This article argues that behavioral differentiation of the sexes is minimal when children are observed or tested individually. Sex differences emerge primarily in social situations, and their nature varies with the gender composition of dyads and groups. Children find same-sex play partners more compatible, and they segregate themselves into same-sex groups, in which distinctive interaction styles emerge. These styles are described. As children move into adolescence, the patterns they developed in their childhood same-sex groups are carried over into cross-sex encounters in which girls' styles put them at a disadvantage. Patterns of mutual influence can become more symmetrical in intimate male-female dyads, but the distinctive styles of the two sexes can still be seen in such dyads and are subsequently manifested in the roles and relationships of parenthood. The implications of these continuities are considered.