A new review paper has just been published on the current status of brain structure research in transsexuality. Interestingly, it was submitted to the Archives of Sexual Behavior two full years ago. This suggests that it went through a rather thorough peer review. For myself, the first thing I do when reading a review paper is to see that the reference list is comprehensive to ensure that the authors aren’t cherry-picking. In this case, they are not. The paper looks to be very complete and scientifically honest. You may wish to read it yourself, as it is not behind a paywall, thankfully.
The paper is written rather densely, with a great deal of information and discussion; so much in fact, that I will likely be writing several essays covering a number of topics from it. At the high level, my regular readers will not be surprised at the primary conclusions drawn from the review, as I had already written about a fair number of the brain research papers. The authors offer this chief conclusion at the end of the paper,
“The review of the available data seems to support two existing hypotheses: (1) a brain-restricted intersexuality in homosexual MtFs and FtMs and (2) Blanchard’s insight on the existence of two brain phenotypes that differentiate “homosexual” and “nonhomosexual” MtFs”
The review of all of the available brain structure research fully supports the Two Type Taxonomy. In light of this, the authors recommend that future researchers take care to distinguish between the two types, lamenting that some studies in the review had not made this distinction, and further, that it is important that the control groups also be concordant with sexual orientation,
“The study of mixed samples implicitly assumes that transsexuals are a homogeneous group. This is far from the truth with respect to the onset of GD and sexual orientation. … These observations signify that control groups in studies of the transsexual brain must be homogeneous in regards to sexual orientation.”
The authors did find separate studies of androphilic “homosexual” MTFs and non-gender dysphoric gay men that used the same methods, such that a tentative comparison could be made,
“The only study on the CTh [cortical thickness] of homosexual persons that do not present gender dysphoria is by the Savic group (Abé et al.). If we compare this study with that of Zubiaurre-Elorza et al. on the CTh of homosexual MtFs, we see both studies report sex differences showing an F > M pattern in similar structures of the right hemisphere. But there is only one region, the pars triangularis, in which homosexuals and homosexual MtFs both present differences. However, these changes are in opposite directions. The pars triangularis of homosexual MtFs is thicker than in heterosexual male controls, while for homosexuals it is thinner than in heterosexual males. Thus, it seems that for transsexuals this region is feminized but demasculinized [i.e.: “different that straight men, but not in the heterosexual female direction” – K. Brown] in homosexual individuals. Interestingly, in both studies, the affected pars triangularis is in the right hemisphere. Nevertheless, confirming Blanchard’s prediction still needs a specifically designed comparison of homosexual MtF, homosexual male, and heterosexual male and female people.”
This is interesting, that there is a difference between gay men and androphilic transwomen? But the right hemisphere pars triangularis of all things? For left hemisphere dominant people, this region of the brain is believed to be involved in the understanding and production of prosody, emotionally nuanced speech modulation. We know this because individuals who have serious lesions in this area have trouble with prosody.
For more information, read the Wikipedia page on prosody.
Before anyone gets too excited about the possible implications for a neurological marker for androphilic transsexuality that differentiates them from gay men, we need to note that the brain exhibits neuroplasticity. That is to say, that like a muscle, exercise of particular skills causes the brain to increase in volume and neuron number in those regions used to supply that skill. If this is about language and more particularly, about language production that imparts an emotional / sexual identity / gender identity through one’s voice, the difference in this part of the brain may be caused by experience and practice.
On the other hand, it just might represent a real difference. We need more studies.
Guillamon, A et al., “A Review of the Status of Brain Structure Research in Transsexualism” Arch Sex Behav (2016). doi:10.1007/s10508-016-0768-5