Social cohesion and the evolution of altruism.

In this paper we propose a stylized model to study how cohesion may affect the spread and consolidation of altruism in a large population where agents are involved in a local public-good contribution game with their neighbors. We show that, if the contribution cost is moderate (neither too high nor too low), cooperation can invade and dominate the population if, and only if, group cohesion displays an intermediate value. This reflects an interesting non-monotonicity of cohesion in the evolution of altruism: while some of it is needed to internalize the benefits of cooperation, too much cohesion prevents the spread of altruism among the population at large.