Developmental research has long sought to understand children's social ideas, and particularly how those ideas influence their judgments and behaviors toward other people. We examine the idea of "common humanity," a social idea that has been investigated historically and philosophically, to re-consider what is already known about children's notions of social equality and difference and to suggest how research in this area might be expanded. We review the social cognition literature to argue that studying the idea of common humanity--which combines abstract notions of universal equality with explicit attention to social group differences--could substantially advance our understanding of children's social-cognitive frameworks. We then discuss a subset of the literature related to children's prejudice and describe how the study of common humanity could shed light on the mechanisms behind interventions that successfully reduce prejudice.