Most babies are born with a specific sex—the biological characteristics that determine whether a baby is physically male or female. (Intersex people are born with the biological characteristics of both sexes). Gender, while closely related, is a social construct. It is the role society assigns to an individual based on biological sex. An individual learns how society expects members of a particular sex to behave through gender socialization, the process of acquiring the norms, beliefs, values, and behaviors associated with masculinity and femininity.
Agents of socialization have a significant impact on the development of gender identity. Parents and families begin influencing a baby’s gender identity soon after birth, often without realizing they are doing it. Mothers and fathers behave differently, speak differently, and handle their children differently based on the children's biological sex. They may play more roughly with boys and more gently with girls, for example. They may describe boys as strong and girls as pretty. They may give action figures to boys and dolls to girls. In studies researchers have shown that adults interact with babies differently based on the sex they perceive the baby to be. They will treat a baby dressed in girl clothes as a girl even if the baby is actually a boy. However, adults do not treat boys and girls differently if they do not know what sex the baby is. Peers also play a part in gender socialization. In school, peer groups tend to be single-sex groups. Girls group with girls, and boys group with boys. Consequently, the games and methods of play may be different based on these gender groupings.
Gender roles in many Western societies are not as rigid as they were in the past. The social understanding of gender has been influenced by science and biology, transgender identities, nontraditional gender roles, fashion, art, and ideas regarding social equality. Nonetheless, mass media and culture, including children's books, toys, television, and advertising, often continue to reinforce gender stereotypes––portraying boys and men as active and adventurous and girls and women as passive and domestic. Children's toys, often used by children during the role-playing phase of self-image formation, are generally targeted toward either boys or girls and are shelved separately in stores.