A child's realization that gender is fixed and does not change over time.

The concept of gender constancy, influenced by the cognitive development theory of Jean Piaget, was introduced by Lawrence Kohlberg (1927-1987). Addressing the formation of gender identity in terms of cognitive development, Kohlberg advanced the idea that the development of sex roles depends in large part on a child's understanding that gender remains constant throughout a person's lifetime. Children realize that they are male or female and are aware of the gender of others by the age of three. However, at these ages they still do not understand that people cannot change genders the way they can change their clothes, names, or behavior. Kohlberg theorized that children do not learn to behave in gender-appropriate ways until they understand that gender is permanent, which occurs at about the age of seven. At this point they start modeling the behavior of members of their own sex. Although it has been supported by some research studies, Kohlberg's theory has also been criticized on the grounds that children do show certain types of gender-associated behavior, such as toy and playmate selection, by the ages of two or three. This points to the fact that there are others factors, such as parental reinforcement, that influence the adoption of sex-typed behavior.