Much of the cooperation and altruism observed in nonhuman animals is directed toward genetic relatives. However, cooperation among non-relatives also occurs, particularly in social mammals such as lions and meerkats, as well as in several species of primates. Most examples of cooperation among non-relatives in nonhuman animals are best explained by mutualism, in which both partners gain immediate benefits from their behavior . For example, in wild dogs, cooperation between hunting partners can increase their per capita success in catching or defending prey ). An alternative form of cooperation among non-relatives is reciprocal altruism, in which providing assistance has net costs at the time it is provided which are offset by subsequent benefits . One significant difference between mutualism and reciprocal altruism is that selection can favor cheating and exploitation in the latter but not in the former. It may be this barrier to the evolution of reciprocal altruism that accounts for the limited number of documented cases of reciprocal altruism among nonhuman animals. In contrast to the situation in nonhuman animals, however, reciprocal altruism is pervasive in human society , as evidenced by massive holiday gift exchanges and by the social debt we feel after having accepted a favor, as reflected in the commonly heard phrase, "I owe you one." Our penchant for reciprocity is also the very foundation of the division of labor upon which our economy is based.At the most basic level, altruism toward non-relatives is an evolutionary puzzle since individuals who sacrifice personal fitness to increase the fitness of others will be selected against. However, a number of potential explanations have been offered for the evolution of altruistic behavior in humans. One is that the short-term costs