The sense of identification with either the male or female sex, as manifested in appearance, behavior, and other aspects of a person 's life.

Influenced by a combination of biological and sociological factors, gender identity emerges by the age of two or three and is reinforced at puberty. Once established, it is generally fixed for life.

Aside from sex differences, other biological contrasts between males and females are already evident in childhood. Girls mature faster than boys, are'physically healthier, and are more advanced in developing oral and written linguistic skills. Boys are generally more advanced at envisioning and manipulating objects in space. They are more aggressive and more physically active, preferring noisy, boisterous forms of play that require larger groups and more space than the play of girls the same age. In spite of conscious attempts to reduce sex role stereotyping in recent decades, boys and...