It is not clear what causes gender dysphoria. At first, hypotheses about causation addressed psychological factors and came mostly from a psychoanalytic perspective. Over the decades, however, available evidence has suggested a multifactorial aetiology, with biological factors rising in importance.
There is known to be an elevated rate of gender dysphoria among people with Klinefelter's syndrome. It is also known that born males with disorders of gender identity are more likely than the general population to be left-handed. Family trees of born-male patients show a relative lack of maternal uncles, and these patients are more likely to be lower-down in male birth order. One hypothesis arising from these findings supposes that in these families, women mount an immune response to a Y chromosome in pregnancy, the titre rising with each successive male pregnancy.
Some observations suggest children and adults with disorders of gender identity show more features of autism spectrum disorder (ASD) than the general population. At one gender clinic prevalence of autistic traits consistent with a clinical diagnosis of ASD among individuals undertaking treatment was 5.5%, compared with reports of 0.5% to 2.0% ASD diagnoses in the general population. People with concurrent gender dysphoria and ASD sometimes have an atypical presentation of gender dysphoria, which makes a correct diagnosis and determination of treatment options for gender dysphoria more difficult.
As a general rule, neurological and physiological function in people with disorders of gender identity appear to be the same as in the general population.
However, there does seem to be an increased rate of non-right-handedness and an increased rate of both autism spectrum disorder (ASD) and features of ASD. One small study showed that the size of the bed nucleus of the brain's stria terminalis was in keeping with the patient's sense of gender rather than their biological sex. Another study showed white matter changes in born-female transsexuals prior to hormone treatment that were akin to those seen in males.
Gender dysphoria is slightly more common among people with Klinefelter's syndrome, partial androgen insensitivity syndromes, and congenital adrenal hyperplasia. In these circumstances, an International Statistical Classification of Diseases and Health Related Problems (ICD)-10 diagnosis of gender dysphoria (not otherwise specified) still applies, rather than one of transsexualism, because the latter diagnosis requires absence of any endocrine or chromosomal abnormality. This distinction is to be lost in ICD-11 and management is still as for transsexualism.
International statistical classification of diseases and health related problems (ICD-10)
Dual role transvestism (also known as cross-dressing; the term ‘transvestite’ may be considered pejorative in some parts of the world).
The wearing of clothes of the opposite sex for part of the time in order to enjoy the temporary experience of membership of the opposite sex, but without desire for something more permanent. No sexual excitement accompanies the cross-dressing, which distinguishes it from fetishistic transvestism.
The desire to live and be accepted as a member of the opposite sex, usually accompanied by a sense of discomfort with one's anatomical sex. The desire should have been present for at least 2 years, and must not be a symptom of another mental disorder or associated with any intersex, genetic, or sex chromosome abnormality.
Gender dysphoria of childhood
A persistent and intense distress about assigned sex, together with a desire to be of the other sex. Diagnosis requires a profound disturbance of normal sense of maleness or femaleness. Diagnosis should not be made when the individual has reached puberty.
It has been established that ICD-11 will not feature gender dysphoria in the section devoted to disorders of psychological functioning, the likely destination being a section devoted to disorders of sexual differentiation.
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