Gender dysphoria (GD) is the distress a person experiences as a result of the sex and gender they were assigned at birth. In this case, the assigned sex and gender do not match the person’s gender identity, and the person is transgender. Evidence from studies of twins suggest that people who identify with a gender different from their assigned sex may experience such distress not only due to psychological or behavioural causes, but also biological ones related to their genetics or exposure to hormones before birth.