Article The Geography of

Hammond and Axelrod use an evolutionary agent-based model to explore the development of ethnocentrism. They argue that local interactions permit groups, relying on in-group favoritism, to overcome the Nash equilibrium of the prisoner’s dilemma and sustain in-group cooperation. This article shows that higher levels of cooperation evolve when groups are dropped from the model, breaking the link between ethnocentrism and cooperation. This article then generalizes Hammond and Axelrod’s model by parameterizing the underlying geographical assumptions they make about the evolutionary environment. This more general model shows that their findings are sensitive to these assumptions and that small changes to the assumed geography of reproduction significantly affect the probabilities of finding ‘‘ethnocentric’ ’ behaviors. The model presented here indicates that it is not local interactions, per se, but settings where interactions are highly likely to be with close relatives that lead to ‘‘ethnocentrism’ ’ as modeled by Hammond and Axelrod. Keywords prisoner’s dilemma, ethnocentrism, agent-based models, evolutionary models, contingent cooperation Hammond and Axelrod (2006; hereinafter HA) present an agent-based model to show how ethnocentrism can develop in a system in which multiple groups structure social interaction and find that in-group favoritism can support high levels o