Partner Selection and Evolution of Out-Group Avoidance

Preferential treatment of in-group members is widely observed in nature. This study examines this phenomenon in the context of cooperation in social dilemmas using evolutionary agent-based models that consider the role of partner selection. Particularly, the model considers a repeated prisoner's dilemma game in which agents belong to one of two groups distinguished by the continuation probability of pair interactions. Based on the behavior in the last round and the group affiliation of the partner, each individual selects to cooperate, defect, or stop interactions and search for a different partner. The simulation results demonstrated that agents adopted cooperative strategies, including tit-for-tat and out-for-tat, toward in-group members. In contrast, agents terminate interactions immediately after pair formation without observing their partner's behavior when paired with an out-group individual. This pattern arises even when the continuation probability of pairs comprising members of different groups is sufficiently large to sustain cooperation when applied uniformly to the entire population. A higher continuation probability with in-group partners hinders interaction with out-group individuals. These results indicate the importance of avoidance in intergroup interactions in social dilemmas and might explain in-group favoritism without animosity toward out-groups.