 Gender identity. Gender identity is the personal sense of one's own gender. Gender identity can correlate with assigned sex at birth, or can differ from it. All societies have a set of gender categories that can serve as the basis of the formation of a person's social identity in relation to other members of society. In most societies, there is a basic division between gender attributes assigned to males and females, the 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, gender-car or non-binary. There are some societies that have third gender categories. Gender identity is usually formed by age three. After age three, it is extremely difficult to change, and attempts to reassign it can result in gender dysphoria. Both biological and social factors have been suggested to influence its formation.