Abstract: Evolution and developmental sex differences. From an evolutionary perspective, childhood is the portion of the life span during which individuals practice and refine those competencies that facilitate survival and reproduction in adulthood. Although the skeletal structure of these competencies appears to be inherent, social interaction and play flesh them out during childhood so that they are adapted to local conditions. Darwin's principles of sexual selection, including male-male competition over mates and female choice of mating partners, successfully explain the acquisition and expression of reproductive competencies in hundreds of species. When this perspective is applied to humans, it predicts sex differences that are, in fact, found in the childhood activities of boys and girls and that reflect sex differences in reproductive strategies in adulthood. A few of these differences are described, along with cultural factors that modify their expression. (PsycINFO Database Record (c) 2002 APA, all rights reserved)