Intergroup aggression in primates and humans: The case for a unified theory

Abstract

Human warfare and intergroup aggression among primates have traditionally been considered to be largely unrelated phenomena. Recently, however, chimpanzee intergroup violence has been proposed to show evolutionary continuities with war among small-scale societies because both systems involve interactions among temporary subgroups, deliberate attempts to hunt and maim, and demographically significant death rates. Here, we ask whether the functional similarities between intergroup aggression among humans and chimpanzees can be extended to troop-living primates. In most primates, patterns of intergroup aggression involve brief encounters among stable troops, rare violence, and almost no killing. Although they, therefore, show little behavioral resemblance to warfare, growing evidence indicates that intergroup dominance is adaptively important in primates because it predicts long-term fitness. This suggests that in all primates, including humans, individuals use coalitions to maintain or expand access to resources by dominating their neighbors. Thus, while the style of coalitionary aggression depends on each species' evolutionary ecology, we propose that the essential functional reasons for intergroup competition are consistent across group-living primates and humans: strength in numbers predicts long-term access to resources.