Science recently published a Special Issue on Human Conflict that featured research on the dynamics of inter-group conflict, aggression, and war. For the most part, the special issue does not feature original research, but rather provides a survey of significant research by authors who have been active in these areas. The following is meant to highlight and review a selection of articles from the special issue in order to reveal central debates and considerations.

Warfare and human nature seem inextricably linked. But what do we mean by “human nature,” and what evidence would we require as proof that warfare is an indelible feature of the human condition? Our folk intuitions regarding biology (which appears to us as existing within the realm of things you are “stuck with”) and culture (which seems within the realm of things you can change) suggest that if there is an evolutionary explanation for warfare, then we must be stuck with it and it must be universal. This type of reasoning is about as popular as it is false, but unfortunately the articles by Douglas Fry and by Frans de Waal fall into this categorical trap. In essence, Fry and de Waal believe that the capacity for peacemaking and the existence of peaceful societies disproves an evolutionary explanation for war. In other words, they argue that if warfare has an evolutionary explanation, then it must in some way be “the norm” among human societies, and since warfare does not seem to be the norm, warfare has no evolutionary roots. This is a crucial fallacy that not only reveals an incomplete understanding of modern adaptationism, but also hinders useful research on warfare.

War and Human Nature

Frans de Waal, a primatologist, begins by (correctly) lamenting the fact that the study of human behavior has too often emphasized our aggressive tendencies. According to de Waal, one simple way we can see that aggression is not a “hallmark of humanity” (whatever that means) is that it does not seem to come with any “built-in rewards.” For example, de Waal offers, humans enjoy and find fulfillment in sex and socializing, and these activities are necessary for survival and reproduction. Similarly, if aggression is a natural human tendency, we should willingly engage in warfare and enjoy doing so. Since de Waal finds examples of humans resisting the urge to kill, he argues that there must be no “genetic basis” for lethal aggression. Unfortunately, there are several problems with this logic.

First, there is no sound evolutionary reason why all species-typical traits must come with built-in rewards. To take de Waal’s own examples, both sex and socializing also come with many evolutionarily sensible anxieties regarding inadequacy and relative status, and which do not generate positive affect. To take another set of examples, evolutionary scientists have identified a range of phobias that are reliably-developing species-typical adaptations in humans. Needless to say, fears and phobias tend not to motivate behavior by triggering pleasure centers in the brain. Does it make any sense to say that predator avoidance (and certain forms of error management) are therefore not evolutionary products just because we don’t find fulfillment in escaping deadly attacks by predators? In short, natural selection may engineer a range of rewards and costs to motivate behavior. Second, evidence that many soldiers resist the order to kill is not evidence that there is no evolutionary explanation for when and why humans should want to engage in lethal aggression. For every story of soldiers resisting the order to attack, there are others depicting horrible atrocities committed willingly and with excitement. The former is no more evidence of our “innate peacefulness” than the latter is evidence of our “innate aggressiveness.” More to the point, however, if humans possess adaptations for the regulation of inter-group aggression, these systems would be designed for small-scale engagements. It should therefore be unsurprising that participation in sustained large-scale warfare should elicit psychological trauma given the intensity of such supernormal stimuli. The question is not whether participation in warfare is a variable; clearly it is. Rather, the question is: how can evolutionary theory explain the context-specificity of such decisions?

Frans de Waal devotes much of his article to demonstrating that, on balance, humans spend most of their time not engaging in war. Instead, we often trade, intermarry, and travel between groups. Again, this is absolutely true and uncontested, but it’s unclear what exactly this has to do with the evolution of warfare. Humans spend most of their time not having sex; does this suggest that evolution has designed us for abstinence, or that sex is a cultural invention? Humans who live in equatorial regions probably spend most of their life not shivering; does this imply that shivering does not have a biological basis since some cultures are rarely if ever observed shivering? De Waal’s logic here makes little sense unless you believe that biology can only be invoked to explain behaviors that are unconditional, perpetual, and universal. Modern adaptationism, and the entire field of behavioral ecology, has thrived on the understanding that animals capable of complex behavior possess psychological adaptations that enable a range of context-specific behavior. Importantly, the false dichotomy between biology-as-inflexible and culture-as-conditional is now exposed, yet few are willing to see it. Inability to recognize this fundamental truth necessarily stands in the way of a clearer scientific understanding of our dynamic evolutionary legacy as humans. Although de Waal’s claims regarding the peacemaking abilities of humans and non-human primates are valid and should be taken seriously, evidence of an evolutionary basis for peacemaking is not evidence against an evolutionary basis for conflict and aggression.

Conflict Management in Primates

Chris Boehm, author of Hierarchy in the Forest, describes the similarities among and differences between humans, chimpanzees, and bonobos, and argues that humans have inherited evolved capacities for both conflict and peacemaking. First explaining important similarities, Boehm points out that humans, chimps, and bonobos each live in bounded social groups, display dominance and submission behavior, and engage in cooperative coalitions, especially in response to threats by out-group males. Bonobos, however, are famously the least aggressive of the three, in which females may be co-dominant with males, and conflicts tend to be both less dangerous and less frequent. Chimpanzees live in social groups characterized by clear linear hierarchies, in which male coalitions frequently form, and dyadic and coalitional conflicts can turn deadly. Quite interestingly, among captive groups of chimpanzees, the balance of power between males and females tends to resemble their bonobo cousins, since in these environments females are better able to form strong coalitions. Nevertheless, among both captive and wild populations of chimpanzees, alpha males may intervene in dyadic conflicts, effectively serving as peacemakers and preventing hostility.

Although both conflict and its management are observed in humans, chimps, and bonobos, Boehm argues that in humans these actions are mediated by “moral concerns” such as “a conscience-based sense of right and wrong.” Since this moral sense is unique to humans, it is “not ancestral but derived.” One is left to wonder, however, whether the line between “ancestral” and “derived” is as clear as Boehm implies. As alluded to above, natural selection often designs social behaviors such that they are motivated by a complex set of positive or negative neurophysiological reinforcements, which we may experience as forms of pleasure or pain. Given that chimps and bonobos engage in forms of conflict management, one wonders what the structure of the motivational dynamic is that compels such action? In other words, what do chimps and bonobos experience (i.e. “feel”), neurophysiologically speaking, when engaging in conflict management? This leads to a reformulation of Boehm’s question as the following: In humans, do we truly witness the uniquely and qualitatively new emergence of a moral sense, where none existed before? Or, might it be plausible that the human motivational complex represents the “sharpening” of an inherited but “unrefined” moral sense? These questions are of course speculative, but new research continues at an exciting pace in this area and Boehm’s research provides a nice overview of social dynamics in humans, chimps, and bonobos.

Us and Them

Two articles by Naomi Ellemers and by Richard J. Crisp and Rose Meleady explore the social psychological foundations of human groupishness. Ellemers describes much of what is known as Social Identity Theory (SIT), which, among other things, explains that an individual’s identity is in part a function of the groups to which one belongs. Thus, a better understanding of when these group identities motivate the behavior of individuals can help us to explain the patterns of various forms of inter-group conflict. We rely on group identity to make sense of complex social situations, and when group achievements are tied to an individual’s personal experiences (such as when you help your sports team to victory), the subsequent emotions are often more intense. Importantly, our responses to group threat are not fixed, but rather vary in at least two broad ways. Either individuals can downplay their association with the group, or they may choose to “defend” the group in some way, either because “they care for the group or because escape is difficult.” SIT has revealed remarkably persistent human tendencies (in-group/out-group behavior), but has also revealed that human responses to such dynamics are remarkably variable.

Crisp and Meleady take the analysis a step further by considering the evolutionary roots of human groupishness and its implications for life in modern multicultural nation-states. The authors explain that our inherent tendency to “think categorically” about groups is in part responsible for the observation that “diversity often appears to accompany conflict.” Therefore, if we investigate the psychological processes by which categorizations occur and how they affect motivation, we improve our understanding of how to mitigate conflict in multicultural environments. It seems to follow logically that if “us/them” divisions breed tension, then to the extent that multicultural environments map onto such psychological categories, diversity itself would precipitate conflict. Recent experiments have revealed, however, that cooperation in ethno-religiously diverse societies is very possible given certain types of institutional arrangements. In a sense, this is consistent with Crisp and Meleady’s argument – that conflict born of diversity is not inevitable. However, the psychological mechanisms they provide for arriving at this conclusion are unconvincing.

Crisp and Meleady posit two systems which they believe now constitute human coalitional psychology. The first is an “early-evolved” system for detecting us versus them. The second is “diversity-based cognition,” which acts to inhibit the first system, and promotes “generative thinking” and “creative reconstrual” of others as allies or in-groupers. The authors assert that this second system provided an adaptive advantage and evolved as societies experienced population growth and technological innovation, and as societies became more complex, more mobile, and increasingly diverse. The first system, however, evolved and was adaptive in ancestral environments composed of “clear cut” tribal affiliations, in which “category confusion” was unlikely, and there was therefore little need “to construct cross-cutting tribal alliances.” Unfortunately, the logic of the argument is rendered problematic by much of what we know about ancestral and modern hunter-gatherer socio-ecologies. As Boehm and de Waal have aptly showed, group membership in ancestral societies was in fact not clear-cut. Even bonobos, our most peaceful cousins, engage in complex coalitional maneuvers in order to undercut the efforts of dangerous individuals. Richard Wrangham, who has done much of the leading research on chimpanzee coalitional behavior, describes chimpanzee groups as “fission-fusion” systems, in which alliances form and dissolve regularly. Our ancestral coalitional environment was indeed not one of large-scale multiculturalism, but coalitional plasticity was not something that only became adaptive with the advent of the metropolis. Crisp and Meleady are correct to observe that humans can inhibit and amend many groupish tendencies, but this observation is effectively explained by adaptations designed for ancestral environments, which were anything but “clear cut.” The authors’ narrow conception of system one and the selection pressures that favored it leads them to the false conclusion that a second system is necessary and must have been a response to modern environments. Yet their general argument about the need to better understand our categorical tendencies and their effect on conflict remains important and valid.

War and Peace

Nothing succeeds like success, and according to Sam Bowles, success at waging and winning wars has propelled expansion and innovation in human socio-political organization. War is fundamentally a collective action, and as such, it requires a substantial amount of within-group coordination, cooperation, and altruism. Bowles, as a group selectionist, believes that it is difficult if not impossible to explain human altruism without invoking substantial ancestrally recurrent inter-group conflict, which would have selected for the forms of within-group altruism that we continue to witness today. Bowles argues that our modern achievements of democracy and the attenuation of inequality can be seen in part as the culmination of this evolutionary and historical process. Ironically, therefore, the legacy of war may be a sharpened sense of peace.

Group selection has enjoyed resurgence in the previous two decades, and Bowles has been one of its foremost advocates. There is not space here to review this debate, but one aspect of the argument seems to warrant some scrutiny. Bowles’ group selection model describes two processes: inter-group warfare and reproductive leveling, which together select for altruistic traits over evolutionary time. Reproductive leveling is a process whereby individuals within groups act to “share food and information, to make decisions by consensus, and to gang up on would-be dominants or free riders who would monopolize reproductive and material resources…” In other words, reproductive leveling is a process whereby individuals develop norms to punish unwanted behavior, prevent power monopolies, and enforce egalitarianism. According to Bowles, reproductive leveling is necessary for group selection to operate, and group selection is necessary to explain altruism. The problem with this argument, as others have pointed out, is that reproductive leveling assumes the behavior it seeks to explain. For example, Bowles argues that reproductive leveling results in a “less tilted evolutionarily playing field and thereby [gives] the altruistically inclined a better chance of survival.” However, this amounts to a statement that altruism gives altruism a better chance of survival. But if altruism needs altruism to survive, how do we get started? This is not a new problem, and potential solutions include extremely high rates of between-group mortality, as well as the ability of altruistic “mutants” to form exclusive clusters. Although not insoluble, it would have been useful for Bowles to address this issue, especially since his framing of reproductive leveling invokes a view of the mind as already capable of establishing the types of norms and institutions that reproductive leveling is meant to explain in the first place. The model seems overdetermined.

The Special Issue on Human Conflict features a few other articles on topics such as modeling, post-traumatic stress disorder, and guerilla (not Gorilla!) warfare that are worth taking a look at. Scott Atran and Jeremy Ginges contribute an article on the role of “religious and sacred imperatives in human conflict.” Their article is a useful and engaging discussion of current research in this area, and it explores the two dominant views on the evolution of religion: religion as biological adaptation, serving the function of promoting ingroup cooperation, and religion as the “by-product” of psychological adaptations that are designed for other functions. Since ETVOL will be featuring an interview with Scott Atran in the coming weeks, we postpone a discussion of this research until that time. Stay tuned!

The last decade has seen a renaissance in the evolutionary science of aggression, violence, and war. This year alone Science, Human Nature, and Philosophical Transactions of the Royal Society – B have featured special issues on these questions. Because so much is at stake, and implications are as far reaching as they are complex, readers should be wary of arguments that reduce the question of war or aggression to one side of a general dichotomy such as nature or nurture, good or bad, innate or learned. Such constructs often say more about the lenses we wear to make sense of the world, rather than the world itself.