Four studies examined children's (ages 3-10, Total N = 235) naive theories of social groups, in particular, their expectations about how group memberships constrain social interactions. After introduction to novel groups of people, preschoolers (ages 3-5) reliably expected agents from one group to harm members of the other group (rather than members of their own) but expected agents to help members of both groups equally often. Preschoolers expected between-group harm across multiple ways of defining social groups. Older children (ages 6-10) reliably expected agents to harm members of the other group and to help members of their own. Implications for the development of social cognition are discussed. (Contains 2 tables and 2 figures.)