The behavior of women and men varies greatly depending on situations, cultures, and historical periods. This flexibility emerges as men and women tailor their division of labor to local ecological and socioeconomic demands. The resulting division is supported by childhood socialization practices that, in interaction with sex differences in child temperament, help boys and girls to develop psychologies suited to their likely adult activities. Although responsive to local conditions, the division of labor is constrained by women's childbearing and nursing of infants and men's size and strength. Because these biological characteristics influence the efficient performance of many activities in society, they underlie central tendencies in the division of labor as well as its variability across situations, cultures, and history. Gender roles—that is, shared beliefs about the traits of women and men—track the division of labor because people infer these traits from their observations of the sexes’ behaviors. Social perceivers often essentialize these traits by regarding them as inherent in the biology or social experience of women and men. Gender role expectations, which tend to be consensual within cultures, influence behavior through proximal social psychological and biological processes, whereby (a) other people encourage gender-typical behavior and individuals conform to their own gender identities and (b) hormonal, reward, and cardiovascular mechanisms enable masculine and feminine behaviors.