Cycles of intergroup revenge appear in large scale conflicts. The authors experimentally test the hypothesis that humans practice group-based reciprocity: if someone harms or helps them, they harm or help other members of that person’s group.

Subjects played a trust game, then allocated money between other people. Senders whose partners returned more in the trust game gave more to that partner’s group members. The effect was about half as large as the effect of direct reciprocity. Receivers' allocations to group members were not affected by their partners' play in the trust game, suggesting that group reciprocity was only triggered when the partner’s intentions were unequivocal.