Adults and children around the world seem to treat categories of people as if they have distinct inherent essences, which make the categories incompatible and permanent. In this talk, I will examine some of the factors that may contribute to the development of such essentialist beliefs about social categories. In particular, I will present data from a recent developmental cross-cultural study on children's social categorization. I will then discuss some ideas about the nature and origins of these beliefs, and describe cultural practices that may help sustain them. I will end with speculations about the possible adaptive function of such beliefs.