surrounded only by palisades. These early researchers interpreted
enclosures as fortifications, but the function of these installations was questioned in later
years. A number of recent authors have been highly critical of the assignment of a defensive
function to these places. Whittle, for example, has referred to them as ‘formalized communal
space’ (Whittle 1996: 174), while in a recent textbook it is claimed that ‘not all (indeed,
perhaps very few) bandkeramik enclosures were defensive in nature . . . ’ (Scarre 2005: 411).
The denial of a defensive function has been based on a number of lines of evidence, none of
which are backed by ethnographic or historical data. While comprehensive reviews of such
sites have been published by L¨uning (1988) and H¨ockmann (1990), these are rarely cited
when it comes to making broad statements about ‘all’ such enclosures. Thus, it has been
variously argued that LBK enclosures cannot represent defensive installations because their
ditches are too shallow (in the order of one metre), enclose too small an area, have limited
evidence of internal settlement, or contain evidence of ritual activity (often cannibalism or
other skeletal manipulation). As a result, LBK enclosures have sometimes been interpreted
as cattle kraals or ritual/symbolic enclosures (Kaufmann 1997: 46).
While few researchers now support the ‘cattle kraal hypothesis’, due to the unnecessarily
large amount of labour required to construct them (Keeley&Cahen 1989: 170), many have
assigned a ritual function to them, particularly for enclosures of the type that Kaufmann
labels the ‘Langweiler type’, i.e. those that have little evidence of internal settlement
(Kaufmann 1997: 66-67). However, as demonstrated by a review of historically and ethnographically
known fortifications, none of these arguments necessarily rules out the assignment
of a basic defensive function to LBK enclosures. The Romanmilitary, for instance, dug
the perimeter ditches of their legionary camps to a depth of only c . 0.9m, though the preferred
depth for more permanent installations was over 2m, i.e. slightly deeper than a person
is tall (see Polybius & Pseudo Hyginus 1994; Josephus 1970; Grant 1974: 300; Lawrence
1979: 80-81, 309, 340-341; Keeley et al. in press). There is no particular absolute size below
which an enclosure cannot function as a fortification. In fact, smaller refuge fortifications
without substantial interior settlement may require less manpower to defend than ones
that enclose larger settlements, and are well documented historically and ethnographically
(Keeley 1996: 57-58). The presence of ‘ritual’ activity is well evidenced at enclosed LBK sites,
but the meaning of this ritual is seldom discussed. The practice of ritual at an enclosed site in
no way inherently implies a non-defensive function; the symbolic importance of a particular
location often derives from its prosaic function – if an enclosure symbolises exclusion, social
solidarity, or any of a number of other things, it often does so because it provides a real
physical deterrent against entry by outsiders (Keeley 2003: 252). Furthermore, there are
many examples of fortifications enclosing ritual areas – as these areas may be particularly
important to protect (Keeley et al. in press).
Of particular relevance to the present discussion, are several features for which only a
military function is appropriate: V- or Y-sectioned enclosure ditches, and complex forms
of gates: baffled, offset, crab-claw, labyrinthine or screened. V or Y-sectioned ditches are
impractical for any domestic purpose, as they erode more quickly than any other form and
are more difficult to dig, but they represent an ideal form for purposes of defence against
human attack, since they offer maximum exposure of any would-be attacker to defensive
projectile fire from above. By contrast, U-sectioned or flat-bottomed ditches may serve a
variety of functions (earth extraction, drainage, etc.), one of which may be defence (moats
for instance are often flat-bottomed with straight walls or U-sectioned to minimise erosion).
When backed by an internal berm and/or palisade, it is certain that a defensive purpose was
intended (Keeley et al. in press). At some Neolithic ceremonial sites in Ireland, for instance,
ring ditches with outer berms (a non-defensive arrangement likely indicative of only ritual
function) were altered during the Bronze Age to incorporate an inner berm, palisade, and
other defensive features at the same time that warfare intensified (Champion et al. 1984:
294-95).
Similarly, complex gate arrangements such as baffles or screens are counter-productive as
entrances for cattle kraals or for purposes of daily activity (making it pointlessly difficult to
enter or exit), but are known as classic defensive features at numerous sites stretching across
thousands of years of history. The primary functions of such gates are to limit the number
of attackers that may enter at once, to prevent direct use of projectile weapons against those
inside, and to force attackers to adopt a non-defensive body positioning (i.e. turned to the
side) when entering (Keeley et al. in press).
Our review of the literature indicates that there are at present 84 sites known with evidence
of enclosure that are securely dated to the LBK, six of which evidence multiple phases of
enclosure (see database at http://www.uic.edu/depts/anth/faculty/keeley).We have recorded
data on location, chronological phase, area/length excavated, ditch form, width, and depth,
number and type of gates/interruptions, presence or absence of berms and/or palisades,
settlement and wells or cisterns, available radiocarbon dates, and the presence or absence
of human remains. In some cases, data was available for only certain site features. (When
data was not available, sites were removed from the total list for purposes of computing
percentages).
The greatest number of enclosed sites date to the younger or youngest phases of the LBK,
with fewer dating tomiddle stages, and very few to the oldest LBK (47 per cent v. 41 per cent
v. 12 per cent respectively, n=78). Furthermore, the majority (62 per cent, n=84) of such
sites are found in the western portion of the LBK distribution. Seventy-five per cent (n=75)
of all known enclosures include defensive ditches. Some palisades without associated ditches
may be small enclosures best described as pens or kraals (Bedburg-Garsdorf and Zwenkau-
Harth, for instance), while others clearly enclosed a larger settlement area and included
complex defensive gate arrangements (Sittard, Elsloo, and K¨oln-Lindenthal I (palisade P),
for instance). Limited excavation makes it difficult to determine in some cases.
Fifty-nine per cent of all known enclosure ditches are V- or Y-sectioned (n=56), and
41 per cent U-sectioned or flat-bottomed. Many times, as at sites such as Darion-Colia,
Waremme-Longchamps, or Stephansposhing, ditches are shallower and U-sectioned away
from gates, and deeper and V-sectioned near gates. Limited excavation may therefore be
an issue in terms of identifying defensive features. These ditches average 2.8m wide and
1.6m deep. Given that all LBK sites have experienced some degree of erosion, typically
0.5-1m, most of these ditches would have been easily as deep or deeper than the height
of any potential early Neolithic attacker in their original form. Fifty-four per cent of all
known enclosures at which at least one gate or interruption was excavated (n=48, including
palisaded sites) possess defensive gate arrangements. Combining both lines of data, the total
number of LBK enclosed sites that possess defensive features as here defined is 51, or 70 per
cent of the total number (73) for which sufficient data are available.
Nevertheless, we do not believe that a defensive role can be ruled out for many of the
remaining ditched enclosures that display neither complex gates nor V- or Y-sectioned
ditches. Schletz-Asparn II, at which there is perhaps the most direct evidence of actual
violent conflict, possesses neither of these features, yet we strongly suspect that it was built
to deter human attack, though it tragically failed its occupants in that capacity.
The intentional enclosure of a dependable water supply in the form of wells or cisterns
is also an occasional feature (8 per cent, n=84) of LBK enclosures (Jadin & Cahen 1998:
125), further supporting their probable use as fortifications. Almost all LBK sites are located
within sight of water, usually second- or third-order streams. Only the anticipated denial
of access to adjacent streams would necessitate the labour of digging and lining wells and
cisterns to secure an internal source.
Discussion
Given the almost exact chronological and geographical correlation between prevalence of
burial trauma and the frequency of construction of enclosures with obvious defensive
purpose, such constructions must represent a material response to violence. There is
abundant ethnographic evidence indicating that fortification is a response to violence
often taken by sedentary farming groups (Keeley 1996: 56-57; LeBlanc & Register 2003:
Chapter 6). Where violence in LBK society was relatively subdued (early and in later eastern
contexts), there was a corresponding lesser need to construct fortifications. Where violence
was extremely intense, a greater need for fortification was felt (later western contexts).
There is a distinct association between enclosed sites and not only remains that can be
taken as immediate evidence of conflict (i.e. Schletz-Asparn and Vaihingen), but also with
skeletal material that has been modified in a way described by many researchers as ‘ritualistic’,
as atOber-H¨orgern,Herxheim and elsewhere.However, even if ritual practice were involved,
many researchers seem to view it as an exclusive alternative explanation to warfare: the
implication is that the victims of the violent rituals were come by via peaceful means.
However, ritual is rarely purely epi-phenomenal, but instead relates to other practices within
society (e.g. Malinowski 1961; Turner 1967), and in this case is strongly related to other
evidence for warfare. Turning again to the ethnographic record, there are a number of reasons
for resorting to such gruesome activities. Starvation or culinary cannibalism is documented,
but results in human remains that show butchery and cooking marks identical to those on
other animals consumed. This does not match the data from LBK sites. Far more common
is ‘ritual’ cannibalism, in which portions of an enemy’s body are consumed, typically
in a proscribed way as to which portions are eaten (Keeley 1996: 103-106; LeBlanc &
Register 2003: 60). Given the nature of the remains from the sites mentioned, there emerges
a pattern of consumption and manipulation of particular portions of the human body, for
instance left legs atOber-H¨orgern, or skulls and skull-pates at a number of sites. While these
were undoubtedly ‘ritual’ activities, archaeology indicates that such activities were related
to warfare, and ethnography indicates that the most likely victims of such activity were
captured enemies.
The remains from Schletz-Asparn and Talheim, as well as numerous other sites, indicate
beyond a doubt that, much of the time, these enemies came from other LBK villages.
The crushing blows to the head that killed most individuals at these sites were inflicted by
LBK-style axes and adzes. Furthermore, the researchers who studied the remains at Talheim
were able to determine that the attributes of all the skeletons present were consistent with
those individuals having belonged to an LBK population in terms of skeletal robustness,
dentition, stature and skull form (Wahl&K¨onig 1987).While the topic of causes is difficult
to address archaeologically, it is to be presumed that in a tribal agricultural society, reasons
for fighting were numerous, and motives may have included revenge for prior attacks, land
disputes, poaching, prestige, capture of slaves or capture of women (Keeley 1996: Table 8.1),
as the under-representation of young women at Schletz-Asparn likely indicates. The
intensification of such warfare seen during the latest western LBK has been linked to a
number of causes, including environmental degradation (which would not explain why
violence remained less intense in the east) or over population (Spatz 1998: 14-15).
There is also evidence, however, to justify the argument previously put forward by one of us
that at times this conflict occurred between LBK farming groups and indigenousMesolithic
peoples (Keeley 1998). While many researchers are quite content to cite ethnographic
examples of trade and acculturation between farmers and foragers (Gregg 1988: Scarre
2005: 407), this ignores an equally substantial body of ethnographic data that demonstrates
that conflict is another common form of interaction (Keeley 1996: 131-38; Keeley&Cahen
1989: 171-72). The ‘Mesolithic’ argument is based on a number of lines of archaeological
evidence.Many fortified sites cluster along the limits of LBK settlement, particularly during
the earliest phases (Keeley 1996: 137-39). The short periods of use of many fortifications
indicates that they were constructed to counter a threat that quickly disappeared, which may
have been the case if farmers at far higher settlement density quickly killed or incorporated
local hunter-gatherers living at extremely low population density. The only multi-phase
enclosure for which we have good data regarding trauma, Schletz-Asparn, demonstrates that
the second-phase ditch was built to counter the threat of other LBK communities. There
is a c . 20-25km wide ‘no-man’s land’ between LBK sites and final Mesolithic sites in the
Hesbaye region of Belgium (Keeley 1996: 139). According to LeBlanc (1999: 69) ‘that such
areas existed is an extremely strong line of evidence for warfare, because it is unlikely that people
would have given up their use of an area without a very good reason’. Kaufmann (1990: 25)
mentions the disappearance ofMesolithic sites in the vicinity of Eilsleben at the time of first
LBK settlement, as does Jochim (2000: 195-196) in south-western Germany, in the vicinity
of fortified sites such as Vaihingen. The possibility exists, though is by no means proven,
that buffer zones existed in areas other than the Hesbaye as well.
The only ‘Mesolithic’ style artefacts found at many LBK sites are projectile points, while
the only LBK artefacts typically found at Mesolithic sites are axes or adzes, for both of
which there is evidence to suggest their use as weapons (Keeley 1998: 309). Milisauskas
has previously suggested that the far higher frequency of projectile points in western LBK
assemblages as contrasted with those in the east is indicative of their use as weapons, as
there is no similar evidence to suggest that hunting was more frequently practised in one
area than another (Milisauskas 1986: 4; 143). In fact, the hunting of game declined with
time (Gronenborn 1999: 162-63), while the frequency of projectile points did not. The
frequency of projectile points is, however, correlated geographically with the frequency of
burial trauma and fortification. We would add to the previous points the recent discovery
at Vaihingen that individuals buried in rubbish pits show greater skeletal robusticity than
those recovered from formal burial contexts (Krause et al. 1998: 96), suggesting that they
belonged to a separate population, likely of hunter-gatherers. Strontium isotope analysis of
skeletons from Vaihingen has demonstrated that there are ‘non-local’ individuals present in
both settlement and ditch contexts, but it is unclear whether any of these were the same
individuals found in non-typical burial contexts (Bentley et al. 2003: 479-82).
Conclusion
The archaeological evidence, coupled with ethnographic analogy, demonstrates that warfare
was a frequent occurrence during the earlier phases of LBK expansion, while in later western
contexts its frequency seems to have been comparable to that found amongst the most violent
tribal types of society known ethnographically. Given the correlation that exists between the
level of this violence and the frequency of LBK enclosures both spatially and chronologically,
as well as the presence of defensive features in the majority of such enclosures, it is most
appropriate to speak of them as fortifications. We stress that this does not rule out the
practice of ritual at these sites, nor the use of them for secondary functions such as penning
animals. In fact, much of this ritual (skeletal manipulation and cannibalism) is probably
related to conflict. While much of this violence seems to have involved LBK communities
fighting each other, as indicated by the mass graves at Talheim and Schletz-Asparn, we argue
that a number of lines of evidence point towards conflict during early stages of settlement
with local hunter-gatherers.
Acknowledgements
Our work at the site of Waremme-Longchamps was funded by the National Science Foundation. Many thanks
to Ivan Jadin,Dominique Bosquet (Institut Royal de SciencesNaturelles de Belgique), and RussellQuick (UIC),
as well as to Martin Carver and the two anonymous reviewers who read this paper and offered helpful questions
and comments. Any remaining errors are the sole responsibility of the authors.
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they encountered and then entered a period of increasingly violent warfare against
each other, culminating in an intense struggle in the area of central and western Germany. The
building of fortifications, physical mutilation and cannibalism, while no doubt enacted with
ritual airs, nevertheless had their context and purpose in the slaughter of enemies.
Keywords: Europe, Neolithic, LBK, fortification, warfare, mutilation, cannibalism
Introduction
The prevalence of conflict or warfare between social groups in prehistory is itself a
hotly contested topic at present. Where many prehistorians note evidence for violence
in fortifications, skeletal trauma and weapons, others prefer to assign it to ritual or symbolic
practice. This intellectual thrust and parry is exemplified in the study of the earliest farming
culture of central Europe, the Linearbandkeramik (Linear Pottery or LBK) culture. Here,
evidence includes a large number of enclosed (and likely fortified) village sites, and an
abundance of burial trauma, which might suggest that violence was common and at times
abnormally intense among these early European agriculturalists. However, as in many other
regions of the world, there have been criticisms of the interpretation of this data as relating
to inter-group conflict. We broadly define warfare here as ‘armed conflict between any social
and political units’ (Keeley & Quick 2004: 110; see Keeley 1996: Chapter 1 for a more indepth
discussion).
It is our purpose to review the evidence for warfare found at LBK archaeological sites,
particularly burial trauma and the fortification of sites.We conclude that conflict was highly
prevalent, particularly at later period western sites, and, furthermore, that there is increasing
evidence to support the claim previously put forth by one of the authors (Keeley 1998) that
this conflict not only occurred between LBK communities, but also between LBK farmers
and indigenous hunter-gatherers. We wish to place this violence in its proper prehistoric
context, as to its frequency and social context at the time of the earliest appearance of
agriculture in Central Europe some 7500 years ago.
Evidence for conflict in the Linearbandkeramik
The Linearbandkeramik is perhaps the best-studied Neolithic culture in all of Europe, with
hundreds of sites having been subjected to excavation over the last century. It was initially
believed that the movement of agriculture into central Europe occurred via a process of
peaceful migration of peoples deriving from the Near East. Little was made of the fate of
indigenous hunting-gathering peoples that had previously occupied central Europe, and no
solid evidence existed to demonstrate the occurrence of violence of any kind. It has become
clear in recent years that the earlyNeolithic was in fact a much more complex and sometimes
very violent period.
While the idea of a large-scale migration into central Europe by farmers has been criticised
recently (see Whittle 1996 for instance), many researchers studying the LBK still hold that
physical migration of a substantial number of people offers the best explanation for the
sudden appearance of a radically new material culture and subsistence system between 5700
and 4900 calBC (Bogucki 2000; Gronenborn 1999; Keeley & Golitko 2004). A recent
review of radiocarbon dates shows that, in contrast to some other regions of Europe, the
Mesolithic/Neolithic transition in the LBK region was quite abrupt, with little overlap
between dates for the two traditions, though this does not rule out low levels of mixing
between populations (Gkiasta et al. 2003: 59). While regional chronologies exist, we here
accept a four-period division: the oldest phase (with expansion out of Hungary into Austria,
the Czech Republic and Slovakia, southern Poland, and eastern and central Germany), two
middle phases (expansion through the Rhine Valley into the low Countries, Alsace and the
Paris Basin, and in the east into Poland, Romania and the Ukraine), and the youngest phase
(regional diversification in already settled areas with significant population growth).
Evidence of traumatic injury
Perhaps the most obvious evidence for past conflicts is provided by the presence of certain
types of traumatic injury in burial populations. This is particularly the case if these involve
embedded projectile points or traumas indicating blunt instruments. When no healing is
evident, the trauma was the probable cause of death (Keeley 1999: 340; Keeley & Quick
2004: 110; Milner et al. 1991: 589). In addition, many other types of ‘culturally modified’
human remains that are found at archaeological sites are most plausibly explained as indirect
evidence of inter-group conflict.
There are a number of now well known LBK contexts that have demonstrated that
violence was often quite severe during the early Neolithic of Central Europe. This was first
demonstrated with the publication in 1987 of the mass grave at Talheim (in the middle
Rhine valley) (Wahl&K¨onig 1987). There, a pit containing LBK culturalmaterial (younger
to youngest phase) was found to contain the remains of 34 skeletons, comprising 16 children
and infants, 9 adult males, and 7 adult females, representing, in the opinion of the authors,
the whole population of a small LBK village. All were killed by blunt force trauma to the
head caused by LBK axes or adzes. One adult male skull had evidently been struck by
an arrow. There were no signs of resistance in the form of parry fractures (Vencl 1999:
60-61).
Figure 1. a) Enclosed LBK sites of theOldest Phase b)Older and Younger Phases c) Youngest Phase ( =Sites with V-sectioned
ditches and/or complex gates = Sites without V-sectioned ditches or complex gates, stippling indicates approximate limits
of LBK settlement).
A similar massacre occurred at the enclosed site of Schletz-Asparn, near Vienna. The
small section of ditch excavated there contained the remains of 66 individuals; almost all
had been killed with LBK axes or adzes, though an arrow had killed one individual. The
remains were fragmented and showed signs of gnawing, indicating exposure for some time
after death, and, again, the demography suggests that a full village was wiped out. The
excavators estimate that had the full ditch been excavated, upward of 300 individuals might
have been uncovered (Windl 1999a, 1999b; Teschler-Nicola et al. 1996).
At Herxheim, in the Rhine Valley, some 173 skulls and skull-pates were found within
two enclosure ditches and the interior settlement. In addition to the skulls, upwards of 334
individuals may be represented by scattered remains, while two articulated skeletons were
found sprawled out within the inner ditch. These remains contrast with a number of typical
semi-flexed burials found within the settlement area (H¨außer 2000: 82; Spatz 1998: 18).
The removal of skull-pate at the site seems to have followed a regular procedure involving
cutting and slight burning (Haidle & Orschiedt 2001: 147-153).
At Vaihingen, an enclosed LBK site near Stuttgart, a dozen individuals were deposited
within two large rubbish pits, while further scattered remains were found throughout the
site, contrasting with the typical burials dug into the enclosure ditch at a later time. Most
of these individuals appear to have been somewhat unceremoniously dumped into their
final burial context, and showed signs of various forms of injury and mutilation. Some of
the typical LBK burials in the ditch bore evidence of violence as well, with one individual
having suffered a parry fracture, and another killed by a crushing blow to the skull (Krause
et al. 1998: 93-95; 97-98).
Mutilated remains are known from many sites, for instance skull drinking cups found
in the enclosure ditch at Eilsleben, or cannibalised leg bones at the enclosed site of Ober-
H¨ogern (Kaufmann 1990: 21-22; Kneipp & B¨uttner 1988: 494-96; Spatz 1998: 13).
Remains bearing evidence of cannibalism are known from a number of sites, many of
which were enclosed (Peter-R¨ocher 1994: 104-108; V encl 1999: 64).While these instances
represent quite dramatic examples of violent death, not all are necessarily the direct result of
inter-group warfare, for instance the skull caches at Herxheim.
Were this the only evidence of skeletal trauma present at LBK sites, it would be easy
to dismiss violence as only an occasional or infrequent occurrence. However, evidence
from typical semi-flexed LBK burials points towards a more regular presence of conflict in
LBK life. In a comprehensive study, Petrasch has calculated the total percentages of LBK
individuals who suffered fromtraumatic injury at some point during their lives. Limiting his
study only to those burial populations that have been subjected to pathological examination,
he arrives at the staggering figure of almost 20 per cent. Removing the data from Talheim,
Schletz-Asparn, andHerxheim, 6.2 per cent of all known burials show evidence for traumatic
injury (Petrasch 1999: 508-509). Some of these injuries were survived; several individuals
at Talheim, for instance, bore traces of having survived traumatic head injuries, only to be
killed later in life (Wahl&K¨onig 1987: 177-78). Individuals must have engaged in repeated
violent engagements throughout their lives.
Violence was evidently more intense in the western LBK area (roughly from central
Germany westwards), the upper value being a staggering 32 per cent. This compares with
only 2 per cent of skeletons in the east having suffered injuries (Milisauskas 2002: 178).
While 2 per cent is representative of a society in which conflict is prevalent, the western
LBK is comparable to the most violent known societies, in which conflict is a constant
preoccupation (Keeley 1996: Tables 6.1 & 6.2; LeBlanc & Register 2003: 224). The arrow
wounds at Schletz-Asparn and Talheim suggest that this rate may have been even higher at
times, as upwards of 70 per cent of all arrow wounds produce no skeletal trauma (Milner
2005: 150).While Petrasch does not separate his data by gender, the under-representation of
women in the population at Schletz-Asparn (Windl 1999a: 43) may indicate that they were
taken alive, suggesting that men suffered the majority of injuries. While trauma seems to
have been most frequent in later western LBK contexts, including the material from Talheim
and Herxheim, the material recovered at Vaihingen (Flomborn phase) and Schletz-Asparn
(Notenkopf phase) indicates that violence was not unknown in earlier LBK contexts. It has
even been suggested that violence in the later western LBK was so extreme as to entail a
‘crisis’ period (Spatz 1998).
LBK enclosures: evidence for fortification
The existence of enclosed LBK settlements has been known since the early twentieth
century, with the first large-scale excavation being carried out by Buttler and Haberey at
K¨oln-Lindenthal between 1929 and 1934. While most are ditched enclosures, there are also
a number of sites that are surrounded only by palisades. These early researchers interpreted
enclosures as fortifications, but the function of these installations was questioned in later
years. A number of recent authors have been highly critical of the assignment of a defensive
function to these places. Whittle, for example, has referred to them as ‘formalized communal
space’ (Whittle 1996: 174), while in a recent textbook it is claimed that ‘not all (indeed,
perhaps very few) bandkeramik enclosures were defensive in nature . . . ’ (Scarre 2005: 411).
The denial of a defensive function has been based on a number of lines of evidence, none of
which are backed by ethnographic or historical data. While comprehensive reviews of such
sites have been published by L¨uning (1988) and H¨ockmann (1990), these are rarely cited
when it comes to making broad statements about ‘all’ such enclosures. Thus, it has been
variously argued that LBK enclosures cannot represent defensive installations because their
ditches are too shallow (in the order of one metre), enclose too small an area, have limited
evidence of internal settlement, or contain evidence of ritual activity (often cannibalism or
other skeletal manipulation). As a result, LBK enclosures have sometimes been interpreted
as cattle kraals or ritual/symbolic enclosures (Kaufmann 1997: 46).
While few researchers now support the ‘cattle kraal hypothesis’, due to the unnecessarily
large amount of labour required to construct them (Keeley&Cahen 1989: 170), many have
assigned a ritual function to them, particularly for enclosures of the type that Kaufmann
labels the ‘Langweiler type’, i.e. those that have little evidence of internal settlement
(Kaufmann 1997: 66-67). However, as demonstrated by a review of historically and ethnographically
known fortifications, none of these arguments necessarily rules out the assignment
of a basic defensive function to LBK enclosures. The Romanmilitary, for instance, dug
the perimeter ditches of their legionary camps to a depth of only c . 0.9m, though the preferred
depth for more permanent installations was over 2m, i.e. slightly deeper than a person
is tall (see Polybius & Pseudo Hyginus 1994; Josephus 1970; Grant 1974: 300; Lawrence
1979: 80-81, 309, 340-341; Keeley et al. in press). There is no particular absolute size below
which an enclosure cannot function as a fortification. In fact, smaller refuge fortifications
without substantial interior settlement may require less manpower to defend than ones
that enclose larger settlements, and are well documented historically and ethnographically
(Keeley 1996: 57-58). The presence of ‘ritual’ activity is well evidenced at enclosed LBK sites,
but the meaning of this ritual is seldom discussed. The practice of ritual at an enclosed site in
no way inherently implies a non-defensive function; the symbolic importance of a particular
location often derives from its prosaic function – if an enclosure symbolises exclusion, social
solidarity, or any of a number of other things, it often does so because it provides a real
physical deterrent against entry by outsiders (Keeley 2003: 252). Furthermore, there are
many examples of fortifications enclosing ritual areas – as these areas may be particularly
important to protect (Keeley et al. in press).
Of particular relevance to the present discussion, are several features for which only a
military function is appropriate: V- or Y-sectioned enclosure ditches, and complex forms
of gates: baffled, offset, crab-claw, labyrinthine or screened. V or Y-sectioned ditches are
impractical for any domestic purpose, as they erode more quickly than any other form and
are more difficult to dig, but they represent an ideal form for purposes of defence against
human attack, since they offer maximum exposure of any would-be attacker to defensive
projectile fire from above. By contrast, U-sectioned or flat-bottomed ditches may serve a
variety of functions (earth extraction, drainage, etc.), one of which may be defence (moats
for instance are often flat-bottomed with straight walls or U-sectioned to minimise erosion).
When backed by an internal berm and/or palisade, it is certain that a defensive purpose was
intended (Keeley et al. in press). At some Neolithic ceremonial sites in Ireland, for instance,
ring ditches with outer berms (a non-defensive arrangement likely indicative of only ritual
function) were altered during the Bronze Age to incorporate an inner berm, palisade, and
other defensive features at the same time that warfare intensified (Champion et al. 1984:
294-95).
Similarly, complex gate arrangements such as baffles or screens are counter-productive as
entrances for cattle kraals or for purposes of daily activity (making it pointlessly difficult to
enter or exit), but are known as classic defensive features at numerous sites stretching across
thousands of years of history. The primary functions of such gates are to limit the number
of attackers that may enter at once, to prevent direct use of projectile weapons against those
inside, and to force attackers to adopt a non-defensive body positioning (i.e. turned to the
side) when entering (Keeley et al. in press).
Our review of the literature indicates that there are at present 84 sites known with evidence
of enclosure that are securely dated to the LBK, six of which evidence multiple phases of
enclosure (see database at http://www.uic.edu/depts/anth/faculty/keeley).We have recorded
data on location, chronological phase, area/length excavated, ditch form, width, and depth,
number and type of gates/interruptions, presence or absence of berms and/or palisades,
settlement and wells or cisterns, available radiocarbon dates, and the presence or absence
of human remains. In some cases, data was available for only certain site features. (When
data was not available, sites were removed from the total list for purposes of computing
percentages).
The greatest number of enclosed sites date to the younger or youngest phases of the LBK,
with fewer dating tomiddle stages, and very few to the oldest LBK (47 per cent v. 41 per cent
v. 12 per cent respectively, n=78). Furthermore, the majority (62 per cent, n=84) of such
sites are found in the western portion of the LBK distribution. Seventy-five per cent (n=75)
of all known enclosures include defensive ditches. Some palisades without associated ditches
may be small enclosures best described as pens or kraals (Bedburg-Garsdorf and Zwenkau-
Harth, for instance), while others clearly enclosed a larger settlement area and included
complex defensive gate arrangements (Sittard, Elsloo, and K¨oln-Lindenthal I (palisade P),
for instance). Limited excavation makes it difficult to determine in some cases.
Fifty-nine per cent of all known enclosure ditches are V- or Y-sectioned (n=56), and
41 per cent U-sectioned or flat-bottomed. Many times, as at sites such as Darion-Colia,
Waremme-Longchamps, or Stephansposhing, ditches are shallower and U-sectioned away
from gates, and deeper and V-sectioned near gates. Limited excavation may therefore be
an issue in terms of identifying defensive features. These ditches average 2.8m wide and
1.6m deep. Given that all LBK sites have experienced some degree of erosion, typically
0.5-1m, most of these ditches would have been easily as deep or deeper than the height
of any potential early Neolithic attacker in their original form. Fifty-four per cent of all
known enclosures at which at least one gate or interruption was excavated (n=48, including
palisaded sites) possess defensive gate arrangements. Combining both lines of data, the total
number of LBK enclosed sites that possess defensive features as here defined is 51, or 70 per
cent of the total number (73) for which sufficient data are available.
Nevertheless, we do not believe that a defensive role can be ruled out for many of the
remaining ditched enclosures that display neither complex gates nor V- or Y-sectioned
ditches. Schletz-Asparn II, at which there is perhaps the most direct evidence of actual
violent conflict, possesses neither of these features, yet we strongly suspect that it was built
to deter human attack, though it tragically failed its occupants in that capacity.
The intentional enclosure of a dependable water supply in the form of wells or cisterns
is also an occasional feature (8 per cent, n=84) of LBK enclosures (Jadin & Cahen 1998:
125), further supporting their probable use as fortifications. Almost all LBK sites are located
within sight of water, usually second- or third-order streams. Only the anticipated denial
of access to adjacent streams would necessitate the labour of digging and lining wells and
cisterns to secure an internal source.
Discussion
Given the almost exact chronological and geographical correlation between prevalence of
burial trauma and the frequency of construction of enclosures with obvious defensive
purpose, such constructions must represent a material response to violence. There is
abundant ethnographic evidence indicating that fortification is a response to violence
often taken by sedentary farming groups (Keeley 1996: 56-57; LeBlanc & Register 2003:
Chapter 6). Where violence in LBK society was relatively subdued (early and in later eastern
contexts), there was a corresponding lesser need to construct fortifications. Where violence
was extremely intense, a greater need for fortification was felt (later western contexts).
There is a distinct association between enclosed sites and not only remains that can be
taken as immediate evidence of conflict (i.e. Schletz-Asparn and Vaihingen), but also with
skeletal material that has been modified in a way described by many researchers as ‘ritualistic’,
as atOber-H¨orgern,Herxheim and elsewhere.However, even if ritual practice were involved,
many researchers seem to view it as an exclusive alternative explanation to warfare: the
implication is that the victims of the violent rituals were come by via peaceful means.
However, ritual is rarely purely epi-phenomenal, but instead relates to other practices within
society (e.g. Malinowski 1961; Turner 1967), and in this case is strongly related to other
evidence for warfare. Turning again to the ethnographic record, there are a number of reasons
for resorting to such gruesome activities. Starvation or culinary cannibalism is documented,
but results in human remains that show butchery and cooking marks identical to those on
other animals consumed. This does not match the data from LBK sites. Far more common
is ‘ritual’ cannibalism, in which portions of an enemy’s body are consumed, typically
in a proscribed way as to which portions are eaten (Keeley 1996: 103-106; LeBlanc &
Register 2003: 60). Given the nature of the remains from the sites mentioned, there emerges
a pattern of consumption and manipulation of particular portions of the human body, for
instance left legs atOber-H¨orgern, or skulls and skull-pates at a number of sites. While these
were undoubtedly ‘ritual’ activities, archaeology indicates that such activities were related
to warfare, and ethnography indicates that the most likely victims of such activity were
captured enemies.
The remains from Schletz-Asparn and Talheim, as well as numerous other sites, indicate
beyond a doubt that, much of the time, these enemies came from other LBK villages.
The crushing blows to the head that killed most individuals at these sites were inflicted by
LBK-style axes and adzes. Furthermore, the researchers who studied the remains at Talheim
were able to determine that the attributes of all the skeletons present were consistent with
those individuals having belonged to an LBK population in terms of skeletal robustness,
dentition, stature and skull form (Wahl&K¨onig 1987).While the topic of causes is difficult
to address archaeologically, it is to be presumed that in a tribal agricultural society, reasons
for fighting were numerous, and motives may have included revenge for prior attacks, land
disputes, poaching, prestige, capture of slaves or capture of women (Keeley 1996: Table 8.1),
as the under-representation of young women at Schletz-Asparn likely indicates. The
intensification of such warfare seen during the latest western LBK has been linked to a
number of causes, including environmental degradation (which would not explain why
violence remained less intense in the east) or over population (Spatz 1998: 14-15).
There is also evidence, however, to justify the argument previously put forward by one of us
that at times this conflict occurred between LBK farming groups and indigenousMesolithic
peoples (Keeley 1998). While many researchers are quite content to cite ethnographic
examples of trade and acculturation between farmers and foragers (Gregg 1988: Scarre
2005: 407), this ignores an equally substantial body of ethnographic data that demonstrates
that conflict is another common form of interaction (Keeley 1996: 131-38; Keeley&Cahen
1989: 171-72). The ‘Mesolithic’ argument is based on a number of lines of archaeological
evidence.Many fortified sites cluster along the limits of LBK settlement, particularly during
the earliest phases (Keeley 1996: 137-39). The short periods of use of many fortifications
indicates that they were constructed to counter a threat that quickly disappeared, which may
have been the case if farmers at far higher settlement density quickly killed or incorporated
local hunter-gatherers living at extremely low population density. The only multi-phase
enclosure for which we have good data regarding trauma, Schletz-Asparn, demonstrates that
the second-phase ditch was built to counter the threat of other LBK communities. There
is a c . 20-25km wide ‘no-man’s land’ between LBK sites and final Mesolithic sites in the
Hesbaye region of Belgium (Keeley 1996: 139). According to LeBlanc (1999: 69) ‘that such
areas existed is an extremely strong line of evidence for warfare, because it is unlikely that people
would have given up their use of an area without a very good reason’. Kaufmann (1990: 25)
mentions the disappearance ofMesolithic sites in the vicinity of Eilsleben at the time of first
LBK settlement, as does Jochim (2000: 195-196) in south-western Germany, in the vicinity
of fortified sites such as Vaihingen. The possibility exists, though is by no means proven,
that buffer zones existed in areas other than the Hesbaye as well.
The only ‘Mesolithic’ style artefacts found at many LBK sites are projectile points, while
the only LBK artefacts typically found at Mesolithic sites are axes or adzes, for both of
which there is evidence to suggest their use as weapons (Keeley 1998: 309). Milisauskas
has previously suggested that the far higher frequency of projectile points in western LBK
assemblages as contrasted with those in the east is indicative of their use as weapons, as
there is no similar evidence to suggest that hunting was more frequently practised in one
area than another (Milisauskas 1986: 4; 143). In fact, the hunting of game declined with
time (Gronenborn 1999: 162-63), while the frequency of projectile points did not. The
frequency of projectile points is, however, correlated geographically with the frequency of
burial trauma and fortification. We would add to the previous points the recent discovery
at Vaihingen that individuals buried in rubbish pits show greater skeletal robusticity than
those recovered from formal burial contexts (Krause et al. 1998: 96), suggesting that they
belonged to a separate population, likely of hunter-gatherers. Strontium isotope analysis of
skeletons from Vaihingen has demonstrated that there are ‘non-local’ individuals present in
both settlement and ditch contexts, but it is unclear whether any of these were the same
individuals found in non-typical burial contexts (Bentley et al. 2003: 479-82).
Conclusion
The archaeological evidence, coupled with ethnographic analogy, demonstrates that warfare
was a frequent occurrence during the earlier phases of LBK expansion, while in later western
contexts its frequency seems to have been comparable to that found amongst the most violent
tribal types of society known ethnographically. Given the correlation that exists between the
level of this violence and the frequency of LBK enclosures both spatially and chronologically,
as well as the presence of defensive features in the majority of such enclosures, it is most
appropriate to speak of them as fortifications. We stress that this does not rule out the
practice of ritual at these sites, nor the use of them for secondary functions such as penning
animals. In fact, much of this ritual (skeletal manipulation and cannibalism) is probably
related to conflict. While much of this violence seems to have involved LBK communities
fighting each other, as indicated by the mass graves at Talheim and Schletz-Asparn, we argue
that a number of lines of evidence point towards conflict during early stages of settlement
with local hunter-gatherers.
Acknowledgements
Our work at the site of Waremme-Longchamps was funded by the National Science Foundation. Many thanks
to Ivan Jadin,Dominique Bosquet (Institut Royal de SciencesNaturelles de Belgique), and RussellQuick (UIC),
as well as to Martin Carver and the two anonymous reviewers who read this paper and offered helpful questions
and comments. Any remaining errors are the sole responsibility of the authors.
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