Why Evolution Matters for War

1

Working paper for “Evolution, The Human Sciences and Liberty”

(Special Meeting of the Mont Pelerin Society, Galapagos, June 23-‐28, 2013)

Why evolution matters for war.

Richard Wrangham

Department of Human Evolutionary Biology, Harvard University

The biology of war is a contentious topic. For some scholars our evolutionary

history is irrelevant, including not only those who believe that war was absent in

human evolution (Ferguson 2011) but also others who regard war as having

occurred routinely in the deep past (Keeley 1996). For many people, however, the

psychology of humans has been importantly shaped by an evolutionary history of

war, even if the nature of the effects and the evolutionary processes leading to them

are a matter of debate (Bowles 2009, McDonald et al 2012). Here I use a brief review

of animal and human aggression to suggest some ways in which an evolutionary

history of war does or does not affect contemporary war behavior. I stress that the

conceptual distinction of two types of aggression, predatory and impulsive, because

they are produced by sufficiently different neural pathways to have had different

evolutionary histories and consequences in humans.

Hunter-‐gatherers provide the critical social context for assessing the

evolutionary background of war, defined as lethal conflict between coalitions from

different groups. War can be external (between societies) or internal (within

societies). Compilations of the frequency of war among hunter-‐gatherers have

widely varying results (e.g. 27-‐92% societies having frequent war, Wrangham and

Glowacki 2012). Inconsistencies emerge partly from differences in definition but

perhaps mostly from decisions about which societies should be included in the

assessment.

2

Hunter-‐gatherer “societies” are groups of several hundred individuals within

which there is a common language and set of cultural practices that differ from

those of neighboring groups. When defined broadly to include societies in which

most but not necessarily all of their food comes from traditional foraging (e.g. at

least 90%, with up to 10% coming from farm products), many are found to have

practiced no war against other societies in the decades since they were first

documented. This observation has been used to buttress the Rousseauian view that

nomadic hunter-‐gatherers had little or no war prior to the development of

agriculture about 10,000 years ago, and therefore that war has not been a product of

evolution. Further supporting the Rousseauian view, there is little archaeological

evidence for battles prior to agriculture. Finally low rates of violence and strongly

egalitarian hierarchies within nomadic hunter-‐gatherer societies contradict the

notion of individuals being inherently selfish and competitive. These points have

been taken to suggest that war is a recent invention with no important evolutionary

precursors (Fry 2007).

However most groups of hunter-‐gatherers were already neighbored by

farmers (whether pastoralists or agriculturalists) by the time that their war

histories were recorded. The farmers tended to dominate them numerically,

politically and militarily, so an absence of war by the hunter-‐gatherers is easily

attributed to strategic wisdom rather than requiring an evolutionary explanation.

Only in a few cases have ethnographers or other diarists been able to document

relationships among neighboring societies of hunter-‐gatherers without a dominant

influence of agriculturalists. In these exceptions, which include Alaska, Tasmania,

Australia, Andaman Islands, New Guinea and Tierra del Fuego, the evidence for

warfare between neighboring societies (i.e. external war) is universal (Wrangham

and Glowacki 2012). Confrontations could often be avoided for long periods, and

rates of death were not necessarily high but as Kelly (2000: 118) said of the

Andaman Islanders, external war was “unremitting and constitutes a condition of

existence that defines the boundaries of the niches exploited by two populations.”

The implication is that warfare would have been equivalently omnipresent

among Paleolithic hunter-‐gatherers. Unfortunately the archaeological record offers

3

no way to test this conclusion. There is admittedly some relevant skeletal evidence

in the Upper Paleolithic back to 36000 years ago (such as an apparent spear-‐thrust

between leg and hip), and there are isolated earlier cases such as a cutmark on a rib

about 50000 years (Shanidar III). These suggest “sporadic intraspecific killing”

(Roper 1969: 448), but the earliest evidence of group killing is of 24 Sudanese

skeletons associated with trauma and weapons 12000 years ago. Cave art

portraying intergroup clashes by 6000 ya in Australia and in the European Upper

Paleolithic is likewise provocative but ultimately merely suggestive (Ferguson

2006).

We are left with the contemporary evidence, which indicates that war has

been a sufficiently normal activity among hunter-‐gatherers who were not

dominated by militarily superior farming neighbors that it should be considered to

have been a routine part of the behavioral repertoire of pre-‐agricultural people, and

therefore a product of evolution, presumably throughout the 200,000 year existence

of Homo sapiens and likely earlier also.

So how has an evolutionary history of war affected human psychology?

Anthropologists critiquing the conclusion that the human past was war-‐like have

sometimes caricatured evolutionary thinking as implying that if warfare has been

influenced by biology, it must be inevitable and/or stereotyped; and since in any

given society warfare can be entirely absent for long stretches of time (e.g. over

generations), while in others the patterns of warfare change rapidly over time, they

reject the idea that if we had war in our past, it shaped us at all. But the notion that

behavioral evolution generates uniformity of behavior is wrong. In any animal

species, tendencies for intergroup violence are strongly influenced by behavioral

ecology (the costs and benefits of aggression); and in humans they are further

influenced by social learning, including cultural norms. So variability of the practice

of war is compatible with behavioral evolution.

However, variation in the practice of war also raises in a different way the

question of whether a supposed evolutionary history of war has any significance

today. The variation presumably occurs largely because decision-‐makers make war

when “it is in their practical self-‐interest to do so” (Ferguson 2011: 265). If decision-‐

4

makers are rational, surely an evolutionary history of war contributes nothing to

our theoretical understanding?

Others have discussed this issue by focusing on how an evolutionary history

of war has influenced coalitionary behavior, including sex differences (Tooby and

Cosmides 1988, Lopez et al 2011, McDonald et al 2012). Here I explore this problem

in a complementary way by attempting to identify patterns of aggression that have

evolved specifically in warring species, including humans. To do so I first describe

the style of fighting in simple human warfare.

Simple vs Complex Warfare.

The major difficulty in identifying warfare from skeletal remains is that war

includes two styles of military practice, only one of which can be recognized

archaeologically. In terms of fighting the distinction is between complex and simple

warfare: complex warfare regularly includes battles (escalated conflicts between

committed opponents), whereas simple warfare is largely confined to surprise

attacks such as raiding. In terms of social organization the distinction is between

hierarchy and acephaly (lack of formal leadership).

A society that practices complex warfare and battles, and has a military

hierarchy, is said to have “true warfare” or to lie above the military horizon

(Turney-‐High 1949). In this system soldiers fight under orders from leaders and

battles are frequently lethal. The result of a specific encounter can thus be a large

number of deaths on both sides, which (especially when combined with metal

weapons) is easily detected archaeologically. Such battle evidence currently goes

back to about 8000 BC in the Middle East (Qermez Dere, Iraq, Ferguson 2006). True

warfare is therefore normally thought to begin within a few hundred years of the

origin of agriculture 10000 years ago, resulting from the development of

hierarchically organized states.

“Simple warfare”, by contrast, is practiced by small-‐scale acephalous hunter-‐

gatherer and farmer societies whose warriors fight voluntarily, and whose

communities are not integrated with each other by any political officials. It consists

mainly of raiding and feuding. Simple warfare tends not to include battles, but when

5

battles occur they normally stop after a few deaths. Massacres can occur when one

side has a massive power advantage, such as burning a hut full of opponents, but the

majority of deaths in simple warfare occur when raiders kill victims in a surprise

attack. Raids often kill very few victims, such as only one, followed by the aggressors

immediately making a rapid and complete retreat in order to avoid the risk of being

confronted. The fact that in simple warfare most deaths occur in very small numbers

explains the difficulty of distinguishing archaeologically between murder and war.

The costs and benefits of simple warfare are not well characterized. On the

one hand the cessation of war tends to benefit all parties, not only by saving lives

and effort but also by enabling intergroup buffer zones to be exploited for food

(Kelly 2005). On the other hand societies that conducted successful aggression

certainly sometimes gained territory. Ember and Ember (1992) found that among

interactions involving 30 non-‐state societies, victors drove the defeated from their

land in 77% of cases in which warfare occurred at least every two years. I assume

that in general, societies that were more successful in war tended to benefit, even

though contrary cases occur.

The significance of animal war for reconstructing human war in the Paleolithic.

The claim of war being a recent cultural novelty has traditionally depended

on the notion that war is unique to humans (Lorenz 1963). But we now know that

war (in the sense of systematic intergroup killing by coalitions) is not unique to

humans. It is known best in ants and a few mammals.

Among mammals, regular lethal intergroup interactions occur in at least two

primates and three carnivores. Among primates intergroup killing is widespread

among chimpanzees Pan troglodytes. Several cases of lethal intergroup interactions

have also been recorded in capuchins Cebus capucinus. Among carnivores the most

frequent killers of member of neighboring groups are wolves Canis lupus,

responsible for up to 50% of adult mortality. Deliberate intergroup killing is also

known in lions Panthera leo and spotted hyenas Crocuta crocuta.

The consequences of war are not well known in animals but two studies of

chimpanzees found that aggressor groups obtained benefits of territory and/or

6

females (Mitani et al 2010).

Two main kinds of explanation have been offered for the restriction of

warfare to these few species. ‘Psychological adaptation’ hypotheses claim that

warfare depends on cognitive abilities that most species do not have, such as the

ability to form flexible coalitions, and that such abilities are sufficiently specialized

that they must have evolved through natural selection (Tooby and Cosmides 1988,

Lopez et al 2011). In support, chimpanzees and capuchins appear to have

particularly strong coalitional abilities compared to other primates, and are among

the biggest-‐brained primates. There is little relevant evidence about the cognition of

wolves, lions and spotted hyenas, though their relative brain volumes are no greater

than in other terrestrial carnivores (Swanson et al 2012). Contemporary humans

successfully manage diverse kinds of war tactic and complex cooperation, and even

if such abilities increased during the last 200000 years there is no reason to think

they were ever any poorer than those of wolves. So human cognition has surely

been adequate for effective intergroup killing by coalitions for a long time, e.g. at

least since the origin of Homo sapiens.

The ‘imbalance-‐of-‐power’ hypothesis provides an ultimate rationale for the

‘cognitive constraints’ hypothesis by explaining why many species of cognitively

sophisticated group-‐living primates show no evidence of intergroup killing despite

having regular aggressive intergroup interactions when access to important

resources is at stake. Group-‐living primates without lethal war include macaques

and baboons, in which coalitions occur based both on kin and non-‐kin, and in some

of which there are frequent intraspecific kills in the form of within-‐group infanticide.

The ‘imbalance-‐of-‐power’ hypothesis explains a lack of war in such species by

claiming that natural selection favored motivation to hunt and kill members of

neighboring groups only in species where groups of aggressors have, over

evolutionary time, regularly been able to find rivals alone or otherwise highly

vulnerable. In those circumstances aggressors could kill without being wounded,

and killing was therefore so cheap that it was favored even when benefits to

individual killers were unpredictable. Nomadic hunter-‐gatherers conform to the

principle of the imbalance-‐of-‐power hypothesis because attacks are normally made

7

on lone victims or on groups that are so vulnerable that they can be killed without

being able to retaliate. As in chimpanzees and the war-‐like social carnivores, the

foraging style of nomadic hunter-‐gatherers is “fission-‐fusion”, i.e. individuals travel

each day in parties of variable size, including alone at times. This renders them

vulnerable to surprise attacks by larger groups or lone individuals with an

appropriate weapon. “Fission-‐fusion” foraging is found in all recent hunter-‐

gatherers and seems likely to have been a consistent feature of Homo sapiens. Like

the cognitive constraints hypothesis, therefore, the imbalance-‐of-‐power hypothesis

is likely to have applied throughout the evolutionary history of Homo sapiens

(Wrangham and Glowacki 2012).

In sum, nomadic hunter-‐gatherers practice regular war with neighboring

hunter-‐gatherer societies; a few animal species also practice war (in the sense of

deliberate killing of members of out-‐groups by coalitions); and the cognitive and

ecological factors that appear to favor war in animals seem likely to have applied to

humans during our hunting-‐and-‐gathering past. These points suggest that human

and non-‐human “war psychology” have similar evolutionary foundations.

Predatory vs impulsive aggression in simple warfare.

Similarities between simple human warfare and non-‐human war are most

easily explored by reference to chimpanzees, since not only are chimpanzees well-‐

studied close relatives of humans that live in multi-‐male groups, but they also have a

sister species (bonobos Pan paniscus) that provides a helpful contrast to

chimpanzees by having no war. In the most recent survey, data from 16 habituated

wild social groups of chimpanzees at 9 sites revealed 78 cases of killings, 81% male.

Most killings (68%) involved intergroup attacks by coalitions of males. The number

of killings recorded per site was positively related to the number of males in a

community (and not to measures of human disturbance). Expressed in terms used

for homicide rates, males killed other grown males at a median rate of 4000-‐5000

per annum per million individuals (Wilson et al in prep).

In contrast to chimpanzees, no killings have been observed in studies of

bonobos despite much opportunity for them to be seen. In captivity the behavior of

8

the two species shows clear differences in tendencies for aggression, with bonobos

being much more tolerant and less competitive. This is remarkable given that the

bonobo lineage split from the chimpanzee lineage less than one million years ago,

and in many ways the two species are similar. They have similar body size, and in

both males are 20-‐30% larger than females. They occupy similar kinds of forest

either side of the Congo River. Both live in social communities of 50 or more

individuals in which adult males spend their lives with their male kin, while females

enter a new group at adolescence. In both species groups defend access to a group

territory using calls, chases and physical fights. The fact that lethal aggression is

typical only of chimpanzees is therefore not a result of any obvious feature of group

structure, physical ability or resource defense. One possibility is that the

psychological differences motivating war in chimpanzees and not in bonobos have

evolved because bonobos live in more stable sub-‐groups; but regardless of whether

grouping patterns provide the ultimate cause of the differences in behavior,

differences in motivation certainly contribute to explaining why chimpanzees are

much more violent than bonobos (Hare et al 2012).

The comparison between chimpanzees and bonobos is informative because

several psychological attributes that are more strongly developed in chimpanzees

than in bonobos are also found in humans. They are therefore candidate

consequences of an evolutionary history of war. In each case they are found in

chimpanzees and humans, whereas they are absent or trivial in bonobos.

First is a sex difference in violence (physical aggression): males are much

more violent than females. Sex differences in aggressiveness are driven largely by

sexual selection (mainly by males competing over access to females). Greater male

than female aggressiveness is illustrated in chimpanzees and humans by a higher

frequency of non-‐lethal physical fights among males within groups, higher rates of

killing by males within groups, higher rates of killing by males between groups, and

higher rates of males being killed (Wrangham et al 2006, Riddle et al 2012).

Second, within-‐group alliances with other males are far more prominent in

chimpanzees and humans than in bonobos (Lopez et al 2011). Alliances among male

chimpanzees and humans are used prominently in competition for status and

9

resources, such that males who are more effective at forming alliances are more

successful in competition. The importance of alliances is evident in humans from the

fact that political control of societies is universally exerted by coalitions of males. In

both species sharing of meat occurs among males more than among females, in

contrast to bonobos where females are more likely to possess and share meat (Kano

1992).

Third, males exhibit a strong motivation to hunt and kill mammalian prey.

Male humans and chimpanzees are easily aroused to hunt, and in both cases there is

tentative evidence of a positive association between hunting prey and aggression

towards neighbors (Otterbein 2004; Gilby et al 2013). The claim that male

chimpanzees have a stronger interest in hunting than male bonobos comes from

there being only five records of monkey-‐hunting by bonobos, with no evidence of a

sex difference in hunting effort (Surbeck and Hohmann 2008). By contrast there are

more than a thousand monkey-‐hunts recorded for male chimpanzees, with minimal

female participation. Monkey-‐hunting is dangerous and achieved best by groups

(Gilby et al 2008). Interestingly, captive data indicate no difference between

chimpanzees and bonobos in their rate of killing small prey individually (up to the

size of rabbits, Ross et al 2009). This suggests that the difference in monkey-‐hunting

is not due to a difference in preference for meat, or willingness to kill per se. Instead,

species differences in willingness to engage in risky chases or to rely on other group

members may be responsible. (By contrast within chimpanzees, although monkey-‐

hunting and territorial aggression tend to occur on the same days, different

individuals appeared to be responsible for hunting and aggression respectively

(Gilby et al 2013).)

As with intra-‐specific killing, the fact that bonobos hunt monkeys less than

chimpanzees do is surprising because it is not explained by differences in habitat,

food availability, food preference, strength or any other obvious ecological factor. It

raises the possibility that in bonobos selection against the propensity to hunt-‐and-‐

kill conspecifics led to a reduced propensity for killing monkeys (Wrangham 1999).

In other words, group hunting of animal prey evolved as a by-‐product of selection

for group hunting of conspecifics, because both involve predatory aggression.

10

This idea is controversial because predation on animals is normally assumed

to be a foraging behavior that is regulated in the brain separately from aggression.

However recent work on the neural control of aggression indicates important

distinctions between the neural control of the two major types of aggression,

impulsive (also called reactive, affective, etc) and predatory. The two types are

associated with different levels of arousal, supported by different neuronal

pathways, and influenced in partly different ways by neurotransmitters (Feshbach

1964, Weinshenker and Siegel 2002, McEllistrem 2004, Siegel and Victoroff 2009).

This bimodal distinction has emerged in parallel from studies of children

(‘proactive-‐reactive’), adult humans (‘instrumental-‐reactive’), human

psychopathologies and animals, especially cats, rats and mice (Weinshenker and

Siegel 2002). Within individuals one or other of the two types of aggression tends to

be the predominant form, though they are not mutually exclusive and are often

correlated (Tharp et al 2011).

Predatory aggression is alternately called instrumental, proactive,

premeditated, offensive or cold-‐blooded. It has been studied much less than

impulsive aggression in both animals and humans, and equivalently less is known

about its neurobiology. For example no successful psychopharmacological

interventions have been found for it in humans, unlike impulsive aggression

(Weinshenker and Siegel 2002). In contrast to impulsive behaviour it is

characterized by a lack of emotional arousal, conformity to a purposeful plan,

attention to a consistent target, and being self-‐rewarding rather than serving to

remove an aversive stimulus (Siegel and Victoroff 2009). It does not require the

presence of a perceived threat. Predatory aggression is elicited in cats by electrical

stimulation of the lateral hypothalamus, ventral periaqueductal gray (PAG), central

amygdala and dopamine-‐producing ventral tegmental region of the midbrain (Siegel

and Victoroff 2009, Tulogdi et al 2010). Predatory aggression towards conspecifics

in rodents (e.g. mice, Sandnabba 1995; rats, Nikulina 1991), humans (Meloy 1997),

and various wild species (e.g. infanticidal primates) is behaviourally similar to

predatory aggression in cats, and equally distinct from impulsive aggression

(including responses to neurotransmitters). Furthermore like predatory aggression

11

towards prey by cats or rats, predatory aggression toward conspecifics by rats is

also associated with activation of the lateral hypothalamus and central amygdala

(Tulogdi et al 2010).

Such similarities have led recent investigators to consider predatory

aggression as a unitary phenomenon with respect to the underlying neurobiology

whether directed towards prey or conspecifics (Weinshenker and Siegel 2002). In

humans predatory aggression towards conspecifics can be triggered by a wide

variety of motivating factors, including desires for money, power, control or sadistic

fantasies (Weinshenker and Siegel 2002). In addition to triggers, however,

predatory aggression is controlled by opportunity. For example in chimpanzees,

groups do not attack unless circumstances mean that their risk of being wounded is

minimal.

Rates of intergroup killing are broadly similar in chimpanzees and simple

human warfare, in both of which the style of aggression is almost entirely predatory

(Wrangham et al 2006). In contrast to this species similarity, humans within social

groups are much less impulsively violent than chimpanzees: physical fights occur at

rates hundreds or thousands of times lower than in chimpanzees. Recognition of the

separate neural control of predatory and impulsive aggression suggests that the

combination of difference and similarity in aggression between chimpanzees and

humans is explicable by the two types of aggression having been subject to distinct

evolutionary histories in the two species. The simplest hypothesis is that in both

species there has been strong selection for male predatory violence, responsible for

high rates of intergroup killing. In humans, however, but not in chimpanzees, there

has been selection against male impulsive violence. Given that impulsive aggression

is the principal form of violence within groups (loss of temper being more

responsible than an evil plan for the rate of murder), humans are much less

aggressive within groups than between groups.

In sum, this discussion leads to three proposals for how an evolutionary

history of war has affected human and chimpanzee psychology.

First, humans (like chimpanzees) are subject to selection for male

willingness to use violence, males readily coordinating in groups, and males being

12

motivated to hunt. The evolution of such traits makes human males into effective

coalitionary killers.

Second, selection for increased violence has had separate consequences for

predatory and impulsive aggression. Predatory aggression has been particularly

important in the expression of intergroup killing in an evolutionary context;

impulsive aggression has not.

Third, the premeditated nature of predatory aggression combined with self-‐

interested assessment of risk predicts that if there are no opportunities for safe

attack, predatory violence will not occur.

Uniquely human behavior in simple warfare.

Although simple human warfare is similar to chimpanzee war in being

focused on surprise attacks by small groups of males who maximize their own safety,

it also differs importantly. The factors responsible for unique aspects of human war

behavior are uncertain: biological arguments have been proposed but plausibly all

differences are due to culture.

For example in complex war, warriors sometimes take large risks in order to

help each other. This has led to the idea that risk-‐taking on behalf of other group

members (‘parochial altruism’) is a human cross-‐cultural universal that has been

genetically selected (Bowles 2009). But the hypothesis of parochial altruism being

embedded in human nature is undermined by the fact that in simple war risky

support for fellow fighters has not been documented: warriors’ behavior seems to

be overwhelmingly concerned with their own safety (Wrangham and Glowacki

2012). Parochial altruism in war may therefore depend on societies educating their

youth appropriately rather than on innate tendencies.

Similarly, revenge acts as an important motivator in human war, not only in

feuding but even in complex war (McCullough 2008). By contrast there is no

evidence for any role of revenge in between-‐group interactions among chimpanzees

or other animals. However what makes humans unique is not their use of revenge as

a motive but their extension of it from a within-‐group to a between-‐group system,

since aggression within groups of chimpanzees can be retaliatory, as in humans. As

13

an alternative to the idea that humans have been selected to use revenge in war, the

pattern of revenge killing in war is plausibly explained by social organization and

cultural norms (Boehm 2011). Again, therefore, the role of innate factors is unclear.

Cultural factors favoring war or peace.

In contrast to the uncertain evolutionary status of parochial altruism and

revenge, the human capacity for culture is clearly an evolved trait. It has had

important effects both in promoting and reducing the propensity for war.

Thus in societies with complex warfare reward, punishment and coercion

play obvious roles in promoting militarism: soldiers who perform well get material

benefits and symbolic rewards such as medals, while those who fail risk a variety of

punishments from dishonor to death at the hands of their own officers. Equivalent

systems can occur in societies with simple warfare. A greater diversity of culturally

explicit rewards in small-‐scale societies is associated with a higher death rate from

war (Glowacki and Wrangham 2013), while in the pastoralist Turkana,

uncooperative warriors may be physically punished (Mathew and Boyd 2009).

Cultural systems can thus increase a society’s military power, and hence presumably

its propensity to engage in violent conflict.

Affiliative relationships are unknown between neighboring groups of animals

that have war (chimpanzees, capuchins, lions etc), and are rare in world systems of

hunter-‐gatherers. But peaceful understandings can be reached in most human

intergroup relationships, associated with trade, feasts, alliances or a perception of

the common good. Peace systems are often associated with culturally prescribed

rituals (e.g. in internal war among Andaman Islanders), indicating that the capacity

for culture has promoted human peace-‐making.

How does the claim that humans have an evolutionary history of war help us

understand patterns of war behavior?

The conclusion that human males have evolved to be effective coalitionary

predators is only relevant to the extent that they can express their behavior in

complex war. Since complex war involves hierarchy of (minimally) leaders and

14

warriors, I consider them separately.

(1) Warriors (engaging in predatory aggression).

By warriors I mean individuals who have sufficient independence to decide

for themselves to engage in a war (where war means, as before, coalitionary killing

of outsiders). Examples are hunter-‐gatherers or small-‐scale farmers on a raid,

members of inner-‐city gangs, fighters in a civil war, secret societies, or other

members of anarchic interacting groups. By contrast soldiers are not necessarily

“warriors” if they are acting under orders, against their will. Although warrior

groups are influenced by cultural norms, the men who compose them predictably

echo the behavior of chimpanzees by using easily formed alliances to readily kill

outsiders using premeditated surprise attacks; and it seems likely that for the most

part they gain satisfaction from doing so, such that if killing is profitable, it is not

stressful (Weierstall et al 2013). Thanks to an evolutionary history of selection for

these behaviors it makes sense that coalitionary killing of enemies is thrilling for the

killers, and that the opportunity to kill safely will therefore sometimes take

precedence over longer-‐term considerations.

Thus from a political or moral perspective a problem with understanding

warrior tendencies of this kind as resulting from an evolutionary history of war is

that their behavior is expected to be relatively inert to larger strategic, moral or

cultural constraints. For example if youths on either side of a political division are

atavistically excited by being able to kill vulnerable enemy, their short-‐term

satisfaction can foster a cycle of revenge and heighten tensions in ways that

challenge the long-‐term interests of the larger society of which they are members.

Soldiers are not necessarily warriors, but they can be if they are enthusiastic

supporters of their army’s military goal. Many military engagements take the form

of surprise attack and thereby engage aspects of the male psyche that are well

adapted for killing, and that can be used by leaders. Given the pronounced sex

difference in the evolutionary history of warfare it seems unlikely that women

would be as effective as men at coalitionary killing.

In short: predatory violence by anarchic groups conforms to evolutionary

models, appears atavistic, and will not necessarily be strategically aligned with the

15

political goals of leaders of the larger society.

(2) Leaders (engaging particularly in impulsive aggression).

Strategy in complex warfare depends importantly on decisions by leaders.

Given that war leaders do not occur among nomadic hunter-‐gatherers, there has

been no opportunity for genetic selection of behavior specifically related to being a

war leader. (For a contrary view suggesting that war leadership has been

evolutionarily salient, see Spisak et al 2012.) However leaders can confront other

leaders as individuals, or sub-‐groups confront each other as sub-‐groups. This means

that conflicts among war leaders can echo within-‐group interactions. We can

therefore expect that strategies or responses adopted by individuals (or councils) to

direct the course of war use will include components favored in within-‐group

conflicts. I suggest four examples.

First, as discussed above, revenge illustrates this principle: humans extend to

a between-‐group context an emotion that likely evolved for within-‐group

interactions. Revenge can function to promote a cycle of violence between groups,

and can clearly undermine efforts to achieve peaceful relationships.

Second, over-‐confidence (an individual’s exaggerated belief in their own

abilities, e.g. in fighting) appears to have been positively selected in males because

of its promotion of success in conflict, e.g. by increasing resolve, persistence or the

credibility of bluffing. However, it can also be disadvantageous both to the

individual concerned (because it leads to faulty assessments and hazardous

decisions) and it is ultimately a disservice to the average participant in the system

(because it promotes dangerous fighting and disastrous political decisions). Over-‐

confidence is common among human war leaders and appears to be an

evolutionarily stable trait despite its defying cool-‐headed logic and exacerbating

numerous problems such as costly wars (Johnson and Fowler 2011).

Third, within groups of primates there are evolved mechanisms for

establishing and maintaining tolerant relationships between competing individuals:

differences by species and sex in the expression, context and frequency of these

“reconciliation” behaviors indicate that they reflect innate predispositions even

though they can be modified by experience (de Waal 1993). Reconciliation

16

behaviors among animals are expressed within groups, not between groups,

because they depend on relationships being personal. The human ability to extend

reconciliation to the between-‐group context increases the opportunities for

reduction of tension between competing groups.

Finally, policing within primate groups reduces the frequency of conflict:

dominant individuals or alliances support subordinates by confronting their

aggressors. In a parallel but more egalitarian way, among hunter-‐gatherers group

members tend to stop fights by intervening, often against both parties. Primate

groups that have effective policers in control roles experience reduced conflict and

are more demographically successful (Flack et al 2006).

(3) Group dynamics.

This selective overview of biological and cultural influences on war has

focused on the style of aggression. An equally important topic for understanding

evolutionary influences on war is the formation and dynamics of social groups.

Even in a species in which intergroup hostilities are not warlike, individuals

spontaneously treat in-‐group members positively and out-‐group members

negatively (rhesus macaque Macaca mulatta, Mahajan et al 2011). This suggests that

the tendency to discriminate in-‐groups and out-‐groups as evolutionary roots and

raises important questions about the ease with which hatred of the category of

“enemy” emerges (Michener 2012). Biology matters to war in many ways other than

by its effects on aggressive interactions.

Discussion/Conclusion.

The transition of the human species from countless small, independent,

anarchic groups at permanent war with each other 10,000 years ago into today’s

series of large, mutually dependent and hierarchically organized groups varying

between peace and war has not eliminated the influence of our evolutionary biology,

but it has changed the context in which it has effects. Instead of treating the rest of

the world as an automatic out-‐group, every state is now part of a “world in-‐group,”

because parties who might have an interest in intervening are now witnesses in all

wars.

17

The result is a series of changes in the costs and benefits of aggression,

including a reduced opportunity for extreme imbalances of power. Predatory

aggression might seem a tolerable legacy if it is routinely suppressed by systems of

balanced power, as it normally is. The fact that the world is to a large extent a single

in-‐group is therefore an optimistic observation. But unfortunately power is not

always balanced. Power asymmetry can lead to massacre, as it did in Nazi Germany

and Rwanda. The legacy remains dangerous.

The legacy of predatory aggression may have exposed humans to the dangers

of absolute power in a particularly direct way, but even without it our evolutionary

biology as a social primate would have left us with potent tendencies for intergroup

hostility. Rhesus macaques are not known to kill each other, but like most primates

they have hostile inter-‐group relationships, discriminate sharply between in-‐group

and out-‐group members, and benefit from success in conflicts. Even if humans had

not had an evolutionary history of war, our primate background would be expected

to push us towards inter-‐group conflicts. Descent from almost any social primate

would likewise have given us vengeful, over-‐confident leaders. Sometimes the result

of such traits would benefit the group; sometimes not. But the fact that opposing

leaders tend to over-‐estimate their own group’s military power exacerbates

conflicts, creating more violence than justified by a rational analysis. Most of the

routes by which we became human would have left us ready to be violent.

The radical alternative to the perspective presented in this paper is the

Idealist view that wars represent disturbances to a default system of adaptive peace.

According to this concept humans are by nature altruistic and benevolent, so groups

tend not to compete, which is clearly a wrong idea. War supposedly occurs when the

system is disturbed by a novel or malign influence. In inter-‐state systems, for

example, war is supposed to emerge from “historical circumstances, evil leaders,

flawed sociopolitical systems, or inadequate international understanding and

education” (Holsti 2004: 54). Similarly, in small-‐scale society war has been argued

to derive from the influences of agriculture, imperial powers, patriarchal ideology or

advanced weaponry (Lorenz 1966, Lerner 1986, Ferguson 2011). Whatever the

importance of such factors, however, they always involve an evolutionary

18

psychology that (particularly in males) has been selected to use power selfishly.

Given that aggression today has been favored by our past, the strong

evidence of a sustained reduction in violence over historical times is impressive. The

fact that humans come equipped with peace-‐making mechanisms that have evolved

within social groups is not an explanation on its own, since we have always had that

capacity. The multiple sources to which Pinker (2011) attributes the decline include

strong governments, increased trade, empowerment of women, extension of

empathy, and rationality. These and other factors, such as stable demography and

monogamy, are surely vital for continued control and reduction of violence. “Nature,”

as Katherine Hepburn’s heroine said in The African Queen, “is what we are put in this

world to rise above.”

Bibliography.

Boehm, C. 2011. Retaliatory violence in human prehistory. British Journal of Criminology, 51, 518-‐534.

Bowles, S. 2009. Did warfare among ancestral hunter-‐gatherers affect the evolution of human social behaviors? Science, 324, 1293-‐1298.

de Waal, F. B. M. & Johanowicz, D. L. 1993. Modification of reconciliation behavior through social experience: an experiment with two macaque species. Child Development, 64, 897-‐908.

Ember, C. R. & Ember, M. 1992. Resource unpredictability, mistrust, and war. Journal of Conflict Resolution, 36, 242–262.

Ferguson, R. B. 2006. Archaeology, cultural anthropology, and origins and intensifications of war. In: The archaeology of warfare: Prehistories of raiding and conquest. (Ed. by Arkush, E. & Allen, M.), pp. 469–523. Gainesville, FL: University of Florida Press.

Ferguson, R. B. 2011. Born to Live: Challenging Killer Myths. In: Origins of Altruism and Cooperation (Ed. by Sussman, R. W. & Cloninger, C. R.), pp. 249-‐270. New York: Springer.

Feshbach, S. 1964. The function of aggression and the regulation of aggressive drive. Psychological Review, 71, 257−272.

Flack, J. C., Girvan, M., de Waal, F. B. M. & Krakauer, D. C. 2006. Policing stabilizes construction of social niches in primates. Nature, 439, 426-‐429.

Fry, D. P. 2007. Beyond war: the human potential for peace. Oxford: Oxford University Press. Gilby, I. C., Eberly, L. E. & Wrangham, R. W. 2008. Economic profitability of social predation

among wild chimpanzees: individual variation promotes cooperation. Animal Behaviour, 75, 351-‐360.

Gilby, I. C., Wilson, M. L. & Pusey, A. E. 2013. Ecology rather than psychology explains co-‐

19

occurrence of predation and border patrols in male chimpanzees. Animal Behavior, in press.

Glowacki, L. & Wrangham, R. W. 2013. The role of rewards in motivating participation in simple warfare: A test of the cultural rewards war-‐risk hypothesis. Human Nature, in press.

Hare, B., Wobber, V. & Wrangham, R. W. 2012. The self-‐domestication hypothesis: bonobos evolved due to selection against male aggression. Animal Behavior, 83, 573-‐585.

Holsti, O. R. 2004. Theories of International Relations. In: Explaining the History of American Foreign Relations (Ed. by Hogan, M. J. & Paterson, T. G.), pp. 51-‐90. New York: Cambridge University Press.

Johnson, D. P. P. & Fowler, J. H. 2011. The evolution of overconfidence. Nature, 477, 317-‐320.

Kano, T. 1992. The Last Ape: Pygmy Chimpanzee Behavior and Ecology. Stanford, CA: Stanford University Press.

Keeley, L. H. 1996. War before civilization. New York: Oxford University Press. Kelly, R. C. 2000. Warless societies and the origins of war. Ann Arbor, MI: University of

Michigan Press. Kelly, R. C. 2005. The evolution of lethal intergroup violence. Proceedings of the National

Academy of Sciences, 102, 15294-‐15298. Langergraber, K. E., Schubert, G., Rowney, C., Wrangham, R., Zommers, Z. & Vigilant, L. 2011.

Genetic differentiation and the evolution of cooperation in chimpanzees and humans. Proc. R. Soc. B, 278, 2546-‐2552.

Lerner, G. 1986. The Creation of Patriarchy. New York: Oxford University Press. Liddle, J. R., Shackelford, T. K. & Weekes-‐Shackelford, V. W. 2012. Why Can’t We All Just Get

Along? Evolutionary Perspectives on Violence, Homicide, and War. Review of General Psychology, 16, 24-‐36.

Lopez, A. C., McDermott, R. & Petersen, M. B. 2011. States in mind: Evolution, coalitional psychology, and international politics. International Security, 36, 48-‐83.

Lorenz, K. 1966. On Aggression. New York: Harcourt, Brace. Mahajan, N., Martinez, M. A., Gutierrez, N. L., Disendruck, G., Banaji, M. & Santos, L. R. 2011.

The evolution of ingroup bias: perceptions and attitudes in rhesus macaques (Macaca mulatta). Journal of Personality and Social Psychology, 100, 387-‐405.

Mathew, S. & Boyd, R. 2011. Punishment sustains large-‐scale cooperation in prestate warfare. PNAS, 108, 11375-‐11380.

McCullough, M. E. 2008. Beyond Revenge: The Evolution of the Forgiveness Instinct. San Francisco, CA: Jossey-‐Bass.

McDonald, M. M., Navarrete, C. D. & Van Vugt, M. 2012. Evolution and the psychology of intergroup conflict: the male warrior hypothesis. Philosophical Transactions of the Royal Society, 367, 670-‐690.

McEllistrem, J. E. 2004. Affective and predatory violence: A bimodal classification system of human aggression and violence. Aggression and Violent Behavior, 10, 1–30.

Meloy, J. R. 1997. Predatory violence during mass murder. Journal of Forensic Sciences, 42, 326–329.

Michener, W. 2012. The individual psychology of group hate. Journal of Hate Studies, 10, 15-‐48.

Mitani, J. C., Watts, D. P. & Amsler, S. J. 2010. Lethal intergroup aggression leads to

20

territorial expansion in wild chimpanzees. Current Biology, 20, R507–R508. Nikulina, E. M. 1991. Neural control of predatory aggression in wild and domesticated

animals. Neurosci Biobehav Rev, 15, 545-‐547. Otterbein, K. F. 2004. How War Began: Texas A&M University Press. Pinker, S. 2011. The Better Angels of Our Nature: Why Violence Has Declined. New York:

Penguin. Roper, M. K. 1969. A survey of evidence for intrahuman killing in Pleistocene. Current

Anthropology, 10, 427-‐459. Ross, S. R., Holmes, A. N. & Lonsdorf, E. V. 2009. Interactions between zoo-‐housed great

apes and local wildlife. American Journal of Primatology, 71, 1-‐8. Sandnabba, N. K. 1995. Predatory aggression in male mice selectively bred for isolation-‐

induced intermale aggression. Behavior Genetics, 25, 361–366. Siegel, A. & Victoroff, J. 2009. Understanding human aggression: new insights from

neuroscience. International Journal of Law and Psychiatry, 32, 209–215. Spisak, B. R., Dekker, P. H., Krüger, M. & Van Vugt, M. 2012. Warriors and peacekeepers:

Testing a biosocial implicit leadership hypothesis of intergroup relations using masculine and feminine faces. PLoS ONE, 7(1), e30399.

Surbeck, M. & Hohmann, G. 2008. Primate hunting by bonobos at LuiKotale, Salonga National Park. Current Biology, 18, R906-‐R907.

Swanson, E. M., Holekamp, K. E., Lundrigan, B. L., Arsznov, B. M. & Sakai, S. T. 2012. Multiple determinants of whole and regional brain volume among terrestrial carnivorans. PLoS ONE, 7, e38447, 38441-‐38411.

Tharp, A. L. T., Sharp, C., Stanford, M. S., Lake, S. L., Raine, A. & Kent, T. A. 2011. Correspondence of aggressive behavior classifications among young adults using the Impulsive Premeditated Aggression Scale and the Reactive Proactive Questionnaire. Personality & Individual Differences, 50, 279-‐285.

Tooby, J. & Cosmides, L. 1988. The evolution of war and its cognitive foundations. In: Technical Report, No. 88-‐1. Santa Barbara: Institute for Evolutionary Studies, University of California, Santa Barbara.

Tulogdi, A., Toth, M., Halasz, J., Mikics, E., Fuzesi, T. & Haller, J. 2010. Brain mechanisms involved in predatory aggression are activated in a laboratory model of violent intra-‐specific aggression. European Journal of Neuroscience, 32, 1744-‐1753.

Turney-‐High, H. H. 1949. Primitive war: Its practice and concepts. Columbia: University of South Carolina Press.

Weierstall, R., Schalinski, I., Crombach, A., Hecker, T. & Elbert, T. 2012. When combat prevents PTSD symptoms—results from a survey with former child soldiers in Northern Uganda. BMC Psychiatry, 12, 1-‐8.

Weinshenker, N. J. & Siegel, A. 2002. Bimodal classification of aggression: affective defense and predatory attack. Aggression and Violent Behavior, 7, 237–250.

Wrangham, R. W. 1999. Evolution of coalitionary killing. Yearbook of Physical Anthropology, 42, 1-‐39.

Wrangham, R. W. & Glowacki, L. 2012. War in chimpanzees and nomadic hunter-‐gatherers: evaluating the chimpanzee model. Human Nature, 23, 5-‐29.

Wrangham, R. W., Wilson, M. L. & Muller, M. N. 2006. Comparative rates of aggression in chimpanzees and humans. Primates, 47, 14-‐26.

Documents

Why Evolution Matters for War