This article reviews findings from anthropology, psychology, and other disciplines about the role of biological factors in the development of sex differences in human behavior, including biological theories, the developmental course of sex differences, and the interaction of biological and cultural gendering processes at different ages. Current evidence suggests that major biological influences on individual differences in human gender, to the extent that they exist, operate primarily in early development, during and especially prior to puberty. Biological effects are likely to be mediated by relatively simple processes, like temperament, which are then elaborated through social interactions (as with mother and peers) into more complex gendered features of adult personality. Biological anthropologists and psychologists interested in gender should direct more attention to understanding how social processes influence the development and function of the reproductive endocrine system.