The term gender identity was coined by psychiatry professor Robert J. Stoller in 1964 and popularized by psychologist John Money.
[11] The 2012 book Introduction to Behavioral Science in Medicine says that with exceptions, "Gender identity develops surprisingly rapidly in the early childhood years, and in the majority of instances appears to become at least partially irreversible by the age of 3 or 4".
[12][13] The Endocrine Society has stated "Considerable scientific evidence has emerged demonstrating a durable biological element underlying gender identity.
[15] There are several theories about how and when gender identity forms, and studying the subject is difficult because children's immature language acquisition requires researchers to make assumptions from indirect evidence.
[13] Martin and Ruble conceptualize this process of development as three stages: (1) as toddlers and pre-schoolers, children learn about defined characteristics, which are socialized aspects of gender; (2) around the ages of five to seven years, identity is consolidated and becomes rigid; (3) after this "peak of rigidity", fluidity returns and socially defined gender roles relax somewhat.
Diamond had contributed to research involving pregnant rats that showed hormones played a major role in the behavior of different sexes.
Rebelo et al. argue that the evidence in totality suggests that gender identity is neither determined entirely by childhood rearing nor entirely by biological factors.
[53] Research suggests that the same hormones that promote the differentiation of sex organs in utero also elicit puberty and influence the development of gender identity.
[55] In 1955, John Money proposed that gender identity was malleable and determined by whether a child was raised as male or female in early childhood.
[56][57] Money's hypothesis has since been discredited,[57][58] but scholars have continued to study the effect of social factors on gender identity formation.
[57] In the 1960s and 1970s, factors such as the absence of a father, a mother's wish for a daughter, or parental reinforcement patterns were suggested as influences; more recent theories suggesting that parental psychopathology might partly influence gender identity formation have received only minimal empirical evidence,[57] with a 2004 article noting that "solid evidence for the importance of postnatal social factors is lacking.
[62] However, Emily Kane found that many parents still showed negative responses to items, activities, or attributes that were considered feminine, such as domestic skills, nurturance, and empathy.
[40] Once the child's sex is determined, most children are raised to in accordance with it, fitting a male or female gender role defined partly by the parents.
[73][74] A study by Reiner & Gearhart provides some insight into what can happen when genetically male children with cloacal exstrophy are sexually assigned female and raised as girls,[75] according to an 'optimal gender policy' developed by John Money:[69] in a sample of 14 children, follow-up between the ages of 5 and 12 showed that 8 of them identified as boys, and all of the subjects had at least moderately male-typical attitudes and interests,[75] providing support for the argument that genetic variables affect gender identity and behavior independent of socialization.
Moreover, only "53% of [the] LGBTQ population live in states prohibiting housing discrimination based on sexual orientation and gender identity", while "17% of [the] LGBTQ population lives in states explicitly interpreting existing prohibition on sex discrimination to include sexual orientation and/or gender identity".
"[89] In late-19th-century medical literature, women who chose not to conform to their expected gender roles were called "inverts", and they were portrayed as having an interest in knowledge and learning, and a "dislike and sometimes incapacity for needlework".
[90] During the mid-1900s, doctors pushed for corrective therapy on such women and children, which meant that gender behaviors that were not part of the norm would be punished and changed.
On this basis, he argued that bisexuality was the original sexual orientation and that heterosexuality was resultant of repression during the phallic stage, at which point gender identity became ascertainable.
[94] During the 1950s and '60s, psychologists began studying gender development in young children, partially in an effort to understand the origins of homosexuality (which was viewed as a mental disorder at the time).
In 1958, the Gender Identity Research Project was established at the UCLA Medical Center for the study of intersex and transsexual individuals.
Psychoanalyst Robert Stoller generalized many of the findings of the project in his book Sex and Gender: On the Development of Masculinity and Femininity (1968).
[101] Today, gender-affirming surgery is performed on people who choose to transition so that their external sexual organs will match their gender identity.
The 1987 revision of the manual, DSM-III-R, added a third diagnosis: gender identity disorder of adolescence and adulthood, nontranssexual type.
This remains controversial,[107] although the vast majority of today's mental health professionals follow and agree with the current DSM classifications.
In recent years, however, there has been a "growing chorus of voices contesting the pathologization of transgender lives and the dominance of medical-scientific narratives about trans experience.
No one shall be forced to undergo medical procedures, including sex reassignment surgery, sterilisation or hormonal therapy, as a requirement for legal recognition of their gender identity.
"[111] Principle 18 states that "Notwithstanding any classifications to the contrary, a person's sexual orientation and gender identity are not, in and of themselves, medical conditions and are not to be treated, cured or suppressed.
The pathologization of difference has led to gender-transgressive children and adolescents being confined in psychiatric institutions, and subjected to aversion techniques – including electroshock therapy – as a 'cure'.
In 2015, gender identity was part of the United States Supreme Court case Obergefell v. Hodges in which marriage was no longer legally restricted to be only between man and woman.
There are parts of the community that take "two-spirit" as a category over an identity itself, preferring to identify with culture or Nation-specific gender terms.