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The use of evolutionary theory for explaining human warfare is an expanding area of inquiry, but it remains obstructed by two important hurdles. One is that there is ambiguity about how to build an evolutionary theory of human warfare. The second is that there is ambiguity about how to interpret existing evidence relating to the evolution of warfare. This paper addresses these problems, first by outlining an evolutionary theory of human warfare, and second by investigating the veracity of four common claims made against the use of evolutionary theory for explaining warfare. These claims are: (1) ancestral warfare was not frequent or intense enough to have selected for psychological adaptations in humans for warfare; (2) the existence of peaceful societies falsifies the claim that humans possess adaptations for fighting; (3) if psychological adaptations for warfare exist, then war is an inevitable and universal component of the human condition; (4) modern warfare and international politics is so qualitatively different from ancestral politics that any adaptations for the latter are inoperative or irrelevant today. By outlining an evolutionary theory of war and clarifying key misunderstandings regarding this approach, international relations scholars are better positioned to understand, engage, and contribute to emerging scholarship on human warfare across the social and evolutionary sciences.
International Theory (2016), 8:1, 97139
Cambridge University Press, 2015
doi:10.1017/S1752971915000184 First published online 29 October 2015
The evolution of war: theory and
School of Politics, Philosophy, and Public Affairs, Washington State University, Vancouver, Washington, USA
The use of evolutionary theory for explaining human warfare is an expanding area
of inquiry, but it remains obstructed by two important hurdles. One is that there is
ambiguity about how to build an evolutionary theory of human warfare. The second
is that there is ambiguity about how to interpret existing evidence relating to the
evolution of warfare. This paper addresses these problems, rst by outlining an
evolutionary theory of human warfare, and second by investigating the veracity of
four common claims made against the use of evolutionary theory for explaining
warfare. These claims are: (1) ancestral warfare was not frequent or intense enough to
have selected for psychological adaptations in humans for warfare; (2) the existence of
peaceful societies falsies the claim that humans possess adaptations for ghting; (3) if
psychological adaptations for warfare exist, then war is an inevitable and universal
component of the human condition; (4) modern warfare and international politics is so
qualitatively different from ancestral politics that any adaptations for the latter are
inoperative or irrelevant today. By outlining an evolutionary theory of war and
clarifying key misunderstandings regarding this approach, international relations
scholars are better positioned to understand, engage, and contribute to emerging
scholarship on human warfare across the social and evolutionary sciences.
Keywords: war; violence; evolution; psychology
The use of evolutionary theory for explaining warfare has received
increasing attention in many disciplines such as anthropology, social
psychology, and economics (Choi and Bowles 2007; Ginges and Atran
2011; Gneezy and Fessler 2011; McDonald, Navarrete and Van Vugt
2012). Scholars of international relations have also begun to apply an
evolutionary lens toward the study of specic aspects of warfare such as
territoriality (Johnson and Toft 2014) and aggression (McDermott et al.
2009; Thayer and Hudson 2010). However, few specically generate a
framework for an evolutionary theory of warfare per se, and those that
do tend to leave one component of a pivotal link unexamined: the
selection pressures that have shaped a human coalitional psychology for
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warfare, or the information-processing structure of that coalitional
psychology itself.
For example, Thayer (2004) examines the ancestral selection pressures
that would have favored xenophobia and egoism in humans, yet his
analysis stops short of examining the psychological mechanisms themselves
that render this behavior conditional in specic ways. Rosen (2005)
provides an excellent examination of the operation of the physiological
mechanisms that regulate certain forms of dominance and competitive
behavior, but his analysis tends to focus on individuals and is relatively
disembedded from a consideration of the coalitional dynamics within
which status competition occurs. These and other contributions have been
invaluable in their own right; however, if there is anything that this growing
research area has conrmed, it is that warfare is the complex output of a
human coalitional psychology that regulates behavior across many
domains, from xenophobia and status competition to cooperation and even
peacemaking (De Waal 2012). In short, it is now possible to sketch
an evolutionary theory of warfare that clearly articulates an evolved
coalitional psychology as the link between ancestral selection pressures
and modern political behavior.
As in any eld of inquiry, research on the evolution of warfare is
accompanied by recurrent disagreements relating to denitional or
interpretational issues, such whether certain patterns of behavior can be
generalized across space and time. Although there has been persistent
disagreement about the use of evolutionary theory in the social sciences
broadly and in international relations specically (e.g. see Bell 2006; Lopez,
McDermott and Petersen 2011; Lopez 2014; Johnson 2015), this paper
identies and engages four particularly prominent claims against the use
of evolutionary theory for understanding warfare. These claims are:
(1) ancestral warfare was not frequent or intense enough to have selected
for psychological adaptations in humans for warfare; (2) the existence of
peaceful societies falsies the claim that humans possess adaptations for
ghting; (3) if psychological adaptations for warfare exist, then war is an
inevitable feature of human societies; (4) modern warfare and international
politics are so qualitatively different from ancestral social life that any
adaptations for the latter are inoperative or irrelevant today.
Importantly, it can be unclear what an evolutionary theoryof warfare
is in the rst place. Clarication is required in two senses. First, an
evolutionary theory of warfare examines the set of adaptations in the
human brain designed by natural selection in response to a range of
ancestral selection pressures related to coalitional living and competition
(Gat 2006; McDonald, Navarrete and Van Vugt 2012; Lopez and
McDermott 2012). This approach not only examines the myriad ways in
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which species-typical adaptations regulate internal physiology and external
behavior, but also examines how unique environmental and genetic
variation helps to explain the inevitable plasticity of political behavior
(Sapolsky 2006).
Second, an evolutionary theory of warfare must lay out a
set of testable assumptions and predictions regarding a range of behavior
relating to warfare. However, in contrast to classic IR grand theory that
seeks to explain most if not all behavior under a single parsimonious
framework (e.g. neorealisms structural determinants of behavior) an
evolutionary theory of war explicitly abandons such parsimony. An
evolutionary approach shifts the focus from structure to context. Humans
are creatures of context, and as psychological adaptations were designed in
response to specic contexts, we are left with evolutionary theories of war,
rather than one general framework that explains all behavior, or even all
violent behavior. Therefore, a guiding theoretical challenge will be to
identify the contexts that activate specic psychological adaptations, and to
investigate how those adaptations regulate behavior in those contexts.
This article proceeds in four main parts. In part one, I briey outline the
adaptationist approach, which investigates (1) the selection pressures
that are likely to have prevailed throughout our evolutionary history;
(2) evidence for psychological mechanisms that are likely to have been
designed by natural selection in response to these pressures; and (3) how
these psychological mechanisms are designed to condition behavior upon
ancestrally recurrent and adaptively relevant cues in the contextual
environment. These mechanisms often operate facultatively, meaning that
behavior is irrevocably context dependent by design. In other words,
behavior is a conditional response to the environment, and evolutionary
theory is a tool for analyzing the innate structure of this conditionality.
Parts two and three use this adaptationist framework to help explain the
evolution warfare and explore the range of psychological adaptations
that likely exist for navigating situations of coalitional conict, respectively.
This exercise contributes to a fuller understanding of warfare in interna-
tional relations and reveals that our minds are quite literally designed to
interpret international politics and warfare through the lens of adaptations
designed for ancestral small-scale coalitional politics. In this regard,
A looser conception of evolutionarytheory in international politics exists. For example,
some scholars apply evolutionary dynamics more as a metaphorical instrument for modeling
historical and cultural change, sometimes referred to as cliodynamics(Thompson 2001; Turchin
2003). Although cliodynamic approaches are not mutually exclusive with the evolutionary
framework explored here, ambiguity regarding mechanism vs. metaphorin the application of
evolutionary theory can generate unnecessary confusion if not directly acknowledged. This paper
employs evolutionary theory only in the stricter sense.
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adaptationist research has revealed the psychological underpinnings of
issues of central concern to international relations scholars, such as public
preferences for leadership type in wartime vs. peacetime; the complex
connection between inter-group hostility and within-group cooperation
(e.g. loyalty and nationalism); and the plasticity of out-group coalitional
framing (ally or adversary?) and resultant motivational effects. The fourth
and nal section examines the veracity of four common claims made
against the application of evolutionary theory for understanding human
International relations has beneted greatly from the incorporation
of theory and method from other disciplines, especially economics, but
also sociology and anthropology (Snyder 2002). Given the pace of
developments in evolutionary science regarding the nature of human
competition, aggression, and warfare, greater interdisciplinary exchange at
this intersection can only deepen and improve our understanding of
Adaptationism is an approach for investigating the relationship between
ancestral selection pressures, the functional design of the brain, and
behavior (Williams 1966; Mayr 1983; Cosmides and Tooby 1987;
Godfrey-Smith and Wilkins 2008). Researchers outside of political science
are increasingly turning to evolutionary theory to illuminate the psycho-
logical mechanisms that underlie many areas of interest to international
relations and political science as a whole, such as the design of institutions
(Alexander and Christia 2011; Boyer and Petersen 2011); the nature of
religious belief and its relationship to political violence and extremism
(Atran and Ginges 2012); political ideology (Smith et al. 2012); and
adaptations for warfare (Johnson, Wrangham and Rosen 2002; Choi and
Bowles 2007; Wrangham and Glowacki 2012). Pessimism regarding the
relevance or utility of evolutionary theory for explaining political
phenomena no longer holds sway (Lopez, McDermott and Petersen 2011;
Johnson 2015); instead the question is not whether, but how evolutionary
theory help to explain political processes. Adaptationism is a framework
for addressing this question.
In the rst half of 2012, special issues on the evolution of human conict were featured by
three prominent scientic journals: Science;Philosophical Transactions of the Royal Society B;
and Human Nature. Despite the fact that the range of themes investigated in these special issues
included xenophobia, racism, and warfare, and political scientists were mostly absent among the
group of contributing authors in each instance.
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Species-typical adaptations and genetic differences
The human brain is a complex structure consisting of both universal and
individually variable features. It is beyond dispute that basic substructures
of the brain exist that are species typical. For example, neurological
structures such as the amygdala and hippocampus can be found
cross-culturally, and the dedicated function of these structures, as well as
the operation of behavior-regulatory neuroendocrine pathways such as
testosterone and cortisol, are species typical (Brown 1991; Pinker 1997;
Gazzaniga 2009; Zak and Kugler 2011). Indeed, these structures are
adaptations, built by natural selection, and they serve a specic set of
functions. Although adaptations such as these generate output (increased
testosterone, dominance behavior, etc.) that predictably varies by context,
the adaptations themselves are universal. All complex biological
adaptations have certain hallmark qualities; for example, they are reliably
developing and operate in a specialized manner to solve reproductive
challenges, or adaptive problems, that prevailed over evolutionary time
(Williams 1966).
For example, evolutionary theorists have argued that in species in which
conict between individuals is commonplace, natural selection should
favor adaptations for the assessment of strength in individuals, as well as
adaptations that regulate the adaptive use of aggression and violence in
such conicts (Parker 1974; Archer 1988). Subsequently, evolutionary
psychologists have tested hypotheses relating to the existence of psycho-
logical adaptations in humans and found evidence that humans auto-
matically and cross-culturally detect and attend to ancestrally reliable cues
of formidability and strength in others, and that these cues predict
propensity toward aggression at both an inter-personal and coalitional
level (Sell, Tooby and Cosmides 2009; Sell et al. 2009). These results are
also consistent with ndings that suggest that humans unconsciously track
cues in the face and voice that would have ancestrally correlated with threat
potential and formidability, and furthermore, that such cues seem to affect
leadership preference in wartime and in peace (Puts, Apicella and Cárdenas
2012; Spisak et al. 2012). This is an example of adaptationist analysis that
examines the existence of psychological adaptations that were favored by
natural selection and became species typical in the population because they
embodied strategies that produced tness benets, on average, in ancestral
However, in addition to complex adaptations in the brain that are species
typical in humans, there also exists a tremendous amount of genetic varia-
tion among individuals that affects the way these adaptations process
information and regulate behavior (Tooby and Cosmides 1990; Buss and
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Greiling 1999). For example, although the operation of testosterone in
regulating aggression- and dominance-related behavior is a human
universal (Archer 2006), genetic differences between individuals may have
the effect of setting basal testosterone levels at person-specic levels,
making some individuals more or less sensitive to status challenges, for
Overall, therefore, we have two important biological features to consider
when examining the function of the brain in regulating behavior. First,
there are species-typical psychological adaptations designed by natural
selection that regulate behavior in response to specic environmental cues.
Second, genetic variation among individuals helps to explain some of
the individual differences in the operation of species-typical adaptations
(Verweij et al. 2012). These points help to reveal that an adaptationist
approach does not seek to explain all behavior as necessarily adaptive
(some behavior is clearly maladaptive in modern contexts as will become
clearer below), or even as the product of universal adaptations (some
behavior may be better explained by, inter alia, genetic variation between
individuals). Thus, this approach is distinct from pan-adaptationism,
which falsely argues that all biological and behavioral traits are adaptive.
Pan-adaptationism has been clearly rejected and research has progressed
beyond these challenges (Gould and Lewontin 1979; Godfrey-Smith and
Wilkins 2008).
Explaining individual differences
The above discussion emphasizes two major biological sources of behavior:
species-typical adaptations and individual genetic uniqueness. We can
extend this discussion by examining how both sources can explain
individual differences in behavior.
First, individual differences in behavior can be the result of species-typical
psychological adaptations that interact with unique environments (Orbell
et al. 2004). To take a simple example from the aggression system described
above, an evolved decision-making algorithm such as if bigger than your
opponent, advance; if smaller retreat, may generate aggression in certain
circumstances by certain individuals, but submission in others (Dawkins
1980). The result is that a species-typical adaptation generates behavioral
variation upon interaction with variable environments.
Second, individual differences can be the result of genetic variation that
affects the operation of species-typical adaptations across individuals. For
example, the presence in individuals of a genetic allele dubbed the Warrior
Gene, in combination with traumatic early life events, enhances aggressive
responses to provocation in those individuals (McDermott et al. 2009;
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Tiihonen et al. 2014). This implies that certain thresholds of the species-
typical aggression system can vary across individuals as a direct result of
genetically heritable variation (i.e. genes or allelesthat some individuals
possess but others do not, which may make them more or less likely to
respond aggressively to threat).
Understanding the interplay of species-typical adaptations and heritable
variation is an important theoretical starting point for examining human
behavior in the context of evolutionary theory, although typically these two
perspectives are examined independently of each other (Lopez and
McDermott 2012). There has been growing attention to genetic explana-
tions for political behavior, and these models often utilize theoretical
frameworks that focus on heritable variation among individuals (Alford and
Hibbing 2004; Alford, Funk and Hibbing 2005; Hatemi et al. 2009a, b).
There has been relatively less emphasis on adaptationist explanations of
political behavior, although this approach is also steadily growing in
political science (Sidanius and Kurzban 2003; Thayer 2004; Gat 2006;
McDermott, Fowler and Smirnov 2008; Petersen 2009; Thayer and
Hudson 2010; Johnson and Fowler 2011; Lopez, McDermott and Petersen
2011). Importantly, both of these approaches reveal that human behavior is
at once both deeply plastic and highly malleable not despite, but rather
because of this biological inheritance.
Humans as conditional strategists
Although adaptationism as an approach to explaining political behavior is
increasingly prominent, many do not directly examine the operation of
psychological adaptations as the necessary intermediary between ancestral
selection pressures and modern behavior. Instead, the trend has been to
infer explanations of modern behavior based on examination of broad
evolutionary trends (e.g. Somit and Peterson 1997). Unfortunately, this has
left evolutionary-minded social scientists vulnerable to the critique that
evolutionary theory is not a useful tool for analyzing social behavior, since
applications of evolutionary theory seem to explain a variable (selshness
and altruism) with a constant (evolution). Specically, for example, some
research seems to suggest that our evolutionary history has made us
egoistic and selsh (Thayer 2004), while other research suggests that our
evolutionary history has made us cooperative and even altruistic (Goodwin
2010). Bell (2006) sees this as evidence of the political indeterminacyof
evolutionary theory as a framework for explaining political behavior.
This theoretical challenge is resolved by the recognition that neither
evolution nor natural selection affects behavior directly; instead, natural
selection builds adaptations, which themselves are the proximate cause of
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behavior at the individual level (Symons 1992). Furthermore, an organism
does not confront a single reproductive challenge in its environment; rather,
the environment is composed of myriad reproductive challenges, each
with a potentially unique adaptive solution (Cosmides and Tooby 1994).
Given the diversity of adaptive problems in an organisms environment,
and given that natural selection designs adaptations that are specialized
and often conditional solutions to such adaptive problems, adaptationists
expect that the human brain contains a diversity of psychological adapta-
tions designed to generate adaptively useful and behaviorally conditional
outcomes in specic contexts (Daly and Wilson 1983; Mayr 1983;
Barkow, Cosmides and Tooby 1992; Pinker 1997; Buss 2005; Chiappe and
MacDonald 2005; Forgas, Haselton and Hippel 2007; Gazzaniga 2009;
Morgan et al. 2011).
The aggression system described previously is again a useful example: the
regulation of conicts of interest among animals is often resolved through a
series of provocative displays in which individuals assess each other and
effectively decidewhether to challenge or submit (Eibl-Eibesfeldt 1979;
Archer 1988; Silverberg and Gray 1992). In the context of agonistic
contests, an organism that possessed adaptations instantiated with the
strategy always challengewould have been quickly replaced by those that
possessed adaptations instantiated with conditional strategies such as if
bigger, challenge; if smaller, submit(Dawkins 1980). An analogous
situation holds in the domain of exchange and cooperation. For example,
one signicant adaptive problem that would have faced a social species such
as ours would have been choosing whether, and with whom, to cooperate.
Evolutionary theorists have argued that if reciprocity and exchange are
the products of adaptations designed to regulate behavior in these contexts,
these adaptations ought to attend to cues that ancestrally were reliably
associated with deception in others and defection-related behavior, as
well as the likelihood that the interaction will be repeated (Trivers
1971; Axelrod and Hamilton 1981). Subsequent research by evolutionary
psychologists conrmed that humans are surprisingly good at
detecting and discriminating against actual and potential defectors, often
unconsciously relying on physical cues such as those embedded in facial
features to make such determinations (Yamagishi et al. 2003; Cosmides and
Tooby 2005).
Despite the general statement that adaptations likely exist that equip humans with the
ability to detect cooperation and deceit in others, it is manifestly true that humans often make
assessment mistakes. Such errors can have many origins. It may be the case that person-specic
events in ones life history may have calibrated relevant assessment mechanisms in such a way as
to raise or lower ones baseline level of social trust. Or it may also be the case that such individuals
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There are several lessons that emerge from careful consideration of
adaptationism for the study of political behavior. First, evolution does not
cause behavior; brains do. That is, adaptations emerged as solutions to
specic adaptive problems in the environment, and these psychological
adaptations are the proximate cause of behavior. Thus, the functional t
between adaptations and the selection pressures that explain their design
should be the starting point of analysis. Second, psychological adaptations
are myriad and likely exist for many domains, ranging from mating
behavior and foraging, to social exchange and warfare. Third, psycho-
logical adaptations often operate according to a specialized conditional
logic because the operational structure of this exibility correlated with
reproductive tness in ancestral environments. Fourth, individual
differences can be explained as the product of species-typical adaptations
interacting with variable environments, and can also be explained as the
product of genetically heritable phenotypic variation between individuals.
The evolution of war
Adaptationist analyses of behavior typically consist of at least two major
considerations. The rst is to examine, to the extent possible, the character
of ancestral selection pressures that may have favored the emergence of the
adaptations in question. The second is to apply a task analysis (e.g. with
evolutionary game theory) that examines the logical features that a putative
adaptation should possess if it is indeed designed for solving a specic
adaptive problem. This section (The evolution of war) will engage the rst
consideration, while the following section (Adaptations for warfare) will
address the second.
To begin, an adaptationist explanation for warfare, as a type of
coalitional behavior, must begin with the recognition that individual-level
aggression is fundamentally distinct from (although related to) coalitional-
level aggression. Nevertheless, theory that helps explain individual-level
aggression can be a useful starting point for explaining aggression in
groups. More precisely, the adaptive problems represented by individual
and coalitional aggression are distinct and likely have resulted in distinct
adaptations for regulating behavior in these domains, even if the operation
of these adaptations functionally overlaps. Indeed, although competition
and aggression is not zoologically uncommon between individuals of the
same species, it is much less common to observe coordinated aggression
possess a genetic allele that causes them to be overly trusting. The neuropsychophysiology of
cooperation and trust is in fact a ripe example of the depth of evolutionary explanations of social
behavior. For example, see Zak (2006, 2011).
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among groups (Archer 1988; Tooby and Cosmides 1988). Very few species
demonstrate this behavior, which in part suggests that the necessary
adaptations that enable such behavior are absent. Warfare, as the output of
facultative adaptations for coalitional competition, is necessarily a strategic
response to the environment. However, this should not be confused with
the claim that, given the right environmental conditions, any species can be
made to exhibit coalitional aggression. There is no evidence to support this
latter claim. The environment cannot causebehavior on its own; rather,
behavior is a conditional response to the environment, and adaptations
necessarily structure the conditionality of this response. Thus, we must
interrogate the information-processing structure of those adaptations to
gain a deeper understanding of both the prevalence and variability of
warfare. I begin with a brief look at adaptations for individual-level
aggression and then proceed to explore coalitional aggression and its
implications for warfare.
Individual-level aggression
There has been a tremendous amount of research on the evolutionary ori-
gins of anger and aggression between individuals, especially in humans and
primates. This research has shown that aggression represents the output of
adaptations designed to negotiate conicts of interest (Daly and Wilson
1988; Buss and Shackelford 1997; Wrangham 1999a; Sell, Tooby and
Cosmides 2009), and that there are substantial sex differences in the
incidence of aggression (Daly and Wilson 1983; Van Vugt 2009). These
ndings are largely explained using sexual selection theory and parental
investment theory, which have proven remarkably successful for explaining
the distribution of aggression across a number of species, including
our own.
Sexual selection occurs when adaptations are favored by natural
selection that aid directly in the competition for mates, and patterns of
parental investment in offspring determine the nature and intensity of that
competition. In humans, as with most primates and mammals in general,
female parental investment in offspring has on average been much greater
than male parental investment. The greater the parental investment by one
sex, the more the opposite sex competes over access to the sex that invests
more (Trivers 1972; Symons 1979). This asymmetry means that each sex
faces a unique adaptive problem. From the perspective of the high investors,
one must be choosyabout the right investment because once the
investment is made, the cost of being wrong (i.e. having chosen a poor
investment) is great. The adaptive problem from the perspective of the
low investors is to access as many investment opportunities as
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possible because if investing is cheap, you should invest as often as
possible. Low investors compete over access to high investors, and the
greater the investment asymmetry between the high and low investors,
the more intense the competition, and the more selection tends to favor
violence and aggression for use in the competition between low investors
for access to high investors.
An early obligate investment asymmetry between males and females has
been evolutionarily recurrent (e.g. sperm production vs. egg production and
9-month gestation period), and cross-cultural evidence consistently reveals
a greater relative incidence of aggression among men than women. As
just a few examples, men are more likely than women to be the targets and
perpetrators of violence (Daly and Wilson 1988; Van Vugt 2009); to
respond to threat with xenophobic reactions to out-groups (Schaller, Park
and Mueller 2003); to endorse warfare after being primed with pictures of
attractive members of the opposite sex (Chang et al. 2011); to show-off
via conspicuous self-sacrice (McAndrew and Perilloux 2012); and to
overestimate their chances of success in violent encounters (Johnson,
Wrangham and Rosen 2002; Johnson et al. 2006). The interaction of sexual
selection and parental investment has tended to render aggression relatively
more reproductively worthwhile for males than females. Theory also
suggests that where aggressive encounters between individuals is a
signicant adaptive problem, selection should favor adaptations for
assessment that adaptively regulate aggression in these contests (Parker
1974; Archer 1988). Subsequently, it has been found that cues in the
body, face, and voice of males, but not females, reliably correlates with
aggressiveness, and that people are surprisingly good at inferring the
formidability of males based on these cues (Sell et al. 2009; Haselhuhn and
Wong 2011; Puts, Apicella and Cárdenas 2012; Short et al. 2012).
Coalitional aggression
Theory and evidence presented above help us to understand the selection
pressures that have favored aggression as a reproductive strategy at the
individual level that is relatively more cost-effectivefor males than
females. Although the issue of individual-level aggression is certainly not
irrelevant for international relations (particularly as it relates to issues of
terrorism, for example), this alone does not yet help us understand
the nature of coalitional aggression. We must begin, however, with the
recognition that war, or coalitional aggression, can take a variety of forms.
There are two particularly common forms of coalitional aggression that
have been well studied: the raid and the pitched battle. The former tends to
be characterized by nighttime attacks on rival groups that exploit stealth,
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the element of surprise, and quick retreat. Raids can exist in the context of
feud spirals or as isolated events. Battles, however, occur when relatively
larger groups of combatantsfrom rival groups, that are relatively equal in
size, engage each other in attrition-style ghting. Battles are deadlier and
less common than raids, and therefore an evolutionary theory of coalitional
aggression must take account of this distinction (Johnson, Wrangham and
Rosen 2002; Wrangham and Glowacki 2012). Specically, we must
explain how adaptations could evolve that motivate the sort of risk taking
necessary to sustain deadly pitched battles.
There are two broad conditions that must be satised in order for natural
selection to favor adaptations that make coalitional aggression possible
whether raiding or battles. Furthermore, these two conditions also
simultaneously help to explain why humans are willing to engage in
risky symmetric battles, as well as why such encounters are relatively
uncommon among our chimpanzee cousins.
Condition 1: reproductive benets: First, as a reproductive strategy, the
tness benets of coalitional aggression must outweigh its signicant costs
(Wrangham 1999a; Liddle, Shackelford and WeekesShackelford 2012;
McDonald, Navarrete and Van Vugt 2012). The tness benets of raiding
have been well studied. Primatologists have shown that the outbreak of
chimpanzee coalitional violence reliably occurs in the context of
inter-group hostilities where one coalition outnumbers an opposing
coalition by an approximate ratio of three-to-one (Manson and Wrangham
1991; Wrangham and Peterson 1996). Coalitional violence in these
instances has been reproductively successful because it has correlated with
expanded territory, greater probability of success in future contests, and
facilitates access to both material and reproductive resources (Mitani,
Watts and Sylvia 2010). Chimpanzee coalitional violence occurs almost
exclusively in the context of power asymmetries, and in these contexts it is
relatively easy to see how an overwhelming victory over a weaker group
could be reproductively advantageous. In humans as well, coalitional
raiding remains the most common form of violence among groups, and
appears to mirror the chimpanzee model surprisingly well (Wrangham
and Glowacki 2012). This is further supported by evidence that ancestral
coalitional environments may have approximated Lanchesters square law
of combat, in which the material and reproductive benets of successful
asymmetric aggression were substantial (Johnson and MacKay 2015).
Although the reproductive benets of coalitional raiding are clear,
human coalitional violence also takes the form of pitched battles of attrition
between coalitions of relatively equal size in which victory appears to be
more a function of ghters left standingthan resources gained. This kind
of high-risk/high-loss activity is harder to explain, at rst blush, with an
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evolutionary lens, and it is certainly inconsistent with the chimpanzee
model of coalitional violence. However, a closer look at evolutionary
theory reveals that it is indeed possible for natural selection to favor
coalitional violence even if victoryis purchased at the cost of the death of
all coalition members minus one (Tooby and Cosmides 1988; McDonald,
Navarrete and Van Vugt 2012). This is because, as mentioned above,
male reproductive success is limited only by access to mates; therefore, if
reproductive resources are zero-sum, the missed reproductive opportunities
of the fallen necessarily equal the gained reproductive opportunities of the
surviving victors. In other words, a genetically heritable decision rule
that motivated participation in these types of coalitional violence
would successfully be passed to future generations and spread so long as
(1) the coalition is victoriouson average; (2) reproductive opportunities
are reallocated among the survivors; and (3) the perceived pre-battle
distribution of individual risk is effectively random.
Having identied the conditions under which both coalitional raiding
and relatively symmetric conict can evolve, the next logical question is:
have these conditions in fact prevailed ancestrally? Although I will return to
this question in the nal section below, archeological, anthropological, and
game theoretic evidence suggests that these selection pressures have indeed
endured over human evolution (Manson and Wrangham 1991; Gat 2000;
LeBlanc and Register 2003; Bowles 2009; Wrangham and Glowacki 2012).
The result is that ancestral coalitional aggression has represented a
reproductively worthwhile endeavor in which the average tness benets
(measured over evolutionary time) have tended to outweigh the somatic
(or immediate, physical) risks. Combined with male overcondence in
warfare, which has been hypothesized to serve the function of bluff delivery
in the context of pitched battles (Wrangham 1999b; Johnson, Wrangham
and Rosen 2002), it is in fact unsurprising, from an evolutionary
perspective, that risk taking, brinkmanship, overcondence, and symmetric
aggression between coalitions seem to be staples of human inter-group
conict. The result of this analysis is to turn the original puzzle on its
head: the puzzle is not why humans display patterns of pitched battle, but
why chimpanzees do not. Indeed, part of the answer may be suggested
by the second condition necessary for the evolution of coalitional violence.
Condition 2: cognitive adaptations for n-person cooperation: If the rst
condition that must be satised for the evolution of war in humans is that
it is reproductively benecial, the second is that, as a cognitive task,
coalitional aggression as a collective action requires sophisticated
information-processing systems in the brain that allow the simultaneous
tracking of n-person variables, such as the level of othersparticipation,
distribution of risk, formidability of out-groups, and the probability of
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success (Tooby and Cosmides 1988; Tooby, Cosmides and Price 2006). In
other words, given that coalitional aggression is reproductively worthwhile,
its execution assumes and requires the presence of a host of adaptations
necessary to perform the associated cognitive tasks of cooperation
and coordination. Do such adaptations exist? Indeed, research in social
psychology, biosocial anthropology, and cognitive neuroscience increas-
ingly demonstrates that humans have been equipped by natural selection
with a reliably developing evolved coalitional psychology(Kurzban,
Tooby and Cosmides 2001; Wagner, Flinn and England 2002; Navarrete
et al. 2004; Cosmides and Tooby 2005; Kurzban and Neuberg 2005;
Schaller, Simpson and Kenrick 2006; Forgas, Haselton and Hippel 2007).
I have elsewhere explored many of the components of human coalitional
psychology relevant for international relations (Lopez, McDermott and
Petersen 2011), and the rest of this discussion below will illuminate to those
aspects of coalitional psychology that help explain human behavior in war.
The preceding discussion establishes two important evolutionary condi-
tions necessary to explain the unique pattern of coalitional aggression
demonstrated by humans. However, some evolutionary theorists argue that
warfare, particularly human warfare, poses a very special kind of selection
pressure that is distinct from orthodox natural selection that shapes
adaptations for the benet of individual organisms. This alternative is
known as group selection. Once this approach is discussed, we can then
turn to analysis of the products of these evolutionary processes
adaptations for warfare themselves.
The role of group selection: Adaptationist analyses of warfare examine
the evolved mechanisms in the human brain that were designed by natural
selection in response to the ancestral selection pressures relating to
inter-group conict. However, the above discussion has mostly taken for
granted that these adaptations were shaped by natural selection occurring
at the level of the individual. This means that heritable traits are passed to
future generations and spread to the extent that they generate relative
tness benets for the individuals that possess them.
In this case, we would
say that adaptations are designed to operate for the benet of the indivi-
dual. However, it is theoretically possible that psychological adaptations
exist that operate for the benet of the group. These adaptations would be
built by natural selection operating at the level of the group, or simply, by
Strictly speaking, all adaptations benet the genes that code for them; however, it is con-
vention to operate under the assumption that adaptations exist for the benet of the individual
as it is the differential survival and reproduction of individuals that affects gene frequencies in
future generations. Thus, the unit of selection is always and inevitably the gene, however, the level
at which selection operates tends to be the level of individual organisms.
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group selection. Although these processes are clearly not mutually
exclusive, they are analytically distinguished by the following question:
are adaptations built by natural selection because of the benets they confer
to the individuals that posses them, or because of the benets they confer
to the groups within which individuals live? Or, alternatively, what is
determining directional change in gene frequencies across generations the
success of individuals (individual selection), or the success of groups (group
Group selection was originally offered by Darwin and Wynne-Edwards
to help explain certain forms of altruism that seemed puzzling or difcult
to explain as a function of genetic self-interest. These are cases in which
individuals accept a net cost to themselves in order to confer a benet
on another individual within ones group (Wynne-Edwards 1962). By
the middle of the 20th century, however, game theoretic models had
demonstrated that the conditions that must hold in order for group
selection to occur are uncommon (Hamilton 1964b; Williams 1966;
Maynard Smith 1976). Simultaneously, research on inclusive tness and
reciprocal altruism helped explain how certain forms of altruism could
indeed be explained by individual-level selection (Hamilton 1964b;
Williams 1966; Trivers 1971; Krasnow et al. 2013). Nevertheless, by the
late 20th century, group selectionists argued that these restrictions had
been overstated (Wilson 1997; Sober and Wilson 1999). Apart from the
often-intense scientic turf battles this debate has generated, this resurgence
has in fact led to a greater degree of useful nuance among evolutionary
approaches for explaining aspects of warfare.
One example of this productive nuance is the emerging distinction
between geneticgroup selectionists and culturalgroup selectionists.
Genetic group selectionists continue to argue that ancestral warfare was
frequent and intense enough to have selected for biological adaptations in
individuals that regulate behaviors for the benet of the group even though
they may be individually disadvantageous, such as forms of heroism,
extreme risk taking in battle, or other forms of within-group altruism
(Choi and Bowles 2007; Lehmann and Feldman 2008; Bowles 2009).
Culturalgroup selectionists focus on explaining the content and
distribution of social norms as a cultural by-product of inter-group warfare
(Ginges and Atran 2011; Mathew and Boyd 2011). These scholars argue
that in the context of intense and repeated coalitional conict, groups that
cultivated social norms of sacrice, sharing, and pro-sociality would have
prevailed in conict over other groups whose ability to cooperate
was hindered by the absence of such norms. Thus, for geneticgroup
selectionists, warfare exerts a biological selection pressure for pro-social
psychological adaptations in the brain, while for culturalgroup
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selectionists, warfare operates through cultural selectionto select for
pro-social cultural norms. Importantly, these approaches are not mutually
exclusive, and warfare is often the independent variable, not the dependent
A second example of this nuance among evolutionary approaches is the
greater attention given to the plausibility of multi-level selection, or, natural
selection operating at more than one level simultaneously. Although space
limits a full consideration of this paradigm here, it is sufcient to note that
in effect it represents a tactical retreat from classical arguments that insist
on the necessary prominence of selection pressures operating at only one
level. From this perspective, the task for understanding the design of a given
adaptation, is to identify which levels operate and what their relative impact
is on changing gene frequencies (Lopez 2014).
Having now explored the ancestral selection pressures that help to
explain the emergence of warfare in humans, we can turn now to an
investigation of the products of those selection pressures; namely, the
design and operation of psychological adaptations for warfare.
Adaptations for warfare
Warfare, or coalitional aggression generally, is rst and foremost a
collective action problem. Even simpler examples of stealth raiding entail
collective action problems such as labor recruitment and enemy targeting
that are often resolved and directed by an individual of disproportionate
inuence in the group (Glowacki and Wrangham 2013). Thus, to the extent
that ancestral inter-group violence posed a set of selection pressures that
have favored adaptations for warfare in humans, the general prediction is
that modern contexts of inter-group violence continue to activate a host
of evolved psychological heuristics regarding the propersolution to
collective action problems in those contexts.
In other words, our modern-
day social, moral, and political intuitions about how people ought to
behave when our group or country is threatened are likely the product of
adaptations that were designed to facilitate adaptively successful collective
action in response to out-group threat in ancestral environments.
From this general proposition, we must proceed to identify the specic
ways in which collective action problems must have been solved in ancestral
contexts in order to successfully prevail in, or prevent, inter-group violence.
The result of this analysis is to outline the architecture of coalitional
One critique of this theoretical claim might be to accept, prima facie, the proposition that
such adaptations exist, but then to refute that they continue to operate in a relevant way in
modern contexts. I will return to this below.
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motivation in warfare, which represents the set of psychological adapta-
tions that exist to solve evolutionary challenges related to the outbreak of
war (ante bellum), the conduct of ghting (in bello), and post-conict
relations (post bellum). For the sake of scope, I will focus on the rst two
sets of challenges, while also noting that research on the psychological
challenges and strategies of post-conict forgiveness and reconciliation is
well underway (McCullough 2008; Burnette et al. 2012; McCullough,
Kurzban and Tabak 2013).
Psychologie, ante bellum
Although there is extensive debate on the causes of war, a useful point of
departure is the bargaining theory of war in international relations (Wagner
2000; Reiter 2003). In addition to offering the most prominent set of
explanations for the outbreak of war, it is also particularly useful because it
offers a taxonomy within which to incorporate psychological motivations for
warfare. Lake, for example, argues that psychological and rational approaches
to warfare are indeed compliments, not substitutes(2010, 10). Thus, I will
briey explore the rationalist explanation for war, and provide examples of how
an adaptationist approach to psychology and behavior can expand upon it.
According to Fearon (1995), given that wars are costly, if bargains short
of war exist that will allow states to arrive at their goals without paying the
cost of ghting, states should be able to identify and agree to these terms
and avoid a costly war. If war erupts, therefore, it is necessarily a bargaining
failure, which can occur for at least three reasons. First, anarchy may
undermine the ability of self-interested actors to commit to war-avoiding
bargains in the long run, which by extension undermines the success of any
agreement in the present (Powell 2006). Second, even though war is costly,
rational actors also wish to do well in the pre-war bargaining phase; thus,
they face strong incentives to misrepresent their capabilities and interests to
their adversaries in order to prevail in negotiations, which can inadvertently
eliminate the range of war-avoiding options available to states. Third, when
rational actors deliberate over issues that they perceive to be indivisible,
the bargaining spaceover which they might compromise is effectively
eliminated, and warfare is the result of this bargaining failure.
Fearon admits that issue indivisibilities should not exist in a rationalist
world, since any material object is, in principle, subject to division. Thus,
upon closer inspection, this third category of rationalist explanations for
war in fact opens the door to several non-rational motivations for war.
This is not to say that there is no room for evolutionary insight in the other two rationalist
explanations. For example, anarchy has been an enduring force not just in international relations,
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The result is an expanded taxonomy that incorporates both rationalist
and non-rationalist explanations for war. Adaptationist approaches to the
outbreak of war suggest at least two ways in which issue indivisibilities
spark the ames of war: sacred values and territory. Furthermore, an
adaptationist lens reveals that another assumption of the rationalist
framework for war may be violated more often than acknowledged: the
assumption that war is costly. I discuss each in turn.
Sacred value and territory: According to Tetlock, sacred values
are treated as possessing transcendental signicance that preclude
comparisons, trade-offs, or indeed any mingling with secular values
(2003, 320). Although the issues, objects, and symbols that groups hold as
sacred vary tremendously across space and time, the nature of sacred values
and peoples reactions to the violation of these values are cross-culturally
universal. In other words, the adaptations that shape these intuitions are
universal, even while their output across space and time is predictably
variable. According to the sacred value protection model, individuals react
to violations of sacred values with a combination of moral outrage and
moral cleansing the former meant to target violators with a range of
punitive sentiment and behavior, and the latter meant to reafrm sacred
values and strengthen the bonds of within-group solidarity (Tetlock et al.
2000). Sacred values need not be religious, but when they are, the predicted
consequences are all the more dangerous, as is witnessed by the Danish
cartoon depictions of Mohammad in 2005 and the proposed burning of the
Quran by Terry Jones in 2010.
Sacred values are particularly signicant when they are imbued in
territorial claims. Territory, by its very nature, should easily lend itself to
rationalist division. Indeed, great powers have historically found the
problem of territorial division as easy as drawing lines on a map, such as
during the Berlin Conference of 1885 or the division of the Middle East
under the 1916 SykesPicot agreement. Yet, these foreign territorial
divisions often remain a source of incredible tension, particularly under
two conditions: when rival inhabitants each view themselves as long-time
residents (Johnson and Toft 2014), and when they view the land as sacred.
In these instances, the perceived indivisibility of territory thwarts movement
toward war-avoiding bargains, by at best foreclosing their existence, and
at worst sparking outrage and aggression at even the prospect of such a
division. Such outrage could not be predicted if the issue were merely that
the land is perceived as indivisible for some reason having to do with its
but also especially throughout the vast majority of human evolution on the lawless Pleistocene. Its
impact on our evolved psychology cannot be underemphasized (Gat 2009).
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material properties alone (and not because it is sacred). In other words, if
the problem were merely that an object by its nature precluded division,
there would be no reason that the consideration or proposal of its division
would ignite hateful outrage. Instead, the prospect of dividing the indivi-
sible would simply be received as puzzling and illogical. Thus, knowing
simply that rational actors may somehow stumble upon issue indivisibilities
offers a necessarily incomplete explanation of both the causes and contours
of the problem.
War is costly but revenge is sweet: The human drive for revenge
represents an undoing of a central assumption of the rationalist framework
for war; namely, that war is costly. This undoing is partially demonstrated
by the fact that the experienced motivation for revenge seeking is not
the future benet of deterrence, but merely the imposition of harm and
suffering for its own sake in the present (Schelling 1966; Löwenheim and
Heimann 2008). Revenge seeking against an aggressor can be favored by
natural selection because its effect, when successful, is to reverse the tness
benets gained by the aggressor from exploitation or theft, for example
(Barash and Lipton 2011; Boehm 2011). Successful revenge can also
indirectly solve the adaptive challenge of deterrence by inspiring caution in
would be aggressors through the establishment of a reputation for engaging
in costly retaliatory violence (McCullough, Kurzban and Tabak 2013). In
this sense, a motivation to seek revenge operates as a commitment device
(Frank 1988), designed specically to overwhelm considerations of cost
and future deterrence for the sake of returning suffering upon an enemy.
Thus, although revenge can evolve because it in part solves adaptive
problems of deterrence, its operation generates a desire to cause harm for its
own sake, independent of any expected deterrence value.
Apropos, neurological evidence has revealed that the prospect of
inicting retaliatory punishment triggers pleasure centers in the brain,
suggesting that, at least when revenge is the motivation, violence may be
perceived as a benet rather than as a cost (de Quervain et al. 2004). At the
very least, it suggests that the war-is-costly assumption merits revision by
examination of the contexts in which adaptive heuristics operate to
endogenously incentivize behavior that may otherwise appear irrational to
homo economicus. Importantly, revenge is not merely in the error term;
rather, it remains among the most common motivations for violence
cross-culturally and continues to be particularly salient among states and
non-state actors (Barash and Lipton 2011).
Hitlers rise to power is a well-known example of the ability of revenge to
compel great power behavior, but it would be false to imagine that revenge
might only be relevant for non-democratic states captured by despotic
elites. Churchill, for example, was persuaded by the opposite opinion, and
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proposed that democracy is more vindictive than Cabinets [sic]. The wars
of peoples will be more terrible than those of kings(Levy 1988, 659).
Indeed, Gaddis argues that in fact the very foundations of American identity
have been shaped by its reaction to surprise attacks on its homeland, such as
the attack on Pearl Harbor as well as the terrorist attacks of 11 September
2001. The latter is particularly emblematic of the central characteristics
of revenge, such as enhanced xenophobia and indiscriminate targeting of
individuals that t out-group proles (Michener 2012), as well positive
jubilation at the discovery that the suspected mastermind of the terrorist
attacks had been killed (Adams 2015).
The desire for revenge is often at the heart of intractable conicts that
remain hot, such as the ArabIsraeli conict, and it can also remain at the
heart of conicts that remain frozen, where it festers in cultural narratives of
defeat and in the minds of resentful elites (Holbrooke 1999). Therefore,
although the consequences of revenge have been well studied, its
antecedents are equally important to recognize, particularly the role of
national humiliation. For example, in the context of the 1973 Yom Kippur
War, Kissinger recognized the need to allow the Arabs a limited victory, even
if largely mythical. Such action was meant to relieve them of enough shame
and humiliation to allow for making peace(Harkavy 2000, 348). Savvy
elites may occasionally recognize the potential for conict-escalating events
and act to forestall or mitigate them, such as in this episode. However, a
deeper understanding of the role of humiliation and revenge in international
relations can only broaden this understanding and even provide
policymakers with clearer guidance during times at which our psychology is
specically designed to cloud rather than clarify cost-benet analysis.
Psychologie, in bello
The bargaining theory of war has provided scholars with a useful set
of propositions about the conditions under which bargaining failures
may lead to war. Adaptationist perspectives on psychology and behavior
broaden our explanations of war in novel ways by, for example, explaining
the origin and nature of issue indivisibilities as well as explaining why and
when the war-is-costly assumption of rationalist models may not be
ecologically valid. Another feature of rationalist models of war has been
their tendency to treat actors as unitary (Lake 2010). Although a unitary
actor assumption for international relations is not always invalid
(Lopez, McDermott and Petersen 2011), we certainly deepen our under-
standing of war by interrogating the within-group dynamics that may
sustain or mitigate inter-group violence. It is to these dynamics that we
now turn.
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Rally round the group: All politics, from the local to the international, is
a story of group identity. There is signicant evidence that humans
instinctively categorize themselves and others according to socially relevant
groups with associated identities, and these identities are active in shaping
behavior across levels of analysis, and perhaps most dramatically in the
case of inter-group violence (Lopez, McDermott and Petersen 2011;
Pietraszewski et al. 2015). Some of these dynamics have been explored
through the application of social identity theory to international relations,
which has made important contributions regarding warfare and co-
operation (Mercer 1995; Curley 2009), and scholars have applied an
evolutionary perspective on social categorization to explain the intensity
and recurrence of inter-ethnic conict (Alexander and Christia 2011;
Crisp and Meleady 2012).
Although these within-group biases are powerful determinants of moti-
vation and behavior in the context of warfare, the nature and scope of
group boundaries are exceedingly plastic. Yet, despite the plasticity of
social and political group borders, the presence of inter-group conict itself
is reliably sufcient to trigger a host of motivational and behavioral
effects, such as increased levels of cooperation within groups (Puurtinen
and Mappes 2009; Burton-Chellew, Ross-Gillespie and West 2010), a
particular eagerness to punish those who do not cooperate, as well as
a heightened sense of shame and guilt among non-participants (Gneezy and
Fessler 2011). In other words, these instinctual categorizations adaptively
direct the magnication of in-group loyalty and cooperation in the context
of out-group conict, in which expediency of action and solidarity of
purpose were pivotal determinants of success (Cikara, Botvinick and Fiske
2011; Mahajan et al. 2011).
The link between within-group cooperation and inter-group conict is a
useful illustration of the way in which natural selection has built
adaptations that are designed to activate a set of cognitive and motivational
systems upon detecting one powerful cue in the environment: inter-group
conict. Although the motivational output of these systems is an important
determinant of behavior, humans are also particularly good at manipulat-
ing the operation of these emotions in others for personal or political gain.
For example, emotional motivators such as shame play a powerful role
in shaping beliefs regarding the urgency and value of participation in
coalitional violence even in modern warfare. One particularly poignant
example of the role of shame (and its manipulation) in warfare is illustrated
by the Order of the White Feather in Great Britain during World War I. As
it became clear the war would drag on longer than anticipated, the Order
sought to manipulate the shame of non-participating service-age males into
participating by having females publicly bestow white feathers upon them
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(Hochschild 2012). Participation in collective violence against an adversary
involves the manipulation of many positive as well as negative emotional
motivations, and investigation into the evolved machinery that structures
such motivations can illuminate debates regarding public support for
warfare as well as questions regarding the recruitment patterns and tactics
of terrorist organizations.
Of course, most instances of war are not examples of perfect
within-group solidarity, and therefore we are left with the necessity
of explaining why, for example, some are compelled to ght while others
resist participation and even rebuke the effort. Although this is a complex
puzzle with many dimensions, evolutionary theory may help to explain
one component of this puzzle. First, recall that not all forms of coalitional
violence are equal two particularly common forms are raids and
battles. However, it may also be the case offensive and defensive coalitional
violence have been important and ancestrally recurrent types of coalitional
violence, each with their own sets of reproductive costs and benets
(Tooby and Cosmides 1988). One important difference between the
two forms of violence is that defensive coalitional violence may have
generated benets in the form of public goods more often than offensive
coalitional violence (Lopez 2010). Therefore, one prediction would be
that the framing of conict as either offensive or defensive would
trigger distinct patterns of support for the coalitional action, leading to
wider support for defensive rather than offensive warfare.
Using this distinction, one could predict a very broad pattern of
within-group coalitional unity and the resultant distribution of emotions
such as shame among non-participants simply by identifying the
predominant framing of the conict along the single dimension of offense
vs. defense. In other words, we might hypothesize that while support for
defensive action is expected to be relatively wide and deep, support
for offensive action would be greatest among (1) those it benets directly,
and (2) those who misperceive the offensive action as defensive. It is perhaps
no surprise, therefore, that much political haggling and arm-twisting
revolves around the rather parochial question of who started it?Elites and
political leaders play a prominent role in framing the answers to these
critical questions.
Rally round the leader: The above discussion illustrates that many
coalitional dynamics within ones group are triggered and shaped
by important and adaptively relevant contextual features such as
the existence and nature of out-group threat. At the center of these
coalitional dynamics, of course, is the issue of political leadership.
Experimental evidence suggests that individuals possess psychological
adaptations that evolved in response to the challenge of identifying
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effective leadership for coalitional action (Van Vugt and Ahuja 2011;
Price and Van Vugt 2014). Indeed, leadership can dramatically reduce
the cost of coalitional aggression by providing focal points and enhancing
coordination. Although people are often willing to defer to leaders
and to allow them to keep a share of the group benets (Hooper,
Kaplan and Boone 2010), people remain sensitive to the prospect of
leadership exploitation (Smith et al. 2007). This indicates that an
evolutionary arms race likely existed between leaders and followers in
which selection favors leadership as a solution to collective action
problems, but there is a counter-selection pressure in which follows
resist the tendency of leadership to drift toward despotism and undermine
collective action (Van Vugt and Ahuja 2011).
Of course, as mentioned above, all collective action is not equal,
and different individuals by virtue of their personality or formidability
may prove to be more effective leaders in wartime than in peace.
To the extent that this has proven true over evolutionary time, we
would hypothesize that adaptations exist that lead individuals to
prefer particular types of leaders contingent upon the immediate
context. Indeed, Spisak et al. (2012) show that support for specic
leaders depends on cues in the face that ancestrally correlated with
success and failure in specic coalitional environments. Regardless of
the sex of the prospective leader being evaluated, people tended to
prefer masculine faces in wartime, and people tended to prefer
feminine faces during peacetime. In a related study, age was also a
signicant predictor of support for a prospective leader in wartime.
Spisak (2012) hypothesized that, if it was the case ancestrally, all else
equal, that greater age tended to correlate with greater status and
dominance leadership, then during wartime, individuals should be
especially supportive of older candidates. Experimental evidence conrmed
these expectations even when controlling for the actual and perceived
political experience of the candidates.
The link between within-group cooperation and inter-group conict,
as well as the challenge of leadership, are two broad domains that allow
us to think more clearly about what adaptations ought to exist to regulate
behavior in wartime, and specically how natural selection builds
adaptations that are designed to operate facultatively upon the detection
of a range of contextual cues specic to the occurrence of inter-group
Importantly, as Spisak (2012) clarify, the hypothesized ancestral relationship between age
and status is likely quadratic, as at very extreme old age, for example, there must have been an
inverted relationship between age and dominance-related status.
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Grand theory in international politics?
The architecture of coalitional motivation sketched above has one major
implication for grand theory in international politics: it must either be built
upon a model of the mind that is not ecologically valid, or grand theory is
simply not possible. Specically, grand theories of international politics that
seek to explain a range of behavior across a multitude of domains by
reference to only a few structural or cognitive principles are likely to be
theoretically hamstrung by their failure to recognize and incorporate
the motivational plurality that actually drives behavior and its exquisite
sensitivity to context. Adaptationism is a framework for uncovering the
range of contexts, or domains, that trigger evolved adaptations, as well as
for uncovering the ways in which evolved adaptations are designed to
respond to context, as presented above. What we are necessarily left with
are theories of warfare, and the tools for expanding our understanding
of the coalitional psychology that shapes inter-group violence in many
contexts. Neither realist anarchy nor the enlightened self-interest of
liberalism is sufcient to explain the range of behavior of interest to
international relations scholars. Constructivism is similarly incomplete to
the extent that it fails to acknowledge the material properties of the mind
that are active in producing and responding to social values and norms,
which, for example, is necessary in order to explain why some norms seem
to be more stickythan others (Adler 1997; Lopez and McDermott 2012).
Previous attempts at grand theorizing in international relations have
generated useful middle-range insight into important dynamics, such as
alliance formation and neoliberalisms solutions to the prisoners dilemma
(Milner 1992). However, future research must incorporate and build upon
these specic insights while abandoning the quest for a single grand theory
of international politics.
Now that the theoretical framework for explaining and exploring the
evolution of coalitional aggression, or warfare, has been established, we can
turn to a closer examination of four critiques that are often levied against
evolutionary approaches to warfare.
Fighting fallacies
There are four prominent claims against the application of evolutionary
theory for understanding warfare. Although the fallacious nature of these
claims should now be largely transparent given the preceding discussion,
This trend is experiencing reversal as constructivists increasingly turn to psychology to solve
this problem. See, for example, Shannon and Kowert (2011).
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these claims deserve to be singled out because they are particularly enduring
and forceful. Consequently, I engage them here in order to expose their
assumptions and evaluate their merit.
Ancestral warfare was not frequent or intense enough to have
selected for psychological adaptations for warfare
Adaptationist analyses depend in large part on our ability to survey and
understand ancestral environments. When experimental results fail to
verify hypotheses regarding the design of adaptations, it can be due to the
fact that we are incorrect regarding the structure of ancestral selection
pressures (e.g. such pressures are either misspecied or non-existent), or it
can be due to the fact that the logic that links the psychological design or
formof adaptations to their ultimate ancestral purpose or functionis
awed (Confer et al. 2010).
Consequently, there are two forms of evidence upon which to rely when
evaluating the prevalence of ancestral warfare: we can survey the current
state of research on direct and indirect evidence of ancestral coalitional
warfare, especially in elds such as archeology, paleoanthroplogy, and
primatology (Tooby and DeVore 1987), and we can survey current
experimental evidence with modern humans to examine the extent to
which hypotheses regarding the structure of psychological adaptations for
warfare are veried given a set of ancestral selection pressures. As I have
discussed above, there is substantial laboratory and cross-cultural evidence
that humans seem to be endowed with a coalitional psychology, and that
such adaptations are specialized for the regulation of behavior, motivation,
and cognition in the context of warfare. However, what is the state of
knowledge regarding direct and indirect evidence of coalitional warfare in
ancestral environments?
Primatology: Primate studies have mostly revealed that if there is an
evolutionary origin of human violence, it can be traced back at least to the
hominidchimpanzee split (Manson and Wrangham 1991; Wrangham and
Peterson 1996; Wrangham 1999a). There are clear selection pressures
favoring coalitional violence, and the tness benets of such activities have
been measured and replicated (Mitani, Watts and Sylvia 2010; Wilson et al.
2014). Chimpanzees appear equipped with psychological mechanisms that
adaptively regulate the expression coalitional aggression based on cues such
as relative power differentials between groups.
This theoretical dynamic is not entirely dissimilar from rational choice models, in which it is
possible to backsolve from revealed preferences to the structure of underlying decision-making
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Nevertheless, two factors complicate our ability to infer a clear
evolutionary link between chimpanzees and humans. The rst is the
existence of relative peace among bonobos, which genetic tests reveal are
nearly as similar to humans as are chimpanzees (Prüfer et al. 2012); the
second is the fact that humans exhibit a form of warfare that chimpanzees
do not sustained battle between coalitions of roughly similar size.
Although genetic tests have revealed that bonobos are as closely related
to humans as chimpanzees, we do not yet know the substance of that
relationship. Indeed, humans share many similarities with both bonobos
and chimpanzees (Rilling et al. 2011; Boehm 2012b). Furthermore,
real differences between chimp and human coalitional aggression do not
contradict the empirical fact that among both chimps and humans, the most
common form of coalitional aggression is the lethal raid (Gat 2006;
Wrangham and Glowacki 2012). Lastly, regarding the fact that chimps do
not engage in large-scale coalitional aggression, recall that the evolution of
warfare in humans requires both reproductive benets and unique
cognitive adaptations for n-person cooperation and coordination. Given
the cognitive requirements for sustaining and operating large n-person
coalitions, it would in fact be more puzzling if chimpanzees did engage in
large coalitional battles but humans did not. In short, coalitional aggression
is reproductively worthwhile for both chimps and humans, but it is
likely that unique cognitive abilities in humans allow greater coalitional
maneuverability and provide the space for cultural innovations to sustain
such coalitions. Indeed, this divergence between the chimp and human
pattern of warfare is to be expected, and not an anomaly. Taken together,
this supports the conclusion that human coalitional violence has roots in
our primate past, but also that this is not the end of the story; we must
investigate aspects of the coalitional environment that have been unique to
human evolutionary history after the hominid split in order to fully
understand the design and structure of putative adaptations in these
Anthropology: The prevalence, intensity, and frequency of hunter-
gatherer warfare has been a topic of intense debate, in part because the
character of warfare in this context is believed to offer a partial window
into our evolutionary past as a species. What is certain is that warfare is not
a perfect human universal, strictly speaking; depending on measurement
techniques, there are usually a handful of cultures that scholars can point to
as existing relatively free of coalitional violence for certain periods of time.
However, despite the absence of perfect universality in the strictest sense, it
is nevertheless also the case that the overwhelming majority of
cultures surveyed do indeed experience some form of coalitional violence
(Ember 1978; Keeley 1996; LeBlanc and Register 2003; Gat 2006).
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Not surprisingly, at the heart of disagreements over the prevalence of
warfare are questions regarding the denition of warfare, since the latter
necessarily affects ones conclusions regarding the former (Palmer 2006). If
it is a useful window into ancestral environments, the character of modern
hunter-gatherer warfare does seem to suggest that coalitional violence was
prevalent in some form during our evolutionary history, but denitional
disagreements will likely prevent any conclusions on the matter.
Denitional disagreements, however, do not hinder our understanding of
the existence of adaptations for warfare; they merely force us qualify our
arguments, which can be useful rather than debilitating. For example, the
Correlates of War (COW) project denes war as sustained combat,
involving organized armed forces, resulting in a minimum of 1000
battle-related fatalities within a 12 month period. If we go by this deni-
tion, then it is clear that there can be no adaptations for warfare of this type,
since this type of warfare was only possible well after the dawn of
agriculture and is evolutionarily recent. Clearly, no adaptations exist for the
quick and effective maneuvering of an M1 Abrams battle tank or phalanx
troop columns. However, if we take a broader denition of warfare, the
implications are predictably different. For example, Levy and Thompson
(2011) dene war as Sustained coordinated violence between political
organizations. Similarly, Bowles (2009) denes war as Relationships in
which coalitions of members of a group seek to inict bodily harm on one
or more members of another group.Dened in this way, it is indeed likely
that warfare existed ancestrally, and this perspective can assist the search
for evidence that can test this claim.
This is not to say that the COW denition of war is wrong, or that
broader denitions of war somehow capture its trueessence; rather,
denitional disagreements only reveal that warfare can take many forms,
and that only a particular form or forms of warfare existed ancestrally that
likely shaped adaptations for motivation and behavior in these contexts.
Whether or not one wants to call the occurrence of this ancestral activity
warfareis really beside the point. The challenge, therefore, is to explore
how adaptations for a form of coalitional behavior that prevailed
ancestrally operate in a modern environment in which the instruments
and scope of this activity have changed drastically. Just as our visual
adaptations do not cease their relevance or operation in the context of
modern instruments that allow the viewing of celestial objects, adaptations
for coalitional violence remain relevant even in the modern world of
mechanized battle.
Archeology: The archeological evidence of ancestral warfare is again
hotly debated, and in this case, the central issue has been the existence of
directevidence of warfare (Ferguson 1997; Otterbein 2004; Fry 2007).
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Direct archeological evidence of warfare consists of bone and skull
indentations and fractures caused by weapons such as spears or other
stone-based instruments, as well as evidence of fortications. However,
these types of material innovations only occurred within the last
30,00040,000 years, and the building of fortications was only possible
after the transition to sedentary lifestyle once the agricultural revolution
had occurred about 12,000 years ago (Ferguson 1997). Nevertheless, as
archeologists LeBlanc and Register have noted, The world was hardly
peaceful until someone in China or Mesopotamia hammered out the rst
bronze sword(2003). Therefore, as we evaluate such evidence, we must be
careful not to confuse the instruments of war with the occurrence of war;
weaponry and forts are not prerequisites of coalitional violence, and it is
therefore false to infer the absence of the latter based on the absence of the
former alone.
Warfare is not the only form of social behavior that leaves a fossil trail
that is open to multiple interpretations. For example, evolutionary scientists
have begun active and productive investigations into the human penchant
for storytelling (Gottschall 2012), as well as religious belief and behavior
(Boyer 2002; Atran and Ginges 2012), as products of underlying
psychological adaptations. Both of these phenomena represent forms of
social behavior that would likely not have left direct fossil evidence of, for
example, an ancestral history of telling stories or religious belief. However,
the absence of literary instruments or religious artifacts from the ancestral
fossil record alone is not sufcient to disprove the possibility that
adaptations exist that are designed to regulate these forms of behavior.
Similarly, it makes little sense to assert that religion or religious belief
emerged only as early as we can nd religious artifacts in the fossil record,
and it would be clearly false to argue that humans could not possess an
adapted talent for storytelling merely because writing utensils or books are
relatively modern inventions. To take the case of chimpanzees, although
there is no fossilized evidence of weapon-related fractures, fortications, or
mass graves among these primate cousins, there is nevertheless clear and
overwhelming evidence that adaptations in chimpanzees exist for
the adaptive regulation of coalitional aggression (Wilson et al. 2012;
Wrangham and Glowacki 2012).
Problematically, it is inevitably the case that evidence from primatology,
anthropology, and archeology considered in isolation of each other are
necessarily incomplete regarding the ancestral presence of coalitional
violence. However, when considered together, the overwhelming evidence
does suggest that inter-group conict was a prevalent and reproductively
signicant feature of ancestral environments, based on evidence of
chimpanzee coalitional aggression, the near universality of hunter-gatherer
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warfare in humans, and direct evidence of warfare as far bask as the
preservation of its instruments allows. Furthermore, experimental research
that investigates the existence of psychological adaptations for warfare only
continues to expand and generate new and novel hypotheses regarding
evolved coalitional aggression, which would also seem to suggest that
warfare was ancestrally recurrent, especially as many of the hypotheses
generated from this framework mentioned above (e.g. overcondence
and brinkmanship; heroism and self-sacrice in battle; perception of and
support for leaders based on facial cues) are unintelligible absent this
The existence of peaceful societies falsies the claim that humans
possess adaptations for ghting
The most prominent recent example of this argument comes from Fry in his
book Beyond War: The Human Potential for Peace (2007). Fry relies in
part on evidence of the non-universality of warfare to argue that humans
could not possess adaptations for warfare. Is the existence of peaceful
cultures evidence against the existence of adaptations for warfare?
As mentioned above, to the extent that psychological adaptations exist
that regulate behavior and motivation in domains of social interaction, it
should also be the case that these adaptations are facultative,
rendering behavioral and motivational output a conditional response to
environmental circumstances. In his analysis of adaptations in chimpanzees
for coalitional aggression, Wrangham writes that, selection has favored a
tendency among adult males to assess the costs and benets of violence, and
to attack rivals when the probable net benets are sufciently high. Also,
regarding human coalitional violence, McDonald, Navarrete and Van Vugt
write: humans may calibrate their responses to out-group males based on
an assessment of the strength of, or threat posed by, a male coalition(2012,
672). Similarly, Tooby and Cosmides argue that putative psychological
adaptations for warfare in chimpanzees and humans are designed in order
to observe, assess, and to regulate the appropriate pattern of response
towards several different males structured into a coalition(1988).
This is a far cry from previous models, such as the killer apehypothesis,
which asserted the existence of a latent and persistent passion for war and
aggression, which perennially fueled the re of inter-group conict (Lorenz
1966). From this antiquated perspective, the existence of variation in
the incidence of warfare is indeed confusing. However, given the innate
conditionality inscribed by natural selection into the design of facultative
adaptations for warfare, it is not surprising that cross-cultural variation
exists in the occurrence of warfare. Indeed, this is exactly what an
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adaptationist perspective would predict. Claims of universality in adapta-
tionist models refer narrowly to the psychological adaptations themselves,
and not the behavior they generate and regulate. To argue that warfare does
not represent the operation of underlying psychological adaptations merely
because warfare is cross-culturally variable is to confuse the variable
output of adaptations with the species typicality of the adaptations
If psychological adaptations for warfare exist, then war is a
necessary and inevitable component of human societies
There are (at least) two reasons why the existence of adaptations for
warfare does not render war inevitable.
First, adaptations for coalitional aggression are facultative, which means
that framing and context are fundamental determinants of inferential,
motivational, and behavioral outcomes. For example, although out-group
threat indeed often triggers hormonal responses that are associated
with dominance reactions and aggressiveness, escalatory responses are
not inevitable. Anthropologist Flinn et al. have demonstrated that the
hormonal responses (particularly testosterone) to coalitional competition
depend critically upon whether the mind receives cues that ones opponent
is an out-grouper or an in-grouper (Wagner, Flinn and England 2002;
Flinn, Ponzi and Muehlenbein 2012). In other words, the severity of conict
can be affected by ones perceptions regarding the coalitional identity of the
other group. Relatedly, despite the well-known studies in which amygdala
response is heightened when subjects are shown pictures of members of
other races, there is also research that shows that this common xenophobic
reaction can be muted through even as simple an exercise as travel through
other cultures (Sapolsky 2006). In short, humans are groupish and we
incessantly dene socio-political interactions along group boundaries
(Ridley 1997; Kurzban, Tooby and Cosmides 2001). These boundaries
affect the way we respond to threat and the prospect of cooperation;
however, these boundaries are also plastic and subject to extensive
Second, the existence of adaptations for coalitional aggression does not
preclude the existence of adaptations for pro-social behavior, such as
cooperation and conict management (Hamilton 1964b; Trivers 1971;
Axelrod 1984; Dugatkin 1997; Cosmides and Tooby 2005). For example,
research has demonstrated evidence for adaptations that regulate
cooperation and social exchange (Fehr and Schmidt 1999; Cosmides and
Tooby 2005; Nowak 2006; Aktipis 2011), altruism toward strangers
(Boyd and Richerson 2009; Delton et al. 2011), and inter-personal and
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group-level trust (Zak 2006; Zak and Kugler 2011). McCullough, Kurzban
and Tabak (2013), for example, nd that humans possess evolved
psychological mechanisms not only to seek revenge, but also to rebuild
relationships and seek forgiveness. These ndings collectively suggest that
humans are equipped with reliably developing psychological machinery
designed to navigate the many challenges of social living from conict to
cooperation. We are armed with the phenotypic weapons of inter-personal
and coalitional competition, yet we also possess sharpened moral intuitions
and cooperative inclinations that enable and facilitate peacemaking and
conict resolution (Boehm 2012a; De Waal 2012).
As the history of human civilization reveals, humans are adept at the
creation and manipulation of cultural innovations that expand the reach of
social and political institutions (Wright 2000). Despite the fact that the
in-group/out-group distinction is a social categorization that appears
deeply ingrained (Mahajan et al. 2011), humans are also noteworthy in
our ability to manipulate the boundaries of these categories, widening
social inclusivity in meaningful and lasting ways (Sapolsky 2006). War is
not inevitable, but peace is not easy.
Modern warfare and international politics is so qualitatively different
from ancestral politics that any adaptations for the latter are
inoperative or irrelevant today
This view has its roots in classic arguments made by Turney-High (1949)
and Wright (1983) in their early and authoritative examinations of
primitive warfare. Turney-High, for example, argued that war below the
military horizonwas qualitatively distinct, and indeed bore little
resemblance at all, to war above the mechanized horizon of modern
warfare. According to this view, below the horizon is the domain of
ritualistic contests, ignited for impractical goals, and in a relatively harmless
fashion. In contrast, war above the military horizon is the realm of
organization and rational policymaking, and it is exceedingly lethal.
Although there can be no doubt that primitive and modern warfare are
differentiated by a great degree of coordination and political organization,
subsequent studies have demonstrated that many of the goals and tactics of
warfare both above and below the military horizon remain similar
regardless of innovations in logistical complexity or command and control
structures (Keeley 1996; LeBlanc and Register 2003; Gat 2006; Gat 2009).
As the psychological adaptations we continue to possess
today were designed in response to recurrent environmental challenges
over evolutionary time, to understand their modern impact in any
given case, we must ask at least two questions: to what extent do
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modern contexts mirror ancestral contexts, and what happens when
they dont?
Modern contexts trigger ancestral logics: To the extent that modern
international conict mirrors domains of ancestral inter-group conict, we
should expect these situations to tap into and trigger these adaptations as if
modern warfare is played out in the ancestral environments that these
adaptations are designed to expect. For example, as mentioned above, the
mere presence of inter-group conict is sufcient to trigger various forms
of within-group pro-sociality, such as a greater willingness to reward
cooperators and punish non-cooperators (Puurtinen and Mappes 2009).
Importantly, these lab results have been complimented by eld experiments
in the context of actual political violence (Burton-Chellew, Ross-Gillespie
and West 2010; Gneezy and Fessler 2011). Ginges and Atran (2011) nd
that decision making in a modern scenario of military intervention often
operates independently of rational expectations regarding the likelihood of
war to achieve its aims, and Johnson, Wrangham and Rosen (2002)
show that states and their leaders may actively self-deceive regarding their
chances of victory in modern strategic environments. It seems clear that
these quick and reliably operating heuristics actively shape the quality of
modern warfare, even if at times their operation appears puzzling from a
rationalist framework, which leads to the next point.
Modern mismatchhelps to explain irrational or surprising behavior:
Adaptations for coalitional behavior affect the shape and character of
international politics even when the latter presents environmental novelties
that adaptations are not designed to expect. In other words, there may be a
mismatchbetween the environment the adaptation expectsand
the environment it actually encounters. For example, our adaptations for
coalitional behavior were designed in small-scale environments that lacked
strong institutionalized hierarchies, where leadership was often informal
and unstable. As discussed above, effective leadership could provide
signicant reproductive benets in the context of coalitional competition in
which quick and coordinated action is critical (Kurzban and Neuberg 2005;
Van Vugt and Kurzban 2007), and there is evidence that individuals
decisions to support a given leader is contingent upon adaptively relevant
cues such as facial features and the status of coalitional conict (Spisak et al.
2012). Given the ancestral importance of personal leadership for many
coalitional endeavors, it is unsurprising that one of the most remarkable
challenges faced by developing countries is that of building trust in
institutions and the rule of law. Democratic transitions the world over are
frustrated by regression to authoritarianism or superpresidentialism, and
often dominated by cults of personality that privilege personal relationships
over trust in state-level institutions (Ishiyama and Kennedy 2001;
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Barany 2008). Although institutional trust is challenging to develop for
many reasons, the ultimate reality is that state-level institutions are an
evolutionary novelty to the minds eye. Combined with the fact that people
tend to prefer stronger leaders in times of inter-group conict, developing
countries face an uphill battle against built-in biases that generate
preference for powerful and charismatic personalities over impersonal
and diffuse institutions.
These dynamics are apparent not only in the case of democracy and
institutions, but also in warfare. For example, historically, large armies
have systematically tended to overestimate their chances of victory,
often undervaluing aspects such as the will to ght by local insurgents.
Ancestrally, relative numbers was indeed a critical variable, perhaps the
critical variable that would have determined the outcome of coalitional
conicts. In such environments, groups were relatively nomadic, and an
adaptive response to invasion by a larger coalition would have been to ee
and surrender territory. However, in modern environments where political
groups are generally tied to territorially xed nation-states, ight is less of
an option, which may inadvertently magnify the will to ght of cornered
adversaries, such as entrenched insurgents.
The study of warfare from an evolutionary perspective is still young, and
research at this point remains focused on identifying adaptations that may
exist and investigating their operation. As this research progresses, it will
become increasingly necessary and central to investigate the question of how
such adaptations interact with evolutionarily novel aspects of the modern
environment. Adaptationists who study human behavior have proven
remarkably successful at explaining various aspects of coalitional behavior and
have demonstrated that these mechanisms continue to operate even in evolu-
tionarily novel environments, such as many aspects of international politics.
The study of international politics has matured in no short measure as a
consequence of its ability to integrate its own perspectives with those of
other disciplines, especially economics, and also social psychology (Mercer
1995) and anthropology (Snyder 2002). Although the scientic study of
warfare is expanding at great pace in the evolutionary sciences, scholars
of international relations have been relatively slow to acknowledge and
integrate such ndings. Two reasons for this reluctance are uncertainty
regarding the proper application of evolutionary theory toward the study
of warfare, as well as uncertainty regarding the proper interpretation of
modern and ancestral evidence of warfare.
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I focus attention on the evolution of warfare for three reasons. First,
the ancestral recurrence of inter-group conict has posed signicant
selection pressures that yield a range of unique hypotheses regarding the
resultant structure of evolved mechanisms for coalitional competition.
Second, research on the evolution of war has already yielded substantial
ndings, such that the state of research is ripe for interdisciplinary
integration (McDonald, Navarrete and Van Vugt 2012; Wrangham and
Glowacki 2012). Third, international relations, as the natural home
eldfor the study of warfare, ought to take the lead in this interdisciplinary
integration. Although there has been some movement toward
integration by international relations scholars (Johnson, Wrangham and
Rosen 2002; Gat 2006; Thayer and Hudson 2010; Lopez, McDermott
and Petersen 2011; Johnson 2015), there is much room for gainful
An adaptationist perspective provides scholars of international relations
with the tools necessary to generate and test evolutionary hypotheses of
behaviors relevant not only to warfare but also to other domains of interest,
such as the design of institutions (Boyer and Petersen 2011). This perspec-
tive dispatches the false notion of an innate passion for violence and
replaces it with the understanding that we are endowed with a strategic
coalitional psychology. Adaptationism provides opportunities for
reconciliation with rationalism by helping to explain apparently irrational
proclivities such as the stubborn persistence of issue indivisibilities, and
may also support constructivist frameworks by helping to explain why
certain social forms may appear more appealing or intuitive to the minds
eye than others.
Adaptationism has established itself as an effective framework for
illuminating the information-processing structure of our evolved coalitional
psychology. As research on the evolution of war continues apace,
international relations scholarship is well positioned to benet from and
contribute to this interdisciplinary merger.
For helpful comments on themes developed in this paper, as well as
feedback on previous drafts, the author is grateful to Rose McDermott,
Richard Wrangham, and Tom Dolan. This paper was presented as
part of the War and Rivalry panel at the International Studies
Association Conference in 2012, and greatly beneted from
participant discussion, especially by Jon DiCicco, William Thompson, and
Brandon Valeriano.
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... Thus, Jones and Allen complain that "…the persistent problem with bioarchaeological evidence is that it can never be unequivocally attributed to warfare" (2014,354). This in part explains the preference of advocates of the long chronology to adopt a definition of warfare that does not hinge on its instruments, though this preference is perhaps better understood as a principled wariness of conflating the instruments of war with its behavioural occurrence (Lopez, 2016b). As LeBlanc (2003) wryly notes: "The world was hardly peaceful until someone in China or Mesopotamia hammered out the first bronze sword." ...
... In this sense, Kelly implicitly argues that warfare cannot be merely opportunistic. In contrast, long chronology scholars allow warfare to target individuals, to occur within kin groups, and do not require that warfare must demonstrate a particular motive or set of motives (Ember and Ember, 1992;LeBlanc and Register, 2003;Gat, 2006;Allen and Jones, 2014;Lopez, 2016b;Glowacki, Wilson and Wrangham, 2017;Hames, 2019). ...
... For example, the cultural evolution of norms and institutions of reward might sufficiently incentivize behaviour toward participation in risky violence (Glowacki and Wrangham, 2013b). Second, it is possible, even if counterintuitive, that natural selection can engineer a motivation to engage in risky violence as long as the rewards are sufficiently high on average over evolutionary time (Tooby and Cosmides, 1988;Enquist and Leimar, 1990;McDonald, Navarrete and Van Vugt, 2012;Lopez, 2016bLopez, , 2016a. Problematically, this would only shift the puzzle back to chimpanzees since we must then explain why chimpanzees do not engage in relatively risker violence. ...
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Introduction: The evolution and history of warfare has been investigated by philosophers, historians, practitioners, social scientists and life scientists. Common questions in this endeavour are: How far back into human evolution and history do we find evidence of warfare? How frequent was warfare in any given historical period? How lethal was warfare? In short, scholarship on the evolution and history of warfare has focused on questions of origins, frequency, and intensity. Despite the fact that scientific interest in these questions is perhaps broader and more methodologically sophisticated than ever, consensus on these questions remains elusive for at least two reasons. First, the archaeological record of warfare is incomplete. Second, we do not agree on what warfare is or how to unambiguously distinguish it from other forms of violence. Beyond an agreement that warfare is something more than violence between two individuals, there is little consensus on the proper scope of our main unit of analysis. Given these hurdles, it would seem that an investigation into the evolutionary origins of human warfare is destined merely to perpetuate academic stalemates, in which old arguments are continuously repackaged with each new discovery of a mass grave or 'peaceful' society. Although this is a rather pessimistic view, I establish it at the forefront of this chapter since my argument will be that these hurdles (e.g. knowledge of ancestral phenomena and consensus on definitions) are not insurmountable. Entire disciplines thrive on their ability to successfully infer and model the unobserved past based on imperfect historical, geological and archaeological evidence. And the question of definitions must be placed in its proper scope-as a methodological, rather than ontological, consideration. One of the core dynamics of the evolutionary process is natural selection, which is the only force known to organise biological design-that is; natural selection builds adaptations. Given that biological adaptations are solutions to recurrent and reproductively significant problems in an organism's environment, these adaptations themselves convey some information about the environment in which they evolved. In other words, the form and function of adaptations contains information about the (socio)ecology in which they were built. Therefore, if there is an argument to be made about the ancestral frequency and intensity of warfare, we should expect that the form and function of our evolved psychology should reflect the ancestral existence of such challenges. This is a way of saying that if warfare was evolutionarily recurrent and reproductively significant for our ancestors, evidence of this fact lies in our very brains.
... 2001). So extensive and crucial is this possibility that research on the "evolutionary mismatch hypothesis" seeks to explore and map its implication for domains such as warfare and leadership (Giphart & Van Vugt, 2018;Li, van Vugt, & Colarelli, 2018) The application of these insights to the dynamics of warfare can help to identify the psychological adaptations that exist and regulate the propensity to participate in coalitional violence (Lopez, 2016;McDonald et al., 2012). This in turn provides a foundation from which to understand how leaders operate within this coalitional landscape. ...
... In other words, how costly was coalitional aggression, and how frequently was this risk present in the lives of our ancestors? In terms of its costliness, and despite resembling the chimpanzee model of intergroup violence in many respects, human coalitional aggression is almost universally recognized as being quite distinct in terms of the degree of risk incurred by participants (Boehm, 1999;Glowacki & Wrangham, 2013;Lopez, 2016;McDonald et al., 2012;Tooby & Cosmides, 1988;Wrangham & Glowacki, 2012;Wrangham, Wilson, & Muller, 2006). The decision of whether to participate is one that does indeed entail significant reproductive consequences. ...
... In addition to paleo-archaeological evidence, the main lines of evidence for such inference are comparisons with nonhuman primates, comparisons with modern hunter-gatherer groups believed to approximate ancestral conditions (Boehm, 2011;Glowacki, Wilson, & Wrangham, 2017;Kissel & Kim, 2018;Lopez, 2016), along with statistical and game theoretic estimations of ancestral regularities (Bowles, 2009;Gómez et al., 2016). Lastly, and crucially, to the extent that the character of biological design reveals information about the world in which it evolved, we can also examine evidence of psychological design itself as a useful source of information for drawing inferences about the character of ancestral environments (Barkow, Cosmides, & Tooby, 1992;Cosmides & Tooby, 1987;Mayr, 1983;Williams, 1966). ...
Warfare is a collective action problem, and groups often stand to benefit from the quick and coordinated action that leaders can provide. This basic principle is as true in modern political contexts as it has been across our evolutionary history, and there is growing evidence that leadership has evolved, in part, to solve such collective action problems. Despite the material and reproductive benefits of leadership for groups, leaders may also seek private gains at the expense of group interests. Drawing upon insights from social and evolutionary psychology, I explain how leaders solve collective action problems in warfare, but also how leaders manipulate audience preferences when their own interests do not align with group interests. Specifically, when leaders anticipate great private gain from foreign aggression while facing steep public resistance at home, leaders will misframe the conflict as defensive rather than offensive in nature. I provide an evolutionary analysis that explains why leaders exploit this framing specifically, and I identify the specific aspects of conflict framing that are most likely to be exploited toward this end.
... There are many evolutionary theories of dyadic aggression (Archer, 2006;Parker, 1974;Sell, Tooby, & Cosmides, 2009), and there have been some influential attempts to explore human coalitional aggression, or warfare, as an evolutionary product (Alexander, 1979;Gat, 2006;LeBlanc & Register, 2003;Lopez, 2016;Potts & Hayden, 2008;Rosen, 2005;Thayer, 2004;Wrangham & Glowacki, 2012). More recently, there is renewed effort in explaining some dynamics of social behavior as products of group selection and multilevel selection, which often take as given, rather than seek to explain, an ancestral history of coalitional violence. ...
... The existence of psychological adaptations for warfare does not allow the inference that humans are naturally war-prone and that warfare is inevitable (Lopez, 2016). Instead, humans seem to possess specialized psychological design that regulates the conditional expression of behavior in response to environmental contingencies. ...
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The study of warfare from an evolutionary perspective has expanded rapidly over the last couple of decades. However, it has tended to focus on the ancestral origins, prevalence, and instruments of war rather than adaptationist analyses of its underlying psychology. I argue that our evolved coalitional psychology may contain a set of distinct evolved heuristics designed specifically for offensive and defensive coalitional aggression. Data from two survey experiments are presented, in which subjects were given scenarios depicting offensive or defensive aggression and were told to make decisions, for example, regarding their willingness to participate in the conflict, their opinions of others who did not choose to participate, and their expectations benefit. The results indicate that humans do indeed distinguish readily between these two domains and that their willingness to participate, as well as their emotional responses toward others, is highly contingent upon this informational cue in adaptively relevant ways. In addition, and consistent with parental investment theory, data reveal a range of sex differences in attitudes toward coalitional aggression in the two conflict domains. Beyond the study of warfare, this project has implications for our understanding of the relationship between individual behavior and group dynamics, as well as for our understanding of the mechanisms by which the psychological framing of political events can lead to important social outcomes.
... These results are consistent with recent work in anthropology emphasizing heterogeneity, variability, and social complexity among populations such as mobile huntergatherers, often characterized as "small-scale" (e.g., Hill et al. 2011;Bird et al. 2019;Singh and Glowacki 2021). Between-group conflicts, although relatively less represented, are consistently documented across all subsistence types including hunter-gatherers, providing additional evidence for the ubiquity of inter-group conflict across the vast majority of human societies (Glowacki, Wilson, and Wrangham 2020;Hames 2019;Lopez 2016). ...
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Conflicts are ubiquitous between individuals as well as between groups. Effective conflict resolution is essential for individual well-being and group functioning and often involves leadership dynamics. The evolutionary human sciences have suggested conflict resolution is shaped by psychological heuristics, norms, and ecology. There are limited empirical data, however, on conflict resolution across cultures. Using a cross-cultural database of 109 leadership dimensions coded from over 1,200 text records from the eHRAF ethnographic database, exploratory analyses investigated correlates of conflict resolution. Results revealed greater evidence of conflict resolution among kin groups than political groups and greater evidence of within-group conflict resolution than between-group, which did not vary across subsistence strategies or group contexts, with two exceptions – military group conflicts were biased towards between-group contexts and religious groups biased towards within-group contexts. The strongest predictors of conflict resolution services were other prosocial functions and included group representation and providing counsel, protection, and punishment, as well as qualities of interpersonal skills and fairness. Followers received social service benefits and reduced risk of harm. For leaders who resolve conflicts, status and social benefits were potential negative predictors. These results provide a comparative view of the correlates of conflict resolution suggesting diversity across social contexts.
... Indeed, emotions constitute a crucial tool for exerting social pressure. Specific emotions have evolved for specific functions, including to organize and maintain group cooperation (e.g., Clifford, 2019;Lopez, 2016;Marcus et al., 2000;Tooby et al., 2008). Many emotions are hypothesized to manage relationships, as we detail next. ...
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Political collective action requires assembling and motivating supporters. Many theories view emotions as functional tools for managing relationships, including within groups. We study what leads citizens to use the emotions anger and gratitude as social pressure. Specifically, we test what determines the use of these emotions to prevent potential exiters from leaving a political group and to encourage potential recruits to join. Because parties are enduring social affiliations (compared to transient or issue-focused groups), we predicted that partisans would express stronger emotions. We tested this proposition in two separate studies—one an observational study featuring a representative sample of US adults and one an experimental study conducted in Denmark. As predicted, people with a partisan mindset, whether naturally occurring or experimentally manipulated, felt more anger and gratitude at potential exiters and recruits. Citizens strive to fortify and expand their ingroups and sometimes use emotions as social pressure to do so.
... So long as terrorism has not fulfilled its political objective, there exists a 'suspended' hostility between the oppressed and their oppressor (Wydra, 2013). The endurance of this feeling is likely drawn from the indivisibility of the 'sacred values' that Lopez (2016) describes. Because such values are necessarily neither negotiable nor subject to compromise, any peace deal made at an organisational level between the state and the terrorist organisation is unlikely to last. ...
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Dominant orthodoxies within the study of terrorism have developed and promoted a strategic understanding of terrorism which has, in effect, reinforced the very conflict it seeks to explain. In response, this article develops the challenge of critical terrorism studies to propose a new strategic approach to terrorism. It posits that, while unable to achieve a Clausewitzian victory over the state per se, terrorism is able to politically paralyse the state's overwhelming power by shifting the moral horizons of that state's constituency. We explore two mutually reinforcing mechanisms by which terrorism achieves this objective and explore a number of strategic policy implications of each. This new approach is needed as current approaches misrepresent the challenge the global community currently faces, creating a self‐perpetuating loop of conflict and grievance which promises no end to terrorist violence. Terrorism poses a fundamental challenge to the state's moral authority and legitimacy. In response, the state mobilises its institutions of meaning‐making to politically exclude and ‘diagnose away’ terrorism and its practitioners in order to delegitimise the challenge its narratives and grievances pose.
... Such conditions are also often associated with high rates of inter-group conflict. Warfare has likely been a recurrent threat over human evolutionary history (Glowacki et al., 2017;Lopez, 2016) and represents a collective action dilemma often associated with strong leadership (Chagnon, 1988;Glowacki & Wrangham, 2013;Otterbein, 1997). Kaplan et al. (2009) integrate several of the foregoing ideas. ...
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Existing approaches within leadership studies often share a bias towards industrialized societies and lack broader cross-cultural and ethological reference. Meanwhile, cross-cultural and evolutionary approaches within anthropology are actively working to unify research on leadership and followership across the biological and social sciences. This review provides a novel and thorough view of political leadership as investigated by evolutionary anthropologists and highlights the benefits of incorporating findings from the evolutionary social sciences into leadership studies generally. We introduce the anthropological approach to leadership; describe evolutionary anthropology, its subdisciplines (including primatology, paleoanthropology, paleogenetics, human behavioral ecology, and gene-culture coevolution), and its complementary disciplines (particularly evolutionary psychology); review leadership and hierarchy in nonhumans, including our extinct hominid ancestors; review female leadership and sex-differences; and, primarily, discuss the relationships between evolution, ecology, and culture as they relate to the observed patterns of political leadership and followership across human societies. Through evolutionary anthropology's diverse toolkit, a deeper insight into the evolution and cross-cultural patterning of leadership is realized.
... territory can always be divided or compensated somehow), which suggests this is in fact not a rationalist explanation for war as such, but rather, an example of the failure of rationality in the bargaining process. An "all or nothing" attitude among policymakers can facilitate violence by rendering war-avoiding bargains impossible to agree on, or even to identify (Fearon 1995;Lopez, 2016). ...
The study of warfare is one of the oldest and most important dynamics of interest for students of politics. This area of research is predominantly − but not exclusively − undertaken within the sub-field of international relations (IR). IR theorists argue that war is the contingent outcome of the interaction among variables operating at three “levels of analysis”: the international level, the domestic level, and the individual level. The international level explores variables that operate exclusively “above” states such as anarchy and the distribution of power. The domestic level explores variables that operate exclusively within states, such as regime type and bureaucratic design. The individual level explores the ways in which individual psychology (i.e. beliefs, culture, personality) contributes to the outbreak of war. We also offer a discussion of new puzzles and challenges in the study of warfare, such as the decline of war hypothesis and the growing prevalence of civil war, insurgency and non-state violence.
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Interest in the evolutionary origins and drivers of warfare in ancient and contemporary small-scale human societies has greatly increased in the last decade, and has been particularly spurred by exciting archaeological discoveries that suggest our ancestors led more violent lives than previously documented. However, the striking observation that warfare is an almost-exclusively male activity remains unexplained. Three general hypotheses have been proposed, concerning greater male effectiveness in warfare, lower male costs, and patrilocality. But while each of these factors might explain why warfare is more common in men, they do not convincingly explain why women almost never participate. Here, we develop a mathematical model to formally assess these hypotheses. Surprisingly, we find that exclusively male warfare may evolve even in the absence of any such sex differences, though sex biases in these parameters can make this evolutionary outcome more likely. The qualitative observation that participation in warfare is almost exclusive to one sex is ultimately explained by the fundamentally sex-specific nature of Darwinian competition-in fitness terms, men compete with men and women with women. These results reveal a potentially key role for ancestral conditions in shaping our species' patterns of sexual division of labour and violence-related adaptations and behavioural disorders.
Why did President John F. Kennedy choose a strategy of confrontation during the Cuban missile crisis even though his secretary of defense stated that the presence of missiles in Cuba made no difference? Why did large numbers of Iraqi troops surrender during the Gulf War even though they had been ordered to fight and were capable of doing so? Why did Hitler declare war on the United States knowing full well the power of that country? War and Human Natureargues that new findings about the way humans are shaped by their inherited biology may help provide answers to such questions. This seminal work by former Defense Department official Stephen Peter Rosen contends that human evolutionary history has affected the way we process the information we use to make decisions. The result is that human choices and calculations may be very different from those predicted by standard models of rational behavior.
The study addresses the causes of fighting among hunter-gatherers, whose way of life represents 99.5 percent of human history. Focusing on somatic and reproductive causes in Part I and on such diverse causes as dominance, revenge, the "security dilemma," and "pugnacity" in Part II, the study seeks to show how all these motives, rather than being separate, come together in an integrated motivational complex, shaped by the logic of evolution and natural selection.
Iver Neumann argues that International Relations (IR) is going to have to engage with a broader set of empirical data and wider inter-disciplinary insights, especially from evolutionary biology. I endorse the debate and extend the challenge, making four key points: (1) our empirical data must extend to the deep origins of human societies, and look more at policy successes rather than failures; (2) our scientific toolkit must integrate rather than differentiate psychology and biology, because the former is in large part a product of the latter; (3) evolution continues to be misinterpreted, especially in the condemnation of functionalism, the myth of biological determinism, the perceived lack of relevance to IR theory, and the idea that social facts cannot have biological roots; and (4) there are other issues of more genuine importance, including the levels of analysis problem (getting from the biology of individuals to the behaviour of states), and the levels of selection problem (the predictions of group selection and individual selection for human nature). I conclude that, despite big challenges of communication and collaboration, the benefits of integration with the natural sciences far outweigh the benefits of disciplinary isolation, offering new knowledge, methods, consilience, and parsimony that will help IR to flourish rather than flounder in the Age of Biology.