On the Science of Changing Sex

Drum keeps pounding… ♫♫

Posted in Brain Sex, Female-to-Male by Kay Brown on April 13, 2013

♫♫…rhythm to the brain…♫♫

shrinking brainData keeps coming in regarding brain imaging studies showing differences between MTF and FtM transsexuals and control males and females.  The question still remains, what does it mean?  Two studies, by two different groups, have measured the cortical thicknesses of transwomen before beginning HRT, to ensure that they are measuring potential organizing effects of endogenous hormones, or other effects, without the confounding activational effects of HRT.  However, the two studies looked at the two different MTF populations.  One, the Zubiaurre-Elorza paper examined only androphilic transwomen (N=18). Given that this group came from Spain, which, as Lawrence demonstrated, has a very low Hofstede Individualism index which is correlated with low percentage of gynephilic transwomen, these 18 were likely to have accurately reported their sexuality as exclusively androphilic.   While the Luders study, as an earlier study reported, examined both gynephilic (N=18) and self reported androphilic (N=6) transwomen.  Given that we know from many other studies of transwomen in high Hofstede Individualism societies, and the fact that the youngest of the Luders group was 23, we would expect that not all six of those who reported that they were androphilic were exclusively so.  In fact, I doubt that more than one or two at most is exclusively androphilic, if any.  Thus arises the question, can we use these two studies to get an early test of Blanchard’s prediction that “non-homosexual” transwomen would show brain difference from controls, but not a shift towards the female typical while the “homosexual” will show just such a shift?

First, some background, since I know that not all readers will be thoroughly up to date on the hypothesis being tested, nor the relevant data supporting it.  I recommend reading the FAQ on the is blog before continuing to read this blog entry, as a starting point.  Given that there is now mountains of data supporting the hypothesis that there are two types of MTF tranwomen, one autogynephilic (AGP) and non-homosexual (with respect to natal sex) and one that is non-autogynephilic and exclusively homosexual (HSTS); and that AGP transsexuals were behaviorally masculine since early childhood, and often even after transition and SRS, while HSTS transwomen had been remarkably feminine in behavior, and often in appearance, since early childhood, Blanchard made the prediction that brain studies would show that BOTH populations would be different than control males, but in different ways.  He further made the prediction that HSTS brains would be shifted toward the female morphology in sexually dimorphic areas of the brain, while AGP transsexuals would not.

This prediction is in opposition to the so called, “feminine essence” hypothesis, which is generally popular among AGP transsexuals, in which they posit that despite their obvious lack of naturally feminine behavior, they are still neurologically “female-like”, at least in some important way.

So far, the very limited data supports Blanchard’s hypothesis and NOT the “feminine essence” hypothesis.  Also so far, no formal study has been conducted that would specifically test these two hypothesis together.

I stated earlier that I didn’t believe that the Luders study group contain many exclusively androphilic (HSTS) individuals.  The reasons I believe this are several fold.  First, they collected most of their group by soliciting via transgender organizations.  As was shown in the Veale study on transsexual sexuality, soliciting through such organizations tends to reach only AGP individuals, as HSTS youth do not tend to be members of such.  Second is the age range of the study entire group.  The youngest was 23.  This would be very unusual for a group of that had a sizable portion of HSTS individuals, given that the median age of transition is 20 years old and the top end is typically 25 years old, and even that old is very rare.  If all six HSTS individuals were 23 to 25, this would be odd.  Also given that the mean age of the entire group is 45, which is a bit higher than the typical mean transition age for a group consisting of only AGP transwomen, this would suggest that there were not many of the total group that was in their early 20’s, certainly not six out of 24 individuals.  Thirdly, as Lawrence has shown, in two different study groups, significant numbers of individuals who claim to be androphilic are inaccurately reporting their sexuality.  Added together, I don’t believe that more than one or at most two, of the Luders study group was actually exclusively androphilic.  I suspect that none of them were.

As we explore the data from these two papers, we need to keep in mind that brains, like bodies, come in different shapes and sizes, more or less.  That is to say, although there are differences between male and female brains, on average, it is difficult to point to a part of the brain and say with absolute certainty, this is a male brain vs. a female brain.  Another thing to keep in mind, if we have a mix of two populations, both may have differences unique to each population… and that when mixed, both of those differences will be detected when we average the data, blurring and blunting the differences, but we should still be able to statistically see a signal in the data, provided we have enough data.

So, lets examine the data.  First, if we look at the high level view, it would appear that there are indeed differences between the control men and women and androphilic transwomen.

Zubiaurre Brain ScansIn general, female brains have thicker cortices (CTh), at least in some areas, as the comparison between male and female controls shows.  As we had expected, MTF  transfolk are different than male controls, but FtM are not that different from female controls.  The authors described the results thus,

“We have found that control females have greater CTh compared with control males in the frontal and parietal regions; in contrast, males have a larger putamen volume than females. With respect to the transsexual groups, we observed that FtMs have greater CTh compared with control males in the parietal and temporal cortices and did not differ from control females. However, FtMs have a larger right putamen than female controls. On the other hand, MtFs did not differ from control females in CTh and had greater CTh than control males in the frontal and occipital regions. In this group, no differences were found in the putamen. All these findings suggest that FtMs have a defeminized putamen, while MtFs have a feminized CTh.

With respect to the CTh of MtFs, we found that this group did not differ from female controls but did from male controls.  These findings suggest that MtFs follow the pattern of cortical thinning typically described for females. Whether the cortical feminization of MtFs depends on a differential cortical androgen receptor distribution, a different efficiency in the androgen receptors or other causes remains to be elucidated.  But what seems clear is that in MtFs the cortical developmental process is affected and follows the direction expected for females. This points out that the developmental approach could help to understand the etiology of transsexualism.”

Let’s compare these results with that of the Luders study, which is mostly (and perhaps entirely) non-exclusively androphilic, which being autogynephilic, Blanchard had previously predicted would show differences from control males, but not in the female like direction:

Luders Brain ScansAt first blush, we see that they are indeed, as predicted, different than control males.  Some areas of the cortices are thicker, but not the same regions as control females nor androphilic transwomen.  So, it would seem unlikely that these brain difference are caused by a feminization of the brain, given that the regions are not those found in control females.  But what does cause these particular differences?  While we might be tempted to conclude that this is caused by autogynephilia, it could also be caused by another attribute that is common in this population, higher IQ.  It has been noted that higher IQ is correlated with thicker cortices.

So where to do we go from here?  This visual comparison of the two studies can only be described as tentative.  But the need for such comparisons are clearly understood by these researchers, as the Zubiaurre-Elorza paper explained,

“On the basis of chromosomal sex and behavior, Blanchard and co-workers (Blanchard et al. 1987, 1989, 1996; Blanchard 1989; see also Smith et al. 2005) have proposed the existence of 2 types of MtFs: 1) MtFs that are attracted to males (“homosexual” transsexuals in Blanchard terminology), and 2) MtFs that are attracted to women (“heterosexual” transsexuals according to Blanchard). Further, Blanchard (2008) hypothesized that homosexual MtFs would differ from heterosexual males in brain sexually dimorphic structures, while in the heterosexual MtFs, the differences might not implicate sexually dimorphic structures. More recently, Cantor (2011) has noted that our findings on the white matter microstructure of (homosexual) MtFs (Rametti, Carrillo, Gómez-Gil, Junque, Zubiarre-Elorza et al. 2011) and that of Savic and Arver (2011) on the cortical volume of (heterosexual) MtFs would support Blanchard’s hypothesis. In the present report, we studied MtF transsexuals erotically attracted to males that show a feminization of CTh but not in the putamen.  …  Consequently, to verify Blanchard’s hypothesis would require a specific design that is beyond the scope of the present study.”

For more essays on trans-brains see Brain Sex.

References:

Eileen Luders, et al., “Increased Cortical Thickness in Male-to-Female Transsexualism”
Journal of Behavioral and Brain Science, July 2011
http://dbm.neuro.uni-jena.de/pdf-files/Luders-JBBS11.pdf

Leire Zubiaurre-Elorza et al, “Cortical Thickness in Untreated Transsexuals”
Cerebral Cortex, August 2012
http://cercor.oxfordjournals.org/content/early/2012/08/30/cercor.bhs267.abstract

Katherine Narr, et al., Relationships between IQ and Regional Cortical Gray Matter Thickness in Healthy Adults
Cerebral Cortex, November 2006
http://cercor.oxfordjournals.org/content/17/9/2163.abstract

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