Gender identity disorder (GID), also known as gender dysphoria, is a formal diagnosis used by psychologists and physicians to describe people who experience significant dysphoria (discontent) with the sex they were assigned at birth and/or the gender roles associated with that sex. Affected individuals are commonly referred to as transsexual or transgender. 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 in the Netherlands and Belgium to 0.5% inMassachusetts to 1.2% in New Zealand. 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.
GID as a birth defect
This argument is supported by physiological evidence, such as the presence of typically-female patterns of white matter and neuron patterns observed in the brains of male-to-female transsexuals and overall longer instances of the androgen receptor gene. (Also see Causes of transsexualism.) However, these markers do not identify every individual who undergoes transition, and that using them to define transsexualism could falsely exclude some people from treatment
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 also result in stigmatization of transgender individuals. Treatment for gender identity disorder is also controversial, as changes made are typically irreversible.
The current approach to treatment for people diagnosed with gender identity disorder is to support them in physically modifying their bodies so that they better match their gender identities, an approach that conceptualizes them as having a medical problem that is corrected through various forms of medical intervention. 
Genetic variation, hormones, and differences in brain functioning and brain structures provide evidence for the biological etiology of the symptoms associated with GID. Twin studies indicate that GID is 62% heritable, evidencing the genetic influence in its development. In male-to-female transsexuals, GID is associated with variations in an individual’s genes that make the individual less sensitive to androgens. Zhou et al. (1995) found that in one area of the brain, male-to-female transsexuals have a typically female structure, and female-to-male transsexuals have a typically male structure. In addition, some aspects of trans women‘s hypothalamus functioning resembles that typical of cisgender women
The American Psychiatric Association permits a diagnosis of gender dysphoria if the criteria in the Diagnostic and Statistical Manual of Mental Disorders (5th Edition), or DSM-5, are met.
The DSM-5 moved this diagnosis out of the sexual disorders category and into a category of its own. The diagnosis itself was renamed from “Gender Identity Disorder” to “Gender Dysphoria,” after criticisms that the former term was stigmatizing. Subtyping by sexual orientation was deleted, and the diagnosis for children was separated from that for adults. The creation of a specific diagnosis for children reflects the supposedly lesser ability of children to have insight into what they are experiencing, or ability to express it in the event that they have insight.
The International Classification of Diseases (ICD-10) list three diagnostic criteria for “transsexualism” (F64.0):