Concepts of national groups (e.g., Americans, Canadians) are an important source of identity and meaning in people's lives. Here, we provide a developmental investigation of these concepts. Across 3 studies involving 5- to 8-year-olds and adults in the United States, we found that (a) compared with older children and adults, young children were more likely to think that national groups have a biological basis, but that (b) other aspects of national group concepts-such as the idea that national group membership is stable and informative about a person-changed less with development. Moreover, with age, the notion that membership in a national group is a meaningful fact about a person (vs. a mere formality) began to link up with attitudes that rationalized the national ingroup's economic advantages and portrayed it as superior to national outgroups. This work contributes to theory on the development of social cognition and provides a unique source of insight into current political trends. (PsycINFO Database Record (c) 2019 APA, all rights reserved).