This chapter argues that rationality and self-interest should be decoupled. It argues that privileging rationality and/or self-interest on the basis of intentional fundamentalism, according to which our folk-psychological intentional attributions provide an inherently understandable fundament. Intentional fundamentalism accords rational and self-interested behavior a status of an ideal of natural order and this sets a specific heuristic for deciding what kinds of phenomena are in need of explanation to begin with. The chapter discusses an idea that could serve as an important element in a naturalistic understanding of prevalence of substantially self-interested behavior. According to this idea, social norms that is, prevalent expectations about social behavior play an important role in the generation of self-regarding actions. There is a long and venerable tradition of thinking about explanation of human behavior linking the explanation of action to rationality. Social psychologist Dale Miller has suggested that self-interested action is a social achievement upheld by context-dependent cultural norms.