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Causes of transsexuality
Several studies have found a correlation between gender identity and brain structure.[6] 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),[7] male-to-female trans women had a female-normal BSTc size (like cisgender women) and female-to-male trans men had a male-normal size. While the transsexuals studied had taken hormones, this was accounted for by including 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.[8]
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 who matched up with the female neuron counts nonetheless.[9]
Several studies have found a correlation between gender identity and brain structure.[6] 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),[7] male-to-female trans women had a female-normal BSTc size (like cisgender women) and female-to-male trans men had a male-normal size. While the transsexuals studied had taken hormones, this was accounted for by including 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.[8]
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 who matched up with the female neuron counts nonetheless.[9]
In a review of the evidence in 2006, Gooren confirmed 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 once again, regardless of hormone exposure, MtF transsexuals lay 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 men than women, but there was a significantly larger volume of gray matter in the right putamen compared to 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 in a group of FtM transsexuals who had not yet received cross-sex hormones: 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]
Hulshoff Pol et al. (2006) studied the gross brain volume of 8 male-to-female transsexuals and in 6 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]
Brain-based research has repeatedly shown that female-to-male transsexuals have several male-like characteristics in neuroanatomy. In 2010, 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.[17] 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 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.[18][19] Studies have also found that circumstance and repeated activities such as meditation modify brain structures in a process called brain plasticity or neuroplasticity. In May 2014, the Proceedings of the National Academy of Sciences reported that parenting "rewires the male brain" for fathers.[20]
