One of the outstanding questions for biologists and social scientists has been the evolution of cooperation. Cooperation is observed both among humans as well as among animals. The question is “How cooperation can evolve”? This was an intriguing question since Darwin’s Origin of species was published. The problem is that the basic tenet of evolution by natural selection is “the survival of the fittest”. This prevents individuals from cooperating since this decreases individual fitness due to the cost cooperation requires. For example, wearing a face mask during a pandemic is an act of cooperation. It causes some discomfort to individuals but benefits the whole society. Mathematical models have played a crucial role in our understanding of mechanisms leading to evolution of cooperation. Cooperation between two individuals is described by the Prisoner’s dilemma. Cooperation in larger groups is described by the Public goods game. To explain evolution of cooperation among members of groups emphasizes group cohesion and cultural norms to explain the “prosocial” outcomes of public goods games. However, the Public goods game shows that defection (i.e., selfish behavior) is still an evolutionary outcome in that all individuals will eventually defect. A new mechanism that promotes cooperation both in the Prisoner’s dilemma and the Public goods game was shown by Krivan and Cressman (2020). This mechanism is based on the assumption that individuals are free to leave their group and form a new group. Experimental work on such opting out behavior shows that, not surprisingly, individuals are more APT to leave groups as the number of defectors in the group increases. Indeed, Krivan and Cressman (2020) show that such behavior promotes evolution of cooperation, provided the time individuals can stay in a group increases. One may assume that cooperation will be promoted if individuals opt out against defectors in the hope of forming a new group with all (or more) cooperators. Surprisingly, Cressman and Krivan (2020) show clearly that when individuals are free to leave their current group, the best opting out rule at promoting cooperation is one whereby the only groups that voluntarily stay together between rounds are those that are homogeneous (i.e., those groups that are either all cooperators or all defectors), when these groups stay together for enough rounds of the game (i.e., long enough). This outcome emerges when defectors are completely intolerant of individuals who cooperate (e.g., defectors exhibit xenophobic behavior toward cooperators) and so opt out whenever their group has a cooperator in it. The strong preference by defectors to be with like-minded individuals then causes all heterogeneous groups to disband after one round.

Křivan, V., Cressman, R. (2020) Defectors’ intolerance of others promotes cooperation in the repeated public goods game with opting out. Scientific reports, to be pblished on November 11, 2020.