Humans are an unusually prosocial species. We volunteer at food banks, recycle, vote, tithe, give blood, and go to war. We care about justice and fairness, and punish those that transgress against social norms. Although altruistic behavior is well-documented in other primates, the range of altruistic behaviors in other primate species, including the great apes, is much more limited than it is in humans. Moreover, when altruism does occur among other primates, it is typically limited to familiar group members—close kin, mates, and reciprocating partners. It is not clear whether some of the most compelling naturalistic examples of “altruistic” behavior among nonhuman primates, such as food sharing, are the product of other-regarding social preferences or more instrumental motives. Silk will discuss a body of experimental research which is designed to reveal the preferences that underlie behavior. These experiments suggest that chimpanzees are not consistently motivated to provide benefits to familiar partners, are tolerant of inequity, and act punitively only after personal losses. Silk will also discuss a body of parallel experiments conducted with children. This work shows that children behave very differently from other apes, and that the social preferences that underlie their behavior are influenced by both their age and the cultural context in which they live. Taken together, these data suggest that human social preferences are derived traits that evolved after the human/ape lineages split 5-8 million years ago.