Self-serving intergroup aggression escalates and prevails over parochial cooperation

Anthropological evidence suggests that intergroup conflict in small-scale societies frequently involves opportunistic attacks by small groups of raiders. These incursions are low-risk and often offer immediate benefits to their participants; however, they can also fuel cycles of revenge between groups, exposing raiders’ in-group members to counterattacks. The existing experimental literature on intergroup conflict has mostly assumed that conflict participation is individually costly but benefits the in-group, leaving the aforementioned form of intergroup aggression understudied. In this research, we used an economic game paradigm that affords both self-serving intergroup aggression, which we term “individual exploitation,” and self-sacrificial conflict participation to investigate the prevalence and determinants of the former and compare it with the latter. In two incentivised behavioural experiments with UK online participants ( N s = 647 and 272), we found prevalent and escalating individual exploitation – consistent with small-scale warfare in pre-state societies – but little self-sacrificial and cooperative conflict participation. We also show that intragroup comparisons can drive such escalation, and costly peer punishment is largely ineffective in containing individual exploitation and promoting in-group cooperation. Our results imply that understanding the human psychology of intergroup aggression requires more consideration of individual exploitation.