The evolutionist sees the influence of phylogeny at all points of the life cycle; and evolutionary theory is both a fruitful source of ideas and a powerful explanatory tool in the study of human behavior. This paper reports various empirical studies that illustrate these points. For example, in pairs of fraternal and identical twins observed over the first year of life identicals were substantially more alike than fraternals in smiling, vocalizing and attending to mother. Newborns of various ethnic groups were demonstrably different on measures of temperament. Differences in the biologically adaptive role of the sexes appear in humans (e.g., females in infancy and in adolescence show more affiliative behavior than males; boys show more preoccupation with dominance hierarchies than girls; men react differentially to pictures in which secondary sex characteristics, beards and breasts, are present or absent).