Gender identity is the personal sense of one’s own gender. Gender identity can correlate with a person’s assigned sex or can differ from it. In most individuals, the various biological determinants of sex are congruent, and consistent with the individual’s gender identity. Gender expression typically reflects a person’s gender identity, but this is not always the case. While a person may express behaviors, attitudes, and appearances consistent with a particular gender role, such expression may not necessarily reflect their gender identity. The term gender identity was coined by Robert J. Stoller in 1964.
In most societies, there is a basic division between gender attributes assigned to males and females, a gender binary to which most people adhere and which includes expectations of masculinity and femininity in all aspects of sex and gender: biological sex, gender identity, and gender expression. Some people do not identify with some, or all, of the aspects of gender assigned to their biological sex; some of those people are transgender, non-binary, or genderqueer. Some societies have third gender categories.
Gender identity is usually formed by age three. After age three, it is extremely difficult to change gender identity. Both biological and social factors have been suggested to influence its formation.