Boys and girls, and men and women show consistent differences, on average, in interests, activity preferences, and social styles. This article summarizes sex differences in human development from infancy through the childhood years and considers how these differences in developmental patterns relate to human evolutionary history. Evidence is reviewed suggesting that the psychological traits that were advantageous differed consistently for men and women during human evolution, consistent with Darwin's (1871) sexual selection as a mechanism through which cognitive and behavioural sex differences evolve and develop during lifetimes. The result is that some sex differences are found very early in development in predispositions to engage in different activities, to attend to different social information, and in methods of social influence. These early differences, in turn, prepare children for somewhat different tasks and roles in adulthood. Although these differences have strong biological origins, developmental experiences serve to flesh out and elaborate on these differences, or to minimize them, depending on the demands of the culture in which the child is situated.