1. Biological Factors

1.1. Genetics

A 2008 study compared 112 male-to-female transsexuals (MtFs), both androphilic and gynephilic, and who were mostly already undergoing hormone treatment, with 258 cisgender male controls. Male-to-female transsexuals were more likely than cisgender males to have a longer version of a receptor gene (longer repetitions of the gene) for the sex hormone androgen, which reduced its effectiveness at binding testosterone.^[1] The androgen receptor (NR3C4) is activated by the binding of testosterone or dihydrotestosterone, where it plays a critical role in the forming of primary and secondary male sex characteristics. The research suggests reduced androgen and androgen signaling contributes to the female gender identity of male-to-female transsexuals. The authors say that a decrease in testosterone levels in the brain during development might prevent complete masculinization of the brain in male-to-female transsexuals and thereby cause a more feminized brain and a female gender identity.^[1][2]

A variant genotype for a gene called CYP17, which acts on the sex hormones pregnenolone and progesterone, has been found to be linked to female-to-male (FtMs) transsexuality but not MtF transsexuality. Most notably, the FtM subjects not only had the variant genotype more frequently, but had an allele distribution equivalent to male controls, unlike the female controls. The paper concluded that the loss of a female-specific CYP17 T -34C allele distribution pattern is associated with FtM transsexuality.^[3]

1.2. Transsexuality among Twins

In 2013, a twin study combined a survey of pairs of twins where one or both had undergone, or had plans and medical approval to undergo, gender transition, with a literature review of published reports of transgender twins. The study found that one third of identical twin pairs in the sample were both transgender: 13 of 39 (33%) monozygotic or identical pairs of assigned males and 8 of 35 (22.8%) pairs of assigned females. Among dizygotic or genetically non-identical twin pairs, there was only 1 of 38 (2.6%) pairs where both twins were trans.^[4] The significant percent of identical twin pairs in which both twins are trans and the virtual absence of dizygotic twins (raised in the same family at the same time) in which both were trans would provide evidence that transgender identity is significantly influenced by genetics if both sets were raised in different families.^[4]

1.3. Brain Structure

General

Several studies have found a correlation between gender identity and brain structure.^[5] A first-of-its-kind study by Zhou et al. (1995) found that in a region of the brain called the bed nucleus of the stria terminalis (BSTc), a region which is known for sex and anxiety responses (and which is affected by prenatal androgens),^[6] cadavers of six persons who were described as having been male-to-female transsexual or transgender persons in life had female-normal BSTc size, similar to the study's cadavers of cisgender women. While those identified as transsexual had taken hormones, this was accounted for by including cadavers of non-transsexual female and male controls who, for a variety of medical reasons, had experienced hormone reversal. The controls still had sizes typical for their gender. No relationship to sexual orientation was found.^[7]

In a follow-up study, Kruijver et al. (2000) looked at the number of neurons in BSTc instead of volumes. They found the same results as Zhou et al. (1995), but with even more dramatic differences. One MtF subject, who had never gone on hormones, was also included and matched up with the female neuron counts nonetheless.^[8]

In 2002, a follow-up study by Chung et al. found that significant sexual dimorphism (variation between sexes) in BSTc did not become established until adulthood. Chung et al. theorized that either changes in fetal hormone levels produce changes in BSTc synaptic density, neuronal activity, or neurochemical content which later lead to size and neuron count changes in BSTc, or that the size of BSTc is affected by the generation of a gender identity inconsistent with one's assigned sex.^[9]

It has been suggested that the BSTc differences may be due to the effects of hormone replacement therapy. It has also been suggested that because pedophilic offenders have also been found to have a reduced BSTc, a feminine BSTc may be a marker for paraphilias rather than transsexuality.^[10]

In a review of the evidence in 2006, Gooren considered the earlier research as supporting the concept of transsexuality as a sexual differentiation disorder of the sex dimorphic brain.^[11] Dick Swaab (2004) concurs.^[12]

In 2008, a new region with properties similar to that of BSTc in regards to transsexuality was found by Garcia-Falgueras and Swaab: the interstitial nucleus of the anterior hypothalamus (INAH3), part of the hypothalamic uncinate nucleus. The same method of controlling for hormone usage was used as in Zhou et al. (1995) and Kruijver et al. (2000). The differences were even more pronounced than with BSTc; control males averaged 1.9 times the volume and 2.3 times the neurons as control females, yet regardless of hormone exposure, MtF transsexuals were within the female range and the FtM transsexual within the male range.^[13]

A 2009 MRI study by Luders et al. of 24 MtF transsexuals not yet treated with cross-sex hormones found that regional gray matter concentrations were more similar to those of cisgender men than to those of cisgender women, but there was a significantly larger volume of gray matter in the right putamen compared to cisgender men. Like earlier studies, it concluded that transsexuality was associated with a distinct cerebral pattern.^[14] (MRI allows easier study of larger brain structures, but independent nuclei are not visible due to lack of contrast between different neurological tissue types, hence other studies on e.g. BSTc were done by dissecting brains post-mortem.)

An additional feature was studied comparing 18 female-to-male transsexuals who had not yet received cross-sex hormones with 24 cisgender male and 19 female gynephilic controls, using an MRI technique called diffusion tensor imaging or DTI.^[15] DTI is a specialized technique for visualizing white matter of the brain, and white matter structure is one of the differences in neuroanatomy between men and women. The study took into account fractional anisotropy values for white matter in the medial and posterior parts of the right superior longitudinal fasciculus (SLF), the forceps minor, and the corticospinal tract. Rametti et al. (2010) discovered that, "Compared to control females, FtM showed higher FA values in posterior part of the right SLF, the forceps minor and corticospinal tract. Compared to control males, FtM showed only lower FA values in the corticospinal tract."^[15] The white matter pattern in female-to-male transsexuals was found to be shifted in the direction of biological males.

Hulshoff Pol et al. (2006) studied the gross brain volume of 8 male-to-female transsexuals and in six female-to-male transsexuals undergoing hormone treatment. They found that hormones changed the sizes of the hypothalamus in a gender consistent manner: treatment with male hormones shifted the hypothalamus towards the male direction in the same way as in male controls, and treatment with female hormones shifted the hypothalamus towards the female direction in the same way as female controls. They concluded: "The findings suggest that, throughout life, gonadal hormones remain essential for maintaining aspects of sex-specific differences in the human brain."^[16]

A 2016 review agreed with the other reviews when considering androphilic trans women and gynephilic trans men. It reported that hormone treatment may have large effects on the brain, and that cortical thickness, which is generally thicker in cisgender women's brains than in cisgender men's brains, may also be thicker in trans women's brains, but is present in a different location to cisgender women's brains.^[10] It also stated that for both trans women and trans men, "cross-sex hormone treatment affects the gross morphology as well as the white matter microstructure of the brain. Changes are to be expected when hormones reach the brain in pharmacological doses. Consequently, one cannot take hormone-treated transsexual brain patterns as evidence of the transsexual brain phenotype because the treatment alters brain morphology and obscures the pre-treatment brain pattern."^[10]

A 2019 review in Neuropsychopharmacology found that among transgender individuals meeting diagnostic criteria for gender dysphoria, "cortical thickness, gray matter volume, white matter microstructure, structural connectivity, and corpus callosum shape have been found to be more similar to cisgender control subjects of the same preferred gender compared with those of the same natal sex."^[17]

A 2020 paper^[18] tried to investigate and differentiate between the two competing hypotheses of a neurodevelopmental cortical hypothesis that suggests the existence of different brain phenotypes vs a functional-based hypothesis in relation to regions involved in the own body perception.^[18] Trans men, trans women, and cisgender women had decreased connectivity compared with cisgender men in superior parietal regions, as part of the salience (SN) and the executive control (ECN) networks.^[18] Trans men also had weaker connectivity compared with cisgender men between intra-SN regions and weaker inter-network connectivity between regions of the SN, the default mode network (DMN), the ECN and the sensorimotor network.^[18] Trans women had lower small-worldness, modularity and clustering coefficient than cisgender men.^[18]

A 2021 review of brain studies published in the Archives of Sexual Behavior found that "although the majority of neuroanatomical, neurophysiological, and neurometabolic features" in transgender people "resemble those of their natal sex rather than those of their experienced gender", for trans women they found feminine and demasculinized traits, and vice versa for trans men. They stated that due to limitations and conflicting results in the studies that had been done, they could not draw general conclusions or identify specific features that consistently differed between cisgender and trangender people. The review also found differences when comparing cisgender homosexual and heterosexual people, with the same limitations applying.^[19]

Androphilic male-to-female transsexuals

A 2015 review reported that two studies found a pattern of white matter microstructure differences away from transsexuals' birth sex and toward their desired sex. In one of these studies, sexual orientation had no effect on the diffusivity measured.^[20]

A 2016 review reported that early-onset androphilic transgender women have a brain structure similar to cisgender women's and unlike cisgender men's, but that they have their own brain phenotype.^[10]

Gynephilic male-to-female transsexuals

While MRI taken on gynephilic male-to-female transsexuals have likewise shown differences in the brain from non-transsexuals, no feminization of the brain's structure have been identified.^[10] Neuroscientists Ivanka Savic and Stefan Arver at the Karolinska Institute used MRI to compare 24 gynephilic male-to-female transsexuals with 24 cisgender female and 24 cisgender male controls. None of the study participants were on hormone treatment. The researchers found sex-typical differentiation between the MtF transsexuals and cisgender females, and the cisgender males; but the gynephilic transsexuals "displayed also singular features and differed from both control groups by having reduced thalamus and putamen volumes and elevated GM volumes in the right insular and inferior frontal cortex and an area covering the right angular gyrus".^[21]

The researchers concluded that:

Contrary to the primary hypothesis, no sex-atypical features with signs of 'feminization' were detected in the transsexual group ... The present study does not support the dogma that [male-to-female transsexuals] have atypical sex dimorphism in the brain but confirms the previously reported sex differences. The observed differences between MtF-TR and controls raise the question as to whether gender dysphoria may be associated with changes in multiple structures and involve a network (rather than a single nodal area).^[21]

Berglund et al. (2008) tested the response of gynephilic MtF transsexuals to two steroids hypothesized to be sex pheromones: the progestin-like 4,16-androstadien-3-one (AND) and the estrogen-like 1,3,5(10),16-tetraen-3-ol (EST). Despite the difference in sexual orientation, the MtFs' hypothalamic networks activated in response to the AND pheromone, like the androphilic female control groups. Both groups experienced amygdala activation in response to EST. Gynephilic male control groups experienced hypothalamic activation in response to EST. However, the MtF subjects also experienced limited hypothalamic activation to EST. The researchers concluded that in terms of pheromone activation, MtFs occupy an intermediate position with predominantly female features.^[22] The MtF transsexual subjects had not undergone any hormonal treatment at the time of the study, according to their own declaration beforehand, and confirmed by repeated tests of hormonal levels.^[22]

A 2016 review reported that gynephilic trans women differ from both cisgender female and male controls in non-dimorphic brain areas.^[10]

Gynephilic female-to-male transsexuals

Fewer studies have been performed on the brain structure of transgender men than on transgender women.^[10] A team of neuroscientists, led by Nawata in Japan, used a technique called single-photon emission computed tomography (SPECT) to compare the regional cerebral blood flow (rCBF) of 11 gynephilic FtM transsexuals with that of 9 androphilic cis females. Although the study did not include a sample of biological males so that a conclusion of "male shift" could be made, the study did reveal that the gynephilic FtM transsexuals showed significant decrease in blood flow in the left anterior cingulate cortex and a significant increase in the right insula, two brain regions known to respond during sexual arousal.^[23]

A 2016 review reported that the brain structure of early-onset gynephilic trans men generally corresponds to their assigned sex, but that they have their own phenotype with respect to cortical thickness, subcortical structures, and white matter microstructure, especially in the right hemisphere.^[10] Morphological increments observed in the brains of trans men might be due to the anabolic effects of testosterone.^[10]

1.4. Prenatal Androgen Exposure

Prenatal androgen exposure, the lack thereof, or poor sensitivity to prenatal androgens are commonly cited mechanisms to explain the above discoveries. To test this, studies have examined the differences between transsexual and cisgender individuals in digit ratio (a generally accepted marker for prenatal androgen exposure). A meta-analysis concluded that the effect sizes for this association were small or nonexistent.^[24]

Congenital adrenal hyperplasia in persons with XX sex chromosomes results in what is considered to be excess exposure to prenatal androgens, resulting in masculinization of the genitalia and, typically, controversial prenatal hormone treatment^[25] and postnatal surgical interventions.^[26] Individuals with CAH are usually raised as girls and tend to have similar cognitive abilities to the typical female, including spatial ability, verbal ability, language lateralization, handedness and aggression. Research has shown that people with CAH and XX chromosomes will be more likely to be same sex attracted,^[25] and at least 5.2% of these individuals develop serious gender dysphoria.^[27]

In males with 5-alpha-reductase deficiency, conversion of testosterone to dihydrotestosterone is disrupted, decreasing the masculinization of genitalia. Individuals with this condition are typically raised as females due to their feminine appearance at a young age. However, more than half of males with this condition raised as females become males later in their life. Scientists speculate that the definition of masculine characteristics during puberty and the increased social status afforded to men are two possible motivations for a female-to-male transition.^[27]

2. Psychological

Psychiatrist and sexologist David Oliver Cauldwell^[28] argued in 1947 that transsexuality was caused by multiple factors. He believed that small boys tend to admire their mothers to such a degree that they end up wanting to be like them. However, he believed that boys would lose this desire as long as his parents set limits when raising him, or he had the right genetic predispositions or a normal sexuality. In 1966, Harry Benjamin^[29] considered the causes of transsexuality to be badly understood, and argued that researchers were biased towards considering psychological causes over biological causes.

Gender dysphoria in those assigned male at birth tends to follow one of two broad trajectories: early-onset or late-onset. Early-onset gender dysphoria is behaviorally visible in childhood. Sometimes gender dysphoria will stop for a while in this group and they will identify as gay or homosexual for a period of time, followed by recurrence of gender dysphoria. This group is usually sexually attracted to members of their natal sex in adulthood. Late-onset gender dysphoria does not include visible signs in early childhood, but some report having had wishes to be the opposite sex in childhood that they did not report to others. Trans women who experience late-onset gender dysphoria will usually be sexually attracted to women and may identify as lesbians or bisexual. It is common for people assigned male at birth who have late-onset gender dysphoria to cross-dress with sexual excitement. In those assigned female at birth, early-onset gender dysphoria is the most common course. This group is usually sexually attracted to women. Trans men who experience late-onset gender dysphoria will usually be sexually attracted to men and may identify as gay.^[30]

Ray Blanchard has developed a taxonomy of male-to-female transsexualism^[31] built upon the work of his colleague Kurt Freund,^[32] which argues that trans women have one of two primary causes of gender dysphoria.^[33][34][35] Blanchard theorizes that "homosexual transsexuals" (a taxonomic category he uses to refer to trans women who are sexually attracted to men) are attracted to men and develop gender dysphoria typically during childhood, and characterizes them as displaying overt and obvious femininity since childhood; he characterizes "non-homosexual transsexuals" (a taxonomic category he uses to refer to trans women who are sexually attracted to women) as developing gender dysphoria primarily because they are autogynephilic (sexually aroused by the thought or image of themselves as a woman^[31]), and as being either attracted to women, attracted to both women and men (a concept he calls pseudo-bisexuality as, considering attraction to males part of the performance of an autogynephilic sexual fantasy), or asexual.

Blanchard's theory has gained support from J. Michael Bailey, Anne Lawrence, James Cantor, and others who argue that there are significant differences between the two groups, including sexuality, age of transition, ethnicity, IQ, fetishism, and quality of adjustment.^{[31][33][36][37][38]} However, the theory has been criticized in papers from Veale, Nuttbrock, Moser, and others who argue that it is poorly representative of MtF transsexuals and non-instructive, and that the experiments behind it are poorly controlled and/or contradicted by other data.^[39][40][41] A 2009 study by Charles Moser of 29 cisgender women in the healthcare field based on Blanchard’s methods for identifying autogynephilia found that 93% of respondents qualified as autogynephiles based on their own responses.^[42] This study was later criticized by Anne Lawrence, who argued that Moser's and Blanchard's scales differed significantly, and that Moser's questions "do not adequately assess the essential element of autogynephilia—sexual arousal simply to the thought of being a female."^[43] Many authorities, including some supporters of the theory, criticize Blanchard's choice of wording as confusing or degrading because it focuses on trans women's assigned sex and disregards their sexual orientation identity.^[10] Evolutionary biologist and trans woman Julia Serano wrote that "Blanchard's controversial theory is built upon a number of incorrect and unfounded assumptions, and there are many methodological flaws in the data he offers to support it."^[44] The World Professional Association for Transgender Health (WPATH) argued against including Blanchard's typology in the DSM, stating that there was no scientific consensus on the theory, and that there was a lack of longitudinal studies on the development of transvestic fetishism.^[45]

A 2016 review found support for the predictions of Blanchard's typology that androphilic and gynephilic trans women have different brain phenotypes. It stated that although Cantor seems to be right that Blanchard's predictions have been validated by two independent structural neuroimaging studies, there is "still only one study on nonhomosexual MtFs; to fully confirm the hypothesis, more independent studies on nonhomosexual MtFs are needed. A much better verification of the hypothesis could be supplied by a specifically designed study including homosexual and nonhomosexual MtFs." The review stated that "confirming Blanchard's prediction still needs a specifically designed comparison of homosexual MtF, homosexual male, and heterosexual male and female people."^[10]

3. Parenting

The failure of an attempt to raise David Reimer from infancy through adolescence as a girl after his genitals were accidentally mutilated is cited as disproving the theory that gender identity is determined solely by parenting.^[46][47] Between the 1960s and 2000, many other newborn and infant boys were surgically reassigned as females if they were born with malformed penises, or if they lost their penises in accidents. Many surgeons believed such males would be happier being socially and surgically reassigned female. Available evidence indicates that in such instances, parents were deeply committed to raising these children as girls and in as gender-typical a manner as possible. Six of seven cases providing orientation in adult follow-up studies identified as heterosexual males, with one retaining a female identity, but who is attracted to women. Such cases do not support the theory that parenting influences gender identity or sexual orientation of those assigned male at birth.^[48]:72–73 Reimer's case is used by organizations such as the Intersex Society of North America to caution against needlessly modifying the genitals of unconsenting minors.^[49]

In 2015, the American Academy of Pediatrics released a webinar series on gender, gender identity, gender expression, transgender, etc.^[50][51] In the first lecture Dr. Sherer explains that parents' influence (through punishment and reward of behavior) can influence gender expression but not gender identity.^[52] She cites a Smithsonian article that shows a photo of a 3 year old President Franklin D. Roosevelt with long hair, wearing a dress.^[51][53] Children as old as 6 wore gender neutral clothing, consisting of white dresses, until the 1940s.^[53] In 1927, Time (magazine) magazine printed a chart showing sex-appropriate colors, which consisted of pink for boys and blue for girls.^[53] Dr. Sherer argued that kids will modify their gender expression to seek reward from their parents and society but this will not affect their gender identity (their internal sense of self).^[52]