By the age of four, boys and girls have developed notable differences in their play and in their choice of playmates. By the age of six, children spend eleven times as much time with same-sex as opposite-sex peers. Although there is always considerable individual variation, on average boys are more likely than girls to engage in rough-and-tumble play, verbal and physical aggression, and large group activities. Girls, on the other hand, prefer activities that involve verbal interaction and fine motor skills and are more sensitive to emotional reactions both in themselves and in other children.

Girls also express aggression differently. They are less likely to use physical violence, but more likely to attempt to undermine other’s friendships, disrupting the network of personal relationships that are so important for girls. Gender-typed behavior is strongly influenced by environment and there is tremendous variation across cultures in the ways that the different genders are socialized. Nonetheless, there is also strong evidence of a biological basis to gender differences. Sex hormones, such as androgens and estrogen, seem to play an important role.