Adaptive foundation of group-based reciprocity

A one-shot sequential prisoner's dilemma game with an in-group and an out-group member was conducted to test the group heuristic hypothesis for the in-group bias in minimal groups. Eighty-nine participants played the role of a second player and faced a fully cooperative first player. The results showed that in-group bias occurred only in the common knowledge condition in which the first player and the second player mutually knew each other's group membership, but not in the private knowledge condition in which the first player did not know the second player's group membership. These results provide support for the group heuristic hypothesis that in-group bias is an adaptive strategy in an assumed generalized exchange system to avoid a bad reputation as a defector.