The scientific study of the evolution of human coalitional aggression has exploded over the last three decades. In four parts, I explore and integrate many of the useful frameworks that have emerged to describe and explain the human practice of intergroup violence. First, we have a clearer understanding of the general conditions required for the evolution of adaptations for coalitional aggression. Second, given an understanding of these conditions, we can more usefully examine the historic and prehistoric record for evidence of the existence of these conditions. Third, I explore and integrate current lab and field evidence for psychological adaptations for coalitional aggression. This section reveals a core dynamic underlying all forms of coalitional aggression: the form of intergroup engagement is functionally linked with the emergent patterns of intragroup dynamics. In other words, how we fight “abroad” determines how we cooperate “at home,” and vice versa. I examine five areas of inquiry that suggest special design for coalitional aggression. These are: the collective action problem of coordinated violence; parochial altruism; attacker-defender asymmetries; leader-follower dynamics; sex differences in the costs and benefits of violence. Fourth, and to conclude, I offer speculation on the historical emergence of modern human warfare. I do not use “coalitional aggression” and “warfare” interchangeably; rather, evolved psychological adaptations for small-scale coalitional aggression are what make the historical emergence of large-scale human warfare possible.

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.

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.