White matter microstructure in female to male transsexuals before cross-sex hormonal treatment. A diffusion tensor imaging study. - Rametti et al, J Psychiatr Res. 2010 Jun 8.
Abstract:n=18 is still too small a sample size to be entirely certain - if the experiment is considered in isolation. But now we have dozens of experiments on humans, hundreds on animals, an aggregate human test sample size in the thousands, and all showing the same pattern.
BACKGROUND: Some gray and white matter regions of the brain are sexually dimorphic. The best MRI technique for identifying subtle differences in white matter is diffusion tensor imaging (DTI). The purpose of this paper is to investigate whether white matter patterns in female to male (FtM) transsexuals before commencing cross-sex hormone treatment are more similar to that of their biological sex or to that of their gender identity.
METHOD: DTI was performed in 18 FtM transsexuals and 24 male and 19 female heterosexual controls scanned with a 3 T Trio Tim Magneton. Fractional anisotropy (FA) was performed on white matter fibers of the whole brain, which was spatially analyzed using Tract-Based Spatial Statistics.
RESULTS: In controls, males have significantly higher FA values than females in the medial and posterior parts of the right superior longitudinal fasciculus (SLF), the forceps minor, and the corticospinal tract. 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.
CONCLUSIONS: Our results show that the white matter microstructure pattern in untreated FtM transsexuals is closer to the pattern of subjects who share their gender identity (males) than those who share their biological sex (females). Our results provide evidence for an inherent difference in the brain structure of FtM transsexuals.
Regional cerebral blood flow changes in female to male gender identity disorder. - Tanaka et al, Psychiatry Clin Neurosci. 2010 Apr 1;64(2):157-61.
Abstract:Again, n=9 does not give me warm fuzzies. Moreover, the controls here were flawed, or rather, incomplete. A comparison was made with women, but not with other men. A better experiment would replicate existing work showing the differences between cis-sexual men and cis-sexual women, and then look at trans-sexual men before and after hormonal treatment. That way we could say something rather more precise than "trans men don't have the neurology of cis women".
AIMS: Despite a range of research on gender identity disorder (GID), at present there is no scientific consensus on whether the etiology of GID is mental or physical. In particular recent advances in the technology of neuroimaging research have led to an increased understanding of the biological basis of various mental disorders. GID also should be evaluated from this perspective. The aim of the present study was therefore to do the first trial to examine the regional cerebral blood flow (rCBF) in GID.
METHODS: Persons considered biologically male fulfilling the GID criteria are termed male to female (MTF) and, conversely, persons considered biological female are termed female to male (FTM). We compared 11 FTM subjects and nine age- and handedness-matched female control subjects. None of the subjects was regularly taking medication and none had any kind of physical or psychiatric comorbidity. To evaluate rCBF in GID subjects and control subjects, statistical parametric mapping analysis of (99m)Tc-ethyl-cysteinate dimer single-photon emission computed tomography was used.
RESULTS: GID subjects had a significant decrease in rCBF in the left anterior cingulate cortex (ACC) and a significant increase in the right insula compared to control subjects.
CONCLUSIONS: The ACC and insula are regions that have been noted as being related to human sexual behavior and consciousness. From these findings, useful insights into the biological basis of GID were suggested.
But it's a start.
Other experiments have shown that in certain, specific parts of the brain, to do with emotional response, sense of smell and some other areas, that trans women are more similar to cis women than cis men. Yet others have shown that in a very few other parts, the opposite is the case. And yet others have shown that trans women tend to have parts of their neurology that correspond with neither a male nor female stereotype, nor anything between. They're just plain unusual.
So "male brain in an otherwise female body" - or the reverse - is true. True-ish, anyway. It's more complicated than that. But close enough.
And other experiments still have shown that those sexually attracted to men have specific parts of the brain tending towards one pattern, those sexually attracted to women tending towards another pattern. Regardless of whether they're straight women / gay men, or straight men / lesbian women. And regardless of whether they're cis or trans. A different, but nearby, part of the brain is involved there.
From my own perspective, I can make certain predictions of what my own brain must look like - if our theories are correct. Feminine, sometimes extremely so, in some areas. Masculine in a few. Just plain Weird in one or two.
To anyone who knows me, that last part at least would be no surprise.
I'd love to get myself tested. While I think it would be likely that my predictions would be born out, so it's just another chunk of confirmatory evidence, one more humdrum data point - if we're lucky, I'm wrong, and then we'd really learn something new. Unlikely though, from my research it appears I'm boringly vanilla normal. For a woman born looking male, who changed spontaneously..... (chuckle).
While, on the whole, I'd really rather be 100% female neurologically... the facts say otherwise. My wishes are immaterial, facts are, or are not.
But then, many standard factory model women, cis-sexual, have many of the same mildly cross-sexed neural circuits. I'm in good company. We should always treat people on their individual merits, and not sexually profile them. That's prejudice.
Curtsy to M.Italiano for bringing the Tanaka paper to my attention, while I was still looking at the one by Rametti