Violent intergroup conflicts cause widespread harm; yet, throughout human history, destructive hostilities occur time and time again1,2. Benefits that are obtainable by victorious parties include territorial expansion, deterrence and ascendency in between-group resource competition3,4,5,6. 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 all7,8,9. Distinguishing offensive and defensive intergroup aggression provides a partial answer: defensive aggression is adaptive under many circumstances10,11,12,13,14. 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 welfare15,16,17. 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.
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We thank M. Albert, R. Böhm, T. Friehe, M. Greiff, M. Hoffman, R. Ligtvoet, F. Morath, K. M. Zimmermann and the audiences at MPI Bonn, NCBEE 2016 Oslo, ESA 2016 Bergen, GfeW 2016 Giessen and MACIE Marburg for helpful comments and suggestions. G.D. gratefully acknowledges financial support by the Dr. Jürgen Meyer Foundation. H.R. gratefully acknowledges financial support by VU Amsterdam. L.G. gratefully acknowledges support from the Eric M. Mindich Research Fund for the Foundations of Human Behavior, the Mind Brain and Behavior Interfaculty Initiative at Harvard University and the ANR Labex IAST. This work was assisted through participation of L.G. and H.R. in the ‘Evolution & Warfare’ Investigative Workshop at NIMBIOS, sponsored by the National Science Foundation through the NSF Award no. DBI-1300426, with additional support from the University of Tennessee, Knoxville, Tennessee, USA. The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript.