Sex Dimorphism of the Brain in Male-to-Female Transsexuals by Savic I, Arver S Cereb. Cortex (2011)
Our group recently employed in vivo investigations and found that smelling of 2 putative pheromones, which in heterosexual subjects normally leads to a sex differentiated activation of the hypothalamus, causes a sex atypical activation in estrogen naïve MtF-TR (Berglund et al. 2008). This observation corroborated with previous anatomical findings in transsexuals but provided no information as to whether and how the hypothalamic networks, which in animal studies are shown to be involved in “sexual behavior,” would be also engaged in the perception of “sexual identity.” Understanding the biology of gender dysphoria requires consideration of additional cerebral networks identified as sexually dimorphic than the hypothalamus. Considering that the sense of discomfort with sex characteristics of the own body is the main feature of gender dysphoria, it is important to also investigate the cerebral networks reported to be involved in own body perception and perception of self. Luders et al. recently addressed some of these issues by an explorative voxel-based morphological (VBM) analysis of the gray matter (GM) fractions. They found that GM volume in the putamen was more pronounced in MtF-TR than in male and female controls, particularly on the right side (Luders, Sanchez, et al. 2009). While the GM in the putamen did not differ significantly between male and female controls, these data, nevertheless, emphasized the possibility that gender dysphoria could be associated with an altered anatomy also in brain regions located outside the hypothalamus.So far, so good.
Only nonhomosexual, transsexual subjects were included. The rationale was the following; Although there is still an ongoing discussion about the classification of transsexual individuals (Blanchard et al. 1987, 1989; Lawrence 2005; Smith et al. 2005), the general view is that there are 2 fundamentally different types of transsexualism: homosexual and nonhomosexual (Blanchard et al. 1987). Homosexual MtF-TR are erotically attracted to the same biological sex and denoted as “androphilic” MtF-TR in the literature. They differ from those attracted to the opposite biological sex (“gynephillic” MtF-TR) and those who are not sexually attracted to other persons; The 2 latter groups are referred to as autogynephillic by the so called Blanchard’s classification (Blanchard 1989; Smith et al. 2005). Blanchard reported that homosexual transsexuals as usually younger at initial presentation and with more pronounced and frequent childhood femininity (Blanchard 1989). Homosexual transsexuals have also been found to be shorter, lighter, and lighter in proportion to their height, also in relation to men in the general population, whereas nonhomosexual transsexuals have been found to be similar in height as controls (Blanchard et al. 1995).*SIGH* In other words, they assume that Transsexual women are actually men. So if attracted to men, must be homosexual. *SIGH* again. Why not just use "androphillic" and "gynephillic"? That way you don't beg the question, especially when dealing with Intersex people.
Group differences in regional GM volume. Colored clusters were calculated from the following group comparisons: (A) HeW-HeM (left), HeM-HeW (right). (B) MtF-TR-HeW (left), HeW-MtF-TR (right). (C) MtF-TR—HeM (left), HeM-MtF-TR (right). Contrast calculated at T corresponding to P = 0.001 at pixel level, P < 0.05 corrected, minimum cluster size 0.4 cm3. Clusters are superimposed on the standard brain, SPM5. The scale is arbitrary showing T-values between 0 and 6.5. The lines indicate the 3 regions in which the gray matter fraction differed between transsexuals and controls. Notably, these regions were similar independently of whether the controls were men or women.
Only right-handed (Oldfield 1971), unmedicated, and healthy subjects were included. Exclusion criteria were history of severe systemic disease, any psychiatric (other than gender dysphoria), or neurological disorder, drug abuse including anabolic steroids or psychiatric medication, and use of hormonal contraceptives in women and hormone supplement in MtF-TR.OK, so we've only included straight women with straight trans women, and are looking at trans women who have had no hormonal therapy. And cut out a lot of trans women, as they have a higher rate of non-right-handedness than the general population as part of the syndrome.
That seems to have shackled a lot of the variables. Sure, the trans women are unrepresentative of trans women as a whole, but with the hormonal balance of males, and the sexual orientation of (straight)females, and the winnowing out so only the least different from the norm regarding handedness are included, that really means that if any neural atypicality is found, we can't look to explanations other than transsexuality for it.
So let's look at what was found in detail:
MR volumetry outside the hypothalamus has, to the best of our knowledge, hitherto not been reported in transsexuals, and the presently observed reductions of the thalamus and putamen volumes in MtF-TR have no precedents. Neither have increases of GM volumes in the temporoparietal junction, and the insular and inferior frontal cortex been described earlier.1. We're looking at areas outside the ones that have been looked at before. Differences have been found that match neither a male nor female stereotype.
It should be emphasized that the present data, being focused on extra hypothalamic structures, do not disapprove previous reports about sex-atypical hypothalamic dimorphism in transsexuals (Swaab et al. 1992; Zhou et al. 1995). Furthermore, they were generated exclusively from investigations of nonhomosexual, gynephillic MtF-TR.2. The results are consistent with previous ones about lesbian trans women having (in some sense) "female brains", though in other senses, not. It's not that simple.
3. That Trans people have brains whose structure in some areas matches neither a Male nor Female stereotype. The differences extend beyond the hypothalamus.
The issue of possible cerebral difference between gynephillic and androphillic (homosexual) MtF-TR and also between androphillic MtF-TR and homosexual healthy men is of special interest and needs to be addressed separately in future studies. Additional studies of the relationship between brain structure and function in transsexual persons and also extending the material to female to male transsexuals are necessary to more precisely interpret the present observations.4. We're only looking at a small subset of Trans people, and we need to sort out what bits are to do with sexual orientation, and what bits are to do with gender identity. And finally, the bottom line:
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).5. It's not simple! The whole is different from the sum of its parts.