C

Conditions Required for Evolution nature, scholars have sought to explain not only of Warfare Adaptations why particular wars are fought but also why humans organize into groups and fight each Anthony C. Lopez other in general. Some, such as the anthropologist Washington State University, Vancouver, WA, Margaret Mead, have argued that war is a cultural USA invention and that it likely developed sometime between the agricultural revolution and the emer- gence of large sedentary proto-states. These Definition authors claim that our evolutionary history and biological makeup can contribute little or nothing Describes the ancestral conditions that could to our understanding of why humans fight. In allow for the evolution of adaptations for warfare other words, historical, not biological, explana- in humans. The evolution of adaptations for war- tions for warfare are needed. fare is possible when there are low-cost/high- Others, however, have argued that group vio- benefit opportunities for violence, and it is also lence in humans is evolutionarily old and that the possible even if violence is costly, as long as origins of collective violence can even be traced fitness benefits can outweigh these costs in partic- back to our chimpanzee ancestors, many millions ular mating contexts. Additionally, in some spe- of years ago (Wrangham 1999; Wilson cies such as humans, the existence of warfare et al. 2014). These authors further argue that the adaptations presumes the existence of a competitive coalitional nature of human evolu- coalitional psychology able to track a broader tionary history had a unique impact on human range of group dynamics. In short, the evolution psychology. Specifically, they argue that due to of adaptations for warfare is possible when repro- the fact that coalitional competition had such a ductive benefits can outweigh the variable costs of significant and recurrent impact on our ancestors’ violence and when those opportunities can be reproductive success, we should expect to see taken advantage of by an evolved coalitional adaptations in the human brain that enable us to psychology. reason adaptively in contexts of coalitional com- petition and violence. These scholars often refer to such adaptations as “adaptations for warfare.” Introduction Labels are useful because they help us catego- rize, define, and understand concepts and dynam- Warfare is a prevalent feature of human social ics; however, research on the evolution of warfare interaction. Given its significant and recurrent is one area in which the labels themselves have

# Springer International Publishing AG 2016 T.K. Shackelford, V.A. Weekes-Shackelford (eds.), Encyclopedia of Evolutionary Psychological Science, DOI 10.1007/978-3-319-16999-6_914-1 2 Conditions Required for Evolution of Warfare Adaptations been the targets of much dispute. Consequently, complex behavior from those that are not is the many disagreements about whether warfare is presence of a complex nervous system and brain. evolutionarily old or culturally recent actually Therefore, the specific adaptations that compose hinge on disagreements about what warfare is the brain are generally designed to regulate some and how it should be defined. aspect of behavior by processing information No consensus exists regarding the definition of from the organism’s environment in specific terms such as “warfare,” but definitional clarity at ways. This is why some evolutionary psycholo- least allows one to evaluate the logical coherence gists will also refer to psychological adaptations of an argument on its own terms. Clearly defining as “information-processing mechanisms.” terms such as “warfare” means disagreements The process of natural selection explains the about what warfare is can be momentarily put existence and design of biological adaptations aside in order to ask a more specific question: (Williams 1966; Maynard Smith 1993). When given a particular definition of warfare, is there some heritable trait emerges in a population, it evidence that this behavioral pattern occurred can be favored by natural selection if it has a ancestrally? This means that different definitions positive effect on reproductive success and when of warfare will result in different conclusions that effect is recurrent over evolutionary time. about its prevalence ancestrally and, by extension, Resultant adaptations will tend to have certain different conclusions about the existence of adap- characteristics; broadly, their design will represent tations for warfare. A logical implication is that a specialized solution to some reproductive chal- some forms of warfare probably have no evolu- lenge or opportunity faced by the organism. With tionary explanation, while other forms of warfare some exceptions (e.g., frequency-dependent probably do. In short, the importance of defini- selection and “drift”), heritable traits that have a tions is not to be overstated and is a necessary positive impact on the organism’s reproductive starting point. success over evolutionary time will tend to spread in the population and become universal or near universal in the population. Defining Terms: “Adaptation” As information-processing mechanisms, adap- and “Warfare” tations in the brain are often designed to produce a range of output (e.g., altered hormone levels) con- What is an adaptation? Like the concept of war- tingent upon a range of input from the environ- fare, there is much discussion on what a biological ment (e.g., having won or lost a contest). The adaptation is, as well as whether adaptations in the conditional – or “facultative”–nature of adapta- brain should be studied in a fundamentally differ- tions suggests that if there are adaptations that ently way than adaptations in other parts of the shape the way humans reason about fighting, phenotype (body). Evolutionary psychologists then variation in the incidence of fighting across generally reject the proposition that adaptations a population will not be evidence that adaptations in the brain (“psychological adaptations”) require for fighting are not universal or don’t exist. different explanations than adaptations found Rather, it is possible that this variation merely elsewhere in the phenotype. Thus, evolutionary reflects the contingent logic of the universal psychologists argue that the design and function adaptation. of specific adaptations in the brain can be What is warfare? A voluminous range of def- explained in much the same way as the design initions of warfare exists, and there is no “right” and function of adaptations in the liver or heart. definition of warfare, only myriad sets of behav- The brain is not one singular adaptation, but iors that can justifiably be referred to as involving rather a collection of specialized adaptations that, violence of some kind among groups. Conse- broadly speaking, function to regulate the com- quently, much depends on one’s area of interest plex behavior of the organism. Indeed, a feature and expertise. Scholars of modern warfare, such that distinguishes organisms that are capable of as those who make use of the massive Correlates Conditions Required for Evolution of Warfare Adaptations 3 of War database, will (appropriately) define war- violence than it is to see the benefits. However, fare as “sustained combat, involving organized there are two types of costs to consider in the armed forces, resulting in a minimum of 1000 context of violence: somatic costs and reproduc- battle-related fatalities within a 12-month period.” tive costs. However, if this is our starting point, then the The somatic costs of violence refer to the phys- question of whether warfare has evolutionary ori- ical damage that an individual or group sustains as gins must be “no” since no one would argue that a direct and immediate consequence of fighting, this form of mechanized combat is anything but such as the loss of limb or the loss of life. In historically recent and evolutionarily novel. contrast, the reproductive costs of warfare refer Consider an alternative definition of war: “rela- to the reproductive opportunities lost as a conse- tionships in which coalitions of members of a quence of fighting. In other words, one cannot group seek to inflict bodily harm on one or more reproduce if they are dead. An evolutionary anal- members of another group” (Bowles 2009; ysis of warfare must attend to the question of its Wrangham and Glowacki 2012). This conceptu- reproductive costs and benefits. Specifically, the alization opens up the possibility that “war” is an reproductive benefits of warfare must outweigh evolutionarily old human practice, which, in turn, the costs of participation in violence. opens up the possibility that adaptations for “war- The fact that warfare can result in loss of life fare” exist. Whether one agrees that this is the means that the theoretical challenge is to explain correct definition of war is ultimately less impor- what reproductive benefits could make up for the tant than whether a form of coalitional violence costs of warfare when such costs entail loss of life can be identified that did exist ancestrally, and by extension a corresponding possibility of enabling specific inferences about how adapta- lost reproductive opportunities for the individual. tions designed in response to those specific ances- There are two pathways by which the reproduc- tral challenges structure our beliefs and tive benefits of collective violence can outweigh motivations in modern environments. these costs. The first pathway intuitively occurs when both the somatic and reproductive costs of warfare are actually quite low relative to the repro- Conditions for the Evolution of Warfare ductive benefits. The second pathway occurs somewhat counterintuitively when, despite very Given the above perspective on adaptation, and high somatic costs to the individual, the reproduc- using the broader definition of war (i.e., Bowles tive benefits of violence are even greater. 2009), a specific question can now be asked: what Pathway 1: Reproductive benefits of low-risk conditions must have held ancestrally in order for raiding. Collective violence can evolve in natural selection to favor psychological adapta- coalitional environments in which low-risk/high- tions in humans that guide motivation and behav- benefit aggressive opportunities exist. When these ior in the context of warfare? opportunities exist, selection can favor raiding behavior between coalitions. Such behavior has Reproductive Benefits specific characteristics and is well explained by The first condition is simultaneously the least the imbalance-of-power hypothesis (Wrangham satisfying (due of its general nature) but also the 1999). Absent weaponry, success in aggressive most important. That is, the reproductive benefits encounters is often determined by relative of fighting in groups must have outweighed its strength, and thus, all else equal, the greater the related costs. This can be difficult to identify in the size asymmetry between coalitions, the more case of aggression – coalitional or likely the larger coalition is to prevail at low cost otherwise – because of the significant physical to its members. In combination with the element costs involved in violence (Allen and Jones of surprise, the attacking coalition can virtually 2014). In other words, it is often easier to see the guarantee a low-cost, high-benefit outcome. substantial risks and physical costs of coalitional 4 Conditions Required for Evolution of Warfare Adaptations

The low costs of asymmetric warfare for the psychological adaptations that motivate high-risk attacking party are clear. However, what are the violence. Each is discussed briefly in turn. benefits? The first benefit is a direct consequence The first problem (i.e., cooperative as well as of the violence – the rival coalition loses one or combative coalitions) is not evidence that human more of its members, which constitutes a relative raiding is not motivated by psychological adapta- power loss. In an environment in which coalitions tions that operate to seize low-risk/high-benefit are small to begin with, the loss of even one aggressive opportunities. Rather, it suggests that individual would not be trivial. A loss of coalition this “raiding psychology” is part of an evolved strength can result in increased attacks from coalitional psychology that is equipped with a neighboring coalitions and an inability to defend broader toolkit of coalitional interaction. To over- the territory controlled by the coalition. Thus, not simplify, one could say that humans have psycho- only is the rival coalition weakened, but the win- logical adaptations not only for coalitional ning coalition can also obtain greater territory and violence but also for inter-coalitional (as well as with it the resources that it contains – both mate- intra-coalitional) cooperation and alliance. This rial and reproductive. The material and reproduc- issue must be put aside and returned to in a sub- tive benefits of low-risk raiding have been well sequent section on coalitional psychology. For studied and measured (Wilson et al. 2014). now, however, what can be said is that the condi- This “chimpanzee model” of warfare tional logic of psychological adaptations for (Wrangham and Glowacki 2012) is a clear illus- intergroup violence in humans likely operates tration of one set of conditions under which within a wider or at least different set of variables coalitional violence can evolve via natural selec- than it does in chimpanzees, encompassing oppor- tion, namely, where low-risk/high-benefit aggres- tunities for aggression as well as alliance and sive opportunities exist. This pattern can be seen trade. in humans, although with important differences. The second problem (participation in high-risk Human raiding, like chimpanzee raiding, occurs violence) must be engaged by careful reconsider- by stealth, is increasingly likely as the imbalance ation of the reproductive costs and benefits of of power reaches a ratio of 3:1, and is accompa- coalitional violence. The resolution of this second nied by non-negligible material and reproductive problem reveals the second pathway by which the benefits such as expanded territory (Johnson and reproductive benefits of warfare can outweigh its Toft 2014). Unlike chimpanzee raiding, however, potentially significant costs. human raiding can be somewhat more risky due to Pathway 2: Reproductive benefits of high-risk the presence of weaponry and the cycles of battles. Collective violence can evolve despite the revenge. existence of significant somatic costs – even indi- It may be the case that chimpanzee warfare vidual death – when reproductive resources in the offers a glimpse at the ancient “roots” of human coalition are subject to zero-sum distribution warfare. However, while some aspects of human (Tooby and Cosmides 1988; McDonald warfare resemble the chimpanzee model, other et al. 2012; Lopez 2016). To understand this argu- aspects do not. In a comprehensive review and ment, the reproductive success of the gene or suite comparison of chimpanzee and human warfare, of genes that would benefit from the risk-taking Wrangham and Glowacki (2012) argue that there behavior of its host must be examined. are two problems with the comparison of chim- All biological adaptations arise initially as a panzee and human warfare. First, relations among consequence of a random genetic mutation that human coalitions can range from hostile and vio- is maintained in a population because of its posi- lent to peaceful and cooperative. Second, human tive effects on the organism’s ability to reproduce. warfare seems to entail greater risks to attackers Taken to the fullest extreme, therefore, Dawkins than the chimpanzee model, which returns us to quips that “a chicken is only an egg’s way for the initial question of how evolution could explain making another egg” (Dawkins 1976). Put simply, natural selection describes the processes whereby Conditions Required for Evolution of Warfare Adaptations 5 a gene arises that has some effect on the body of gene that motivates participation in somatically its host, and when this effect causes more copies costly high-risk battles. Even if many individuals of that gene to make it into subsequent genera- die from the effort, the zero-sum allocation of tions, this gene is under positive selection. Thus, reproductive opportunities within the coalition natural selection can favor maternal altruism in means that the average fitness payoff to the mem- part because a mother’s offspring is likely to share bers of the victorious coalition is not diminished a significant fraction of her own genes (Hamilton by the death of one or even most of its members, 1964). Despite the fact that the “vehicle” in which assuming the above conditions are met. And since the gene finds itself absorbs a cost to confer a natural selection operates to favor strategies that benefit on another vehicle, the reality is that the are successful on average over evolutionary time genes in one vehicle are able to help copies of (i.e., despite individual fitness reversals), a herita- themselves in other vehicles. So what appears to ble propensity for participation in high-risk battles be altruism at the level of the organism or “vehi- can spread in the population. Thus, even if the cle” is actually selfishness at the level of the gene result is occasional death, the on-average repro- (Dawkins 1983). This perspective can help to ductive benefits can outweigh serious and even reveal the underlying genetic benefits of what fatal somatic costs to individual coalition mem- often appear to be somatically costly behaviors, bers. This may be particularly true when a “veil of that is, behaviors in which one organism absorbs ignorance” exists regarding the prebattle distribu- great personal costs to benefit another. tion of risk among the coalition members. With this “gene’s eye perspective” it is possible The Strength and Existence of Selection Pres- to reexamine the reproductive benefits and sures. What has been shown is that it is possible somatic costs of warfare, beginning with some for the reproductive benefits of risky battles to assumptions. First, assume a polygynous system outweigh their related costs. The theoretical pos- in which male parental investment is relatively sibility on its own by no means implies that adap- low and in which the primary limiting factor for tations for warfare exist or that if they do exist, male reproductive success is access to females they must be explained in this fashion. It is merely (Trivers 1972). Second, assume a world of two to suggest that adaptations for warfare can indeed male coalitions that are roughly equal in size. be explained by natural selection operating at the Third, assume that these coalitions fight, resulting level of the gene and the individual. The conclud- in the death of all males in the losing coalition, as ing section briefly engages the question of well as the death of every male except for one whether natural selection operating at other levels individual in the winning coalition. Fourth, (e.g., the group) can explain the evolution of war- assume that reproductive resources and opportu- fare adaptations. nities are allocated in a zero-sum fashion within First, however, a related question arises: given the coalition. This means that the reproductive the possible ancestral existence of conditions that opportunities lost by the fallen equal the repro- would have favored warfare adaptations, how ductive opportunities gained by the victorious strong must this selection pressure be in order survivors (Tooby and Cosmides 1988), which is for such adaptations to emerge? Recall that in made possible by the polygynous nature of the order for selection to build adaptations, there system. The result is that the reproductive benefits must be some challenge or opportunity in the to the winning coalition are not diminished by its environment that is both recurrent and has fitness high mortality, but, rather, are reallocated among consequences for the organism. Thus, one com- the survivor(s). The “last man standing” is there- mon critique against the existence of adaptations fore able to reap tremendously great reproductive for warfare is that even if there may be fitness benefits from this instance of coalitional aggres- benefits associated with coalitional violence, sion, despite the great somatic costs of the effort. such behavior was actually extremely infrequent As Tooby and Cosmides argue, this situation or even nonexistent ancestrally. In other words, therefore means that natural selection can favor a one cannot claim that adaptations exist in 6 Conditions Required for Evolution of Warfare Adaptations response to an environmental event that was rare quantitatively small selection pressures at one or nonexistent. point in time can sum to very large impacts over This challenge leads to two questions: (1) Did evolutionary time (Dawkins 1986). Thus, claims our ancestors engage in coalitional violence? that chimpanzee coalitional violence is not recur- (2) How frequently did they engage in coalitional rent enough for selection to favor adaptations for violence? Unfortunately, the evidence does not warfare fail both theoretically and empirically. In allow a conclusive answer to either of these ques- theory, they underestimate the power of natural tions, forcing researchers to engage in methodo- selection to operate on even the slightest fitness logical triangulation and to make educated differentials. Empirically, clear evidence demon- guesses about the possibility and prevalence of strates that the rate of chimpanzee coalitional vio- ancestral coalitional aggression. Direct evidence lence is sufficient to yield significant reproductive of warfare, such as fossilized bone indentations, benefits (Wilson et al. 2014). mass graves, and weaponry, is exceedingly rare and ultimately nonexistent as one peers further Coalitional Psychology and further back in time. The previous section explored the structure of Proponents of the evolution of warfare, how- reproductive opportunities that could lead selec- ever, point out that the absence of such evidence is tion to favor the emergence of adaptations for not evidence of the absence of warfare; rather, the warfare. However, in addition to the existence of absence of direct evidence of warfare is instead opportunities, the right equipment is necessary in the absence of the instruments of warfare, not of order to take advantage of such opportunities. warfare itself. In that sense, the labeling of such Coalitional aggression is fundamentally a collec- evidence as “direct” is unfortunate and imprecise, tive action problem, and participation in warfare since such evidence is actually indirect, not direct. therefore requires a degree of cognitive design for Swords, mass graves, and fortresses, for example, navigating a host of coalitional challenges. In are not requisites for coalitional violence, and short, adaptations for warfare presume the exis- therefore their absence from the fossil record tence of an evolved coalitional psychology that is after a certain time period does not allow us to designed to solve problems relating to the build- conclude that ancestral warfare was nonexistent. ing, maintaining, and enforcement of within- and As Leblanc and Register wryly note: “The world between-group relationships (Lopez et al. 2011). was hardly peaceful until someone in China or The complexity of this task is not to be Mesopotamia hammered out the first bronze underestimated and requires cognitive mecha- sword” (2003). nisms that perform a range of tasks, such as the This author has elsewhere engaged the ques- following: identify and punish free riders; respond tion of the likelihood of ancestral violence, and it and yield to leadership; and regulate the distribu- is a debate that has been vigorously argued on all tion of resources. In short, collective action for sides (Pinker 2011; Allen and Jones 2014; Lopez aggression presumes cognitive machinery that 2016). However – and somewhat disappointingly quickly and reliably solves a range of related but for both sides in the debate – the very strongest distinct n-person challenges. claim that can be made regarding the existence of One characteristic of the inseparable link ancestral coalitional violence is that the evidence between the unique challenges of warfare and does not allow us to conclude that it did not exist. the broad toolkit of coalitional psychology is the What seems clear is that unless it can be con- immediate and reliable way in which the former cluded (which it cannot) that ancestral coalitional triggers the latter. Experimental research in the lab violence was nonexistent, even the infrequent and in the field has repeatedly and powerfully recurrence of coalitional violence could very demonstrated the effect of intergroup conflict on well have provided enough of a selection pressure within-group cooperation. Specifically, the pros- to favor adaptations for warfare. As has been well pect or actuality of intergroup violence is suffi- explored elsewhere, what appear to be cient to trigger a host of within-group dynamics Conditions Required for Evolution of Warfare Adaptations 7 such as an increase in the overall level of cooper- In short, adaptations for warfare presume the ation, greater desire to reward participants and to existence of an evolved coalitional psychology see noncooperative individuals punished, as well that enables solutions to the complex coordination as the associated emotional displays of within- problems posed by intergroup violence. Taken group solidarity and shame or guilt among together with the previous section, the overall bystanders (Gneezy and Fessler 2011). discussion so far has explored the evolutionary The effect of intergroup conflict on within- feasibility of both low- and high-risk coalitional group collective action is twofold. First, warfare aggression, the power of even weak selection operates as an “effect multiplier” by enhancing pressures to build complex adaptations for war- many of the dynamics necessary for the prosecu- fare, and the role of coalitional psychology for tion of collective action in general, such as puni- facilitating intergroup violence. tive sentiment toward free riders. Second, warfare triggers a unique set of psychological mechanisms designed specifically for intergroup competition Auxiliary Observations Regarding and aggression, such as the tracking of in-group/ the Uniqueness of Human Warfare out-group membership (Kurzban et al. 2001) and relative coalitional formidability (Parker 1974). The above discussion outlines some general con- These two effects of warfare on collective action ditions that make the evolution of warfare adap- dynamics mean that researchers must be careful to tations possible. These conditions alone do not distinguish between those adaptations that are explain or attempt to describe the full range of uniquely designed for warfare and those that are psychological adaptations that possibly exist in designed for collective action but triggered in the humans for engaging in coalitional violence. context of coalitional violence. The two sets of Doing so would be the next logical step and adaptations are necessarily integrated yet distinct. requires careful adaptationist analysis of specific As one example of the integrated nature of reproductive challenges that is beyond the scope adaptations for collective action and adaptations of this brief and general inquiry (McDonald for warfare, Tooby and Cosmides (1988) outline et al. 2012; Lopez 2016). Nevertheless, some the “risk contract of war,” which outlines the brief observations should be made that necessarily psychological conditions that must be met in complement the preceding adaptationist inquiry order for coalitional violence to be initiated into warfare. against an out-group. The risk contract consists Research on the evolution of war and its related of two broad components: first, regulation of the psychological adaptations should not blind individual’s decision to participate and, second, researchers to the very real and powerful “other the enforcement of the agreement (or risk con- half” of the story, namely, the evolved human tract) on others. The individual decision of capacity for peace. The existence of adaptations whether to participate is regulated by variables for warfare does not undermine the case for adap- such as formidability, the degree of perceived tations for cooperation, alliance, or reconciliation. risk, and its expected distribution within the Indeed, the psychology of cooperation and for- group, as well as the expected distribution of giveness among groups is well studied spoils antebellum. The enforcement of the risk (McCullough 2008). The most fruitful way for- contract on others depends on an individual’s ward in these respective areas of research (on war ability and willingness to pay costs to reward and cooperation) is to examine the ebb and flow of and/or punish others for the sake of labor recruit- motivations and dynamics related to each. For ment and alliance maintenance. Thus, while the example, when and why do conflicts escalate or main challenge of the first component is one’s deescalate? Such a question can only be answered own willingness to participate, the main challenge by acknowledging both the human capacity for of the second component is to manipulate the violence and the human capacity for cooperation. willingness of others. 8 Conditions Required for Evolution of Warfare Adaptations

Researchers must also carefully examine sex whither and perish. Warfare is, at least in this differences in aggression. There is tremendous superficial sense, a process of group “selection.” evidence of reliable and cross-cultural differences As a rough illustration, if a heritable trait in the way males and females reason about and emerges within the individuals of a group that engage in warfare (Van Vugt 2009; McDonald has the effect of making that group more likely et al. 2012). The theories of sexual selection and to win battles, then these genes would prosper, parental investment (Trivers 1972) provide a basis while alternative genes in the individuals whose upon which to explain the distribution of sex group was vanquished would disappear. The differences within and between species, and result would be that directional change in gene these theories have also been used to successfully frequencies is driven by its effects on group suc- explain a range of sex differences in humans, cess, not individual success. This conceptualiza- particularly regarding individual and coalitional tion led early critics of group selection to claim aggression (Van Vugt 2009). Importantly, the evi- that this process can only work when there is no dence does not show that females never fight; migration between groups and when the rate of rather, the relevant distinction is to be found in group extinction is high (Maynard Smith 1976). the conditions under which males and females Later proponents of group selection argued that fight (Brooks and Valentino 2011; Lopez both early conceptions of group selection and et al. 2011). restrictions offered by critics were excessive or imprecise (Sober and Wilson 1999; Nowak Research Note: The Levels of Selection et al. 2010). More recently, scholarship has Problem focused on the possibility that the evolution of The above discussion has focused on the fitness warfare has occurred via the simultaneous inter- costs and benefits of warfare for individuals and action of multiple levels of selection. genes. For instance, under certain conditions, Game theoretic evidence has modeled the via- genes that code for behavior-regulatory mecha- bility of warfare evolving via group selection nisms that motivate participation in warfare can given what is known about the likely prevalence spread either because of the associated benefits of ancestral intergroup conflict (Choi and Bowles that accrue to the individual or despite significant 2007). Nevertheless, critics of group selection costs – even death – to the individual. In the latter continue to make two points in particular. First, case, a gene can spread despite costs to the organ- the conditions are rare or absent that would be ism when the on-average result is genetic selfish- necessary for group selection to build complex ness. This process is known as genetic selection adaptations, and modern genetic data are incon- and can result in patterns of behavior as seemingly sistent with predictions from group selection dissimilar as maternal altruism and battlefield her- (Langergraber et al. 2011). Second, although oism. In the former case, however, a gene can group selectionists argue that behaviors such as spread simply because it produces direct positive “prosociality,” heroism, and other forms of social fitness consequence for the organism in which it altruism can only be explained by group selection, finds itself. This is known as individual selection critics argue that these are adequately – and more and is the more commonly understood level of parsimoniously – explained by natural selection selection in evolutionary theory. operating at the individual or genetic level In addition to genetic and individual level selec- (Williams 1966; Price 2012). tion, however, it is theoretically possible for a gene to spread in a population because of the benefits it generates for the group of organisms in which it Conclusion finds itself. This is known as group selection. This is significant for the current discussion because The foregoing discussion is an attempt to outline a warfare can be thought of as a process by which set of conditions that may facilitate the evolution some groups survive and expand while others of adaptations for certain forms of intergroup Conditions Required for Evolution of Warfare Adaptations 9 violence. After defining the relevant terms, this Warfare is a collective action problem and effort consists of several components. First, it is therefore presumes the existence of cognitive necessary to carefully identify the reproductive machinery that enables individuals to solve com- costs and benefits of fighting and to identify the plex coalitional coordination problems. Thus, in conditions under which the benefits of warfare order for warfare to evolve via natural selection, would have outweighed its related costs. Second, the reproductive benefits must outweigh the if current or subsequent research can demonstrate related costs of warfare, but individuals must that these conditions held ancestrally, then support also be able to navigate complex coalitional land- is greater for the possibility that some features of scapes in the first place. human warfare can be explained as products of In addition to an elaboration of the conditions human adaptations for intergroup violence. for the evolution of warfare and the search for Adaptations for warfare can evolve when coa- ancestral evidence that such conditions existed, litions are faced with recurrent and reproductively future research should continue to dissect the significant opportunities for low-risk/high-benefit information-processing structure of adaptations violence. Resultant adaptations should carefully designed for warfare by examining the adaptive monitor relevant variables such as relative coali- logic of related motivations such as revenge, tion size, the element of surprise, and opportuni- intergroup anger and reconciliation, xenophobia, ties for escape – the ancestral analogue of the “exit morality and sacred values, and sex differences in strategy.” Indeed it is known that in chimpanzees aggression. Last, but not least, researchers must such opportunities exist and are reproductively continue to develop models of multilevel selec- beneficial, and the pattern of raiding behavior tion to understand the complex interplay of selec- conforms to this adaptive conditionality tion pressures that operate not only at the level of (Wrangham 1999; Wilson et al. 2014). Raiding the gene but also at the level of individuals and in humans does not perfectly conform to the chim- groups. panzee model due to the fact that human raiding can entail higher physical risk to participants, and inter-coalition relationships are characterized by Cross-References peaceful as well as agonistic relationships. Adaptations for warfare can also evolve even in ▶ Aggression the face of steep somatic and reproductive costs, as ▶ Assessment that Victory will Produce Net long as the fitness benefits of coalitional violence Resources are even greater. This is possible in a world in ▶ Benefits of Aggression which reproductive resources and opportunities ▶ Chimpanzee Raiding are allocated within the victorious coalition on a ▶ Coalitional Relationships zero-sum basis. Although a high-casualty war of ▶ Competition Between Groups attrition between two large coalitions results in ▶ Conditions Define Risk Contract of War sizeable fatalities for both coalitions, the zero-sum ▶ Context-Specificity of Aggression allocation of reproductive resources for the winners ▶ Cooperative Alliances means that the surviving victors stand to reap tre- ▶ Costs of Aggression mendous reproductive benefits. When selection is ▶ Evolved Psychology of Warfare operating at the level of the gene, the result can be ▶ Group-Level Costs of Aggression adaptations that motivate what appear to be indi- ▶ Lethal Violence vidually irrational and even reckless participation ▶ Male warrior Hypothesis in violent conflict. In other words, selfish genes can ▶ Sex Differences in Aggression motivate risky behaviors in individuals when the ▶ Violence reproductive benefits over evolutionary time out- ▶ Warfare weigh the fact that such behavior can occasionally ▶ Warfare as Social Contract Among Coalition lead to individual death. Members 10 Conditions Required for Evolution of Warfare Adaptations

References Maynard Smith, J. (1993). The theory of evolution. Cam- bridge, UK/New York: Cambridge University Press. Allen, M. W., & Jones, T. L. (Eds.). (2014). Violence and Retrieved from http://www.loc.gov/catdir/description/ warfare among Hunter-Gatherers. Walnut Creek: cam025/93020358.html. Routledge. McCullough, M. (2008). Beyond revenge: The evolution of Bowles, S. (2009). Did Warfare among ancestral Hunter- the forgiveness instinct (1st ed.). San Francisco: Jossey- Gatherers affect the evolution of human social behav- Bass. iors? Science, 324(5932), 1293–1298. doi:10.1126/ McDonald,M.M.,Navarrete,C.D.,&VanVugt,M.(2012). fl science.1168112. Evolution and the psychology of intergroup con ict: The Brooks, D. J., & Valentino, B. A. (2011). A war of one’s male warrior hypothesis. Philosophical Transactions of own: Understanding the gender gap in support for war. the Royal Society, B: Biological Sciences, 367(1589), – Public Opinion Quarterly, 75(2), 270–286. 670 679. doi:10.1098/rstb.2011.0301. doi:10.1093/poq/nfr005. Nowak, M. A., Tarnita, C. E., & Wilson, E. O. (2010). The Choi, J. K., & Bowles, S. (2007). The coevolution of evolution of eusociality. Nature, 466(7310), – parochial altruism and war. Science, 318(5850), 1057 1062. doi:10.1038/nature09205. 636–640. doi:10.1126/science.1144237. Parker, G. (1974). Assessment strategy and the evolution fi Dawkins, R. (1976). The selfish gene. Oxford: Oxford of ghting behaviour. Journal of Theoretical Biology, – University Press. 47(1), 223 243. Dawkins, R. (1983). The extended phenotype: The long Pinker, S. (2011). The better angels of our nature: Why reach of the gene. Oxford/New York: Oxford Univer- violence has declined. New York: Viking Adult. sity Press. Price, M. (2012). Group selection theories are now more Dawkins, R. (1986). The blind watchmaker. New York: sophisticated, but are they more predictive? A review of Norton. Samuel Bowles and Herbert Gintis, a cooperative spe- Gneezy, A., & Fessler, D. M. T. (2011). Conflict, sticks and cies: Human reciprocity and its evolution. Evolutionary – carrots: War increases prosocial punishments and Psychology, 10(1), 45 49. rewards. Proceedings of the Royal Society B: Biologi- Sober, E., & Wilson, D. S. (1999). Unto others: The evo- fi cal Sciences. doi:10.1098/rspb.2011.0805. lution and psychology of unsel sh behavior. Cam- Hamilton, W. D. (1964). Genetical evolution of social bridge, MA: Harvard University Press. behaviour I. Journal of Theoretical Biology, 7(1), 1–16. Tooby, J., & Cosmides, L. (1988). The evolution of war Johnson, D. D. P., & Toft, M. D. (2014). Grounds for war: and its cognitive foundations. Institute for Evolutionary – The evolution of territorial conflict. International Secu- Studies Technical Report, 88 1. Retrieved from http:// rity, 38(3), 7–38. doi:10.1162/ISEC_a_00149. www.psych.ucsb.edu/research/cep/papers/ Kurzban, R., Tooby, J., & Cosmides, L. (2001). Can race EvolutionofWar.pdf be erased? Coalitional computation and social catego- Trivers, R. (1972). Parental investment and sexual selec- rization. Proceedings of the National Academy of Sci- tion. In Sexual selection and the descent of man. Chi- ences, 98(26), 15387–15392. cago: Aldine Publishing. Langergraber, K., Schubert, G., Rowney, C., Wrangham, Van Vugt, M. (2009). Sex differences in intergroup com- R., Zommers, Z., & Vigilant, L. (2011). Genetic differ- petition, aggression, and warfare: The male warrior entiation and the evolution of cooperation in chimpan- hypothesis. Annals of the New York Academy of Sci- – zees and humans. Proceedings of the Royal Society B: ences, 1167, 124 134. doi:10.1111/j.1749- Biological Sciences, 278(1717), 2546–2552. 6632.2009.04539.x. doi:10.1098/rspb.2010.2592. Williams, G. C. (1966). Adaptation and natural selection: Leblanc, & Register. (2003). Constant Battles: The Myth of A critique of some current evolutionary thought. the Peaceful, Noble Savage with Katherine E. Register. Princeton: Princeton University Press. St. Martin’s Press: New York. http://us.macmillan.com/ Wilson, M. L., Boesch, C., Fruth, B., Furuichi, T., Gilby, ... books/9780312310905. I. C., Hashimoto, C., , & Wrangham, R. W. (2014). Lopez, A. C. (2016). The evolution of war: Theory and Lethal aggression in Pan is better explained by adaptive controversy. International Theory, 8(1), 97–139. strategies than human impacts. Nature, 513(7518), – doi:10.1017/S1752971915000184. 414 417. 10.1038/nature13727. Lopez, A. C., McDermott, R., & Petersen, M. B. (2011). Wrangham, R. (1999). Evolution of coalitionary killing. – States in mind: Evolution, coalitional psychology, and Yearbook of Physical Anthropology, 42,1 30. international politics. International Security, 36(2), Wrangham, R. W., & Glowacki, L. (2012). Intergroup 48–83. aggression in chimpanzees and war in nomadic Maynard Smith, J. (1976). Group selection. Quarterly hunter-gatherers: evaluating the chimpanzee model. Review of Biology, 51, 277–283. Human Nature. doi:10.1007/s12110-012-9132-1.