Three studies examined the communication of naïve theories of social groups in conversations between parents and their 4-year-old children (N = 48). Parent-child dyads read and discussed a storybook in which they either explained why past social interactions had occurred (Study 1) or evaluated whether future social interactions should occur (Studies 2 and 3). In all 3 studies, the content of parents' and children's explanations reflected an intuitive theory of social groups as markers of intrinsic obligations, whereby individuals are obligated to avoid harm to and direct positive actions toward their in-group members. Furthermore, Studies 2 and 3 suggested that when discussing the normative obligations that guide behavior, parents covertly reinforce their children's developing beliefs about social categories. Implications for the development of social cognition are discussed.