Spoils division rules shape aggression between natural groups.

Violent intergroup conflicts cause widespread harm; yet, throughout human history, destructive hostilities occur time and time again. Benefits that are obtainable by victorious parties include territorial expansion, deterrence and ascendency in between-group resource competition. Many of these are non-excludable goods that are available to all group members, whereas participation entails substantial individual risks and costs. Thus, a collective action problem emerges, raising the question why individuals participate in such campaigns at all. Distinguishing offensive and defensive intergroup aggression provides a partial answer: defensive aggression is adaptive under many circumstances. However, participation in offensive aggression, such as raids or wars of conquest, still requires an explanation. Here, we focus on one condition that is hypothesized to facilitate the emergence of offensive intergroup aggression: asymmetric division of a conflict's spoils may motivate those profiting from such inequality to initiate between-group aggression, even if doing so jeopardizes their group's welfare. We test this hypothesis by manipulating how benefits among victors are shared in a contest experiment among three Ethiopian societies whose relations are either peaceful or violent. Under equal sharing, between-group hostility increased contest contributions. By contrast, unequal sharing prompted offensive contribution strategies in privileged participants, whereas disadvantaged participants resorted to defensive strategies, both irrespective of group relations.