Genetics[ edit ] A study compared male-to-female transsexuals both androphilic and gynephilic , mostly already undergoing hormone treatment, with 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 or testosterone, which reduced its effectiveness at binding testosterone.
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. 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 study found that one third of identical twin pairs in the sample were both transgender: Among dizygotic or genetically non-identical twin pairs, there was only 1 of 38 2. While the transsexuals studied had taken hormones, this was accounted for by including cadavers of non-transsexual male and female 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.
They found the same results as Zhou et al. One MtF subject, who had never gone on hormones, was also included and matched up with the female neuron counts nonetheless. The same method of controlling for hormone usage was used as in Zhou et al.
The differences were even more pronounced than with BSTc; control males averaged 1. Like earlier studies, it concluded that transsexualism was associated with a distinct cerebral pattern. BSTc were done by dissecting brains post-mortem.
An additional feature was studied in a group of FtM transsexuals who had not yet received cross-sex hormones: Compared to control males, FtM showed only lower FA values in the corticospinal tract. They found that hormones changed the sizes of the hypothalamus in a gender consistent manner: In , a team of neuroscientists compared 18 female-to-male transsexuals with 24 male and 19 female gynephilic controls, using an MRI technique called diffusion tensor imaging or DTI.
The study found that the white matter pattern in female-to-male transsexuals was shifted in the direction of biological males, even before the female-to-male transsexuals started taking male hormones which can also modify brain structure.
Similar brain structure differences have been noted between gay and heterosexual men, and between lesbian and heterosexual women. In May , the Proceedings of the National Academy of Sciences reported that parenting "rewires the male brain" for fathers.
In , a German team of radiologists led by Gizewski compared 12 androphilic transsexuals with 12 cisgender males and 12 cisgender females. Using functional magnetic resonance imaging fMRI , they found that when shown erotica, the cisgender men responded in several brain regions that the cisgender women did not, and that the sample of androphilic transsexuals was shifted towards the female direction in brain responses. The androphilic transsexuals differed from both control groups in multiple brain areas, including the superior longitudinal fasciculus, the right anterior cingulum, the right forceps minor, and the right corticospinal tract.
The study authors concluded that androphilic transsexuals were halfway between the patterns exhibited by male and female controls. Researchers of the Karolinska Institute of Stockholm used MRI to compare 24 gynephilic male-to-female transsexuals with 24 cisgender male and 24 cisgender female controls. None of the study participants were on hormone treatment.
The researchers found sex-typical differentiation between the MtF transsexuals and cisgender males, and the cisgender females; 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 ".
These 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.
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. The researchers concluded that in terms of pheromone activation, MtFs occupy an intermediate position with predominantly female features. 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.
Ramachandran found that while nearly two thirds of non-transsexual males who have a penis surgically removed experience the sensation of a phantom penis, only one third of MtF transsexuals do so after sex reassignment surgery.
However, this study overlooks differences between an amputation, where nerves connecting the penis and brain are severed, and reassignment surgery, where some of the penis and scrotum may be reused to create a vaginal canal, labia and a clitoris. In this case, some nerves connecting the genitalia to the brain remain intact.
Two thirds of FtM transsexuals reported the sensation of a phantom penis from childhood onwards, complete with phantom erections and other phenomena. Schneider, Pickel, and Stalla found a correlation between digit ratio a generally accepted marker for prenatal androgen exposure and male-to-female transsexualism. MtF transsexuals were found to have a higher digit ratio than control males, but one that was comparable to control females. Research has shown that people with CAH and XX chromosomes will be more likely to be same sex attracted,  and at least 5.
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. 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 this theory that gender identity is determined by parenting. Sherer explains that parents' influence through punishment and reward of behavior can influence gender expression but not gender identity.
Roosevelt with long hair, wearing a dress. 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. Blanchard's transsexualism typology Ray Blanchard has developed a taxonomy of male-to-female transsexualism  built upon the work of his colleague Kurt Freund,  which assumes that trans women have one of two motivations for transition.
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. Transsexuals themselves overwhelmingly reject the theory,  with evolutionary biologist and trans woman Julia Serano writing 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.
In contemporary clinical practice, sexual orientation per se plays only a minor role in treatment protocols or decisions. It can be difficult to assess sexual orientation in individuals with a GI diagnosis, as they preoperatively might give incorrect information in order to be approved for hormonal and surgical treatment Lawrence, Because sexual orientation subtyping is of interest to researchers in the field, it is recommended that reference to it be addressed in the text, but not as a specifier.
It should also be assessed as a dimensional construct.