Altruists Egoists and Hooligans in a Local Interaction Model

This paper studies a population of agents, each of whom can be either an Altruist or an Egoist. Altruists confer benefits on others at a cost to themselves. Altruism is thus a strictly dominated strategy and cannot survive if agents are rational best-responders. We assume that agents choose their actions by imitating others who earn high payoffs. We also assume that interactions between agents are local, so that each agent a ects (and is affected by) only his neighbors. Altruists can survive in such a world if they are grouped together, so that the benefits of Altruism are enjoyed primarily by other Altruists, who then earn relatively high payo s and are imitated. The addition of mutations to the model threatens this survival by allowing such groups to be invaded by mutant Egoists. However, mutations can also be inimical to Egoists, by removing the Altruists which Egoists exploit,and the net effect of mutations is to ensure that Altruists survive.