Group members often reason egocentrically, believing that they deserve more than their fair share of group resources. Leading people to consider others members' perspectives can reduce these egocentric (self-centered) judgments, such that people claim that it is fair for them to take less, but it actually increases egoistic (selfish) behavior, such that people actually take more of available resources. Four experiments demonstrate this pattern in competitive contexts where considering others' perspectives activates egoistic theories of their likely behavior, leading people to counter by behaving more egoistically themselves. This reactive egoism is attenuated in cooperative contexts. Discussion focuses on the implications of reactive egoism in social interaction, and on strategies for alleviating its potentially deleterious effects.