Hawkish but helpful: When cultural group selection favors within-group aggression

The origin of cooperation is a central problem in evolutionary biology and social science. Cultural group selection and parochial altruism are popular but controversial evolutionary explanations for large-scale cooperation. Proponents of the cultural group selection hypothesis argue that the human tendency to conform—a consequence of our reliance on social learning— maintained sufficient between-group variation to allow group selection (which favors altruism) to overpower individual selection (which favors selfishness), whereupon large-scale altruism could emerge. Proponents of the parochial altruism hypothesis argue that altruism could emerge in tandem with hostility toward other groups if the combination of the two traits increased success in inter-group contests. Proponents of both hypotheses assume that cooperation is altruistic and that within-group conflict is antithetical to cooperation, implying that group selection for cooperation reduces within-group conflict. Yet within-group conflict need not be antithetical to cooperation. This essay uses a mathematical model to show that selection between groups can lead to greater within-group aggression if within-group aggression enhances the value of individually costly public goods contributions. This model may help to explain cross-cultural associations between warfare, socialization for aggression, aggressive sports, and interpersonal violence among humans. It may also apply to other forms of inter-group conflict among humans. Finally, the model suggests that group selection can lead to disharmony within groups, a caveat to the use of group selection models to inform social policy.