Gender identity disorder (GID) or gender dysphoria is the formal diagnosis used by psychologists and physicians to describe people who experience significant dysphoria (discontent) with the sex and gender they were assigned at birth. Evidence suggests that people who identify with a gender different from the one they were assigned at birth, may do so not just due to psychological or behavioral causes, but also biological ones related to their genetics, the makeup of their brains, or prenatal exposure to hormones.
Estimates of the prevalence of gender identity disorder range from a lower bound of 1:2000 (or about 0.05%) in the Netherlands and Belgium to 0.5% in Massachusetts to 1.2% in New Zealand. These numbers are based on those who identify as transgender. It is estimated that about 0.005% to 0.014% of males and 0.002% to 0.003% of females would be diagnosed with gender dysphoria, based on current diagnostic criteria. Research indicates people who transition in adulthood are up to three times more likely to be male assigned at birth, but that among people transitioning in childhood the sex ratio is close to 1:1.
Gender identity disorder is classified as a medical disorder by the ICD-10 CM and DSM-5 (called gender dysphoria). Many transgender people and researchers support declassification of GID because they say the diagnosis pathologizes gender variance, reinforces the binary model of gender, and can result in stigmatization of transgender individuals. The official classification of gender dysphoria as a disorder in the DSM-5 may help resolve some of these issues, because the term gender dysphoria applies only to the discontent experienced by some persons resulting from gender identity issues.
The current medical approach to treatment for persons diagnosed with gender identity disorder is to support the individual in physically modifying the body to better match the psychological gender identity. This approach is based on the concept that their experience is based in a medical problem correctable by various forms of medical intervention.
More information about this disorder can be found at WikiPedia.