Also termed gender dysphoria and gender variance. Gender identity is one's inner sense of masculinity or femininity; gender role is an individual's public expression of being male, female, or a ‘mix’ (androgynous). For most people, gender identity and gender role are the same. A person with a gender identity disorder, however, has a conflict between anatomical sex and gender identity.

Gender is determined by a combination of genetic and environmental factors, in which the influence of family upbringing is an important factor. When physical sexual characteristics are ambiguous, the child's gender identity can usually be established if the child is reared as being clearly male or female. If, however, the child is confused about its sexual identity, the uncertainty may continue into adult life. Transsexuals generally experience conflicts of identity in childhood. In this type of identity disorder, which occurs in one in 30,000 male births and one in 100,000 female births, the person believes that he or she is the victim of a biological accident, trapped in a body different from what is felt to be his or her true sex.

Treatment is difficult: psychotherapy and hormone treatment may be offered, but some affected individuals want surgery to change their body's sexual organs to match their innately felt sexual gender. The decision to seek a physical sex change raises major social problems for individuals, and ethical problems for their doctors. Surgery, which is not always successful in the long term, requires careful assessment, discussion and planning. It is important to preclude mental illness.