Strategy Assortativity and the Evolution of Parochialism

This paper investigates the role of strategy assortativity for the evolution of parochial- ism. Individuals belonging to different groups are matched in pairs to play a prisoner dilemma, conditioning their choice on the identity of the partner. Strategy assortativ- ity implies that a player is more likely to be matched with someone playing the same strategy. We find that, if the degree of strategy assortativity is sufficiently high, then parochialism (i.e., cooperate with your own group and defect with others) spreads over a group, while egoism (i.e., defect with everyone) emerges otherwise. Notably, parochialism is more likely to emerge in a smaller group.