Nature versus nurture Although the formation of gender identity is not completely understood, many factors have been suggested as influencing its development. In particular, the extent to which gender identity is determined by nurture (social environmental factors) versus biological factors (which may include non-social environmental factors) is at the core of the ongoing debate in psychology known as "
nature versus nurture". There is increasing evidence that the brain is affected by the organizational role of hormones in utero, circulating sex hormones and the expression of certain genes. The
social learning theory posits that children furthermore develop their gender identity through observing and imitating gender-linked behaviors, and then being rewarded or punished for behaving that way, thus being shaped by the people surrounding them through trying to imitate and follow them. Large-scale
twin studies suggest that the development of both transgender and cisgender gender identities is due to genetic factors, with a small potential influence of unique environmental factors.
Case of David Reimer and contrasting case A well-known example in the nature-versus-nurture debate is the case of
David Reimer, born in 1965, otherwise known as "John/Joan". As a baby, Reimer went through a faulty circumcision, losing his male genitalia. Psychologist
John Money advised Reimer's parents to raise him as a girl. John Money was instrumental in the early research of gender identity, though he used the term
gender role. He disagreed with the previous school of thought that gender was determined solely by biology. He argued that infants are born a blank slate and a parent could be able to decide their babies' gender. In Money's opinion, if the parent confidently raised their child as the opposite sex from earlier than age two, the child would believe that they were born that sex and act accordingly. Money believed that nurture could override nature. In 1997, sexologist
Milton Diamond published a follow-up, revealing that Reimer had rejected his female reassignment, and arguing against the blank slate hypothesis and infant sex reassignment in general. Diamond was a longtime opponent of Money's theories. Diamond had contributed to research involving pregnant rats that showed hormones played a major role in the behavior of different sexes. Griet Vandermassen argues that since these are the only two cases being documented in scientific literature, this makes it difficult to draw any firm conclusions from them about the origins of gender identity, particularly given the two cases reached different conclusions. However, Vandermassen also argued that transgender people support the idea of gender identity as being biologically rooted, as they do not identify with their anatomical sex despite being raised and their behaviour reinforced according to their anatomical sex.
Other cases One study by Reiner et al. looked at fourteen genetic males who had suffered
cloacal exstrophy and were thus raised as girls. Six of them changed their gender identity to male, five remained female and three had ambiguous gender identities (though two of them had declared they were male). All the subjects had moderate to marked interests and attitudes consistent with that of biological males. Another study, A minority of those raised as female later switched to male. However, none of the males raised as male switched their gender identity. Those still living as females still showed marked masculinisation of gender role behaviour and those old enough reported sexual attraction to women. The study's authors caution drawing any strong conclusions from it due to numerous methodological caveats which were a severe problem in studies of this nature. 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.
Biological factors Several prenatal biological factors, including genes and hormones, may affect gender identity. It has been suggested that gender identity is controlled by prenatal
sex steroids, but this is hard to test because there is no way to study gender identity in animals. According to biologist
Michael J. Ryan, gender identity is exclusive to humans. In a position statement, the
Endocrine Society stated:
Transgender and transsexuality Some studies have investigated whether there is a link between biological variables and
transgender or
transsexual identity. Several studies have shown that
sexually dimorphic brain structures in transsexuals are shifted away from what is associated with their birth sex and towards what is associated with their preferred sex. The volume of the central subdivision of the
bed nucleus of a stria terminalis or BSTc (a constituent of the
basal ganglia of the brain which is affected by
prenatal androgens) of transsexual women has been suggested to be similar to women's and unlike men's, but the relationship between BSTc volume and gender identity is still unclear. Similar
brain structure differences have been noted between gay and heterosexual men, and between lesbian and heterosexual women. Transsexuality has a genetic component. 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. Different amounts of these male or female sex hormones can result in behavior and external genitalia that do not match the norm of their sex assigned at birth, and in acting and looking like their identified gender.
Intersex people Estimates of the number of people who are
intersex range from 0.018% to 1.7%, depending on which conditions are counted as intersex. An intersex person is one possessing any of several variations in
sex characteristics including
chromosomes,
gonads,
sex hormones, or
genitals that, according to the United Nations
Office of the High Commissioner for Human Rights, "do not fit typical binary notions of male or female bodies". An intersex variation may complicate initial
sex assignment and that assignment may not be consistent with the child's future gender identity. Reinforcing sex assignments through surgical and hormonal means may violate the individual's
rights. A 2005 study on the gender identity outcomes of female-raised
46,XY persons with
penile agenesis,
cloacal exstrophy of the bladder, or penile
ablation, found that 78% of the study subjects were living as female, as opposed to 22% who decided to initiate a sex change to male in line with their genetic sex. The study concludes: "The findings clearly indicate an increased risk of later patient-initiated gender re-assignment to male after female assignment in infancy or early childhood, but are nevertheless incompatible with the notion of a full determination of core gender identity by prenatal androgens." A 2012 clinical review paper found that between 8.5% and 20% of people with intersex variations experienced
gender dysphoria. Sociological research in Australia, a country with a third 'X' sex classification, shows that 19% of people born with atypical sex characteristics selected an "X" or "other" option, while 52% are women, 23% men, and 6% unsure. At birth, 52% of persons in the study were assigned female, and 41% were assigned male. 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, according to an 'optimal gender policy' developed by
John Money: Money's hypothesis has since been discredited, but scholars have continued to study the effect of social factors on gender identity formation. A 2008 study found that the parents of
gender-dysphoric children showed no signs of psychopathological issues aside from mild depression in the mothers. It has also been suggested that the attitudes of the child's parents may affect the child's gender identity, although evidence is minimal.
Parental establishment of gender roles Parents who do not support gender nonconformity are more likely to have children with firmer and stricter views on gender identity and gender roles. 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. A study conducted by Hillary Halpern demonstrated that parental gender behaviors, rather than beliefs, are better predictors of a child's attitude on gender. A mother's behavior was especially influential on a child's assumptions of the child's own gender. For example, mothers who practiced more traditional behaviors around their children resulted in the son displaying fewer stereotypes of male roles while the daughter displayed more stereotypes of female roles. No correlation was found between a father's behavior and his children's knowledge of stereotypes of their own gender. Fathers who held the belief of equality between the sexes had children, especially sons, who displayed fewer preconceptions of their opposite gender. ==Gender variance and non-conformance==