In explaining the sources of cooperation and prosocial behavior, psychologists, behavioral economists, and biologists often focus on factors internal to the individual, such as altruistic motivations, other-regarding preferences, and prosocial emotions. By contrast, sociologists typically emphasize social forces external to the individual, including norms, reputation systems, and social networks. Here we review evidence for these rules, reputations, and relations, showing that they have powerful and pervasive effects on cooperation and prosocial behavior. Our discussion highlights two emergent themes of the reviewed literature. First, although these classes of sociological mechanisms typically promote cooperation, their presence can also create ambiguity for individuals regarding the reasons for their own and others’ prosocial acts, and that ambiguity can undermine future prosociality in subsequent settings where the mechanisms are absent. Second, altruistic preferences and social mechanisms often interact, such that the causal significance of altruism is attenuated where these mechanisms are present.