According to a new paper, the brains of male-to-female transexuals are no more "female" than those of men.
The authors write that "The present data do not support the notion that brains of male-to-female transexuals are feminized" and conclude "The present study does not support the dogma that male-to-female transexuals
have atypical sex dimorphism in the brain".
sentence has gained quite a bit of coverage, including a quote on the Wikipedia page for "transgender".
But is it so simple?
Structural MRI scans were used to compare the size of various brain structures between three groups of volunteers: heterosexual men, heterosexual women and the transexuals (or "MtF"s as I will call them for short) who were diagnosed with gender dysphoria and were "genetically and phenotypically males".
There were 24 in each group, which makes it a decent sized study. None of the MtFs had started hormone treatment yet, so that wasn't a factor, and none of the women were on hormonal contraception.
The scans showed that the non-transsexual male and female brains differed in various ways. Male brains were larger overall but women had increases in the relative volumes of various areas. Male brains were also more asymmetrical.
The key finding was that on average, the MtF brains were not like the female ones. There were some significant differences from the male brains, but they weren't the same differences that distinguished the females from the males.
This is a fairly crude approach. It looks at the groups on average. It's a finding, but there's more you could with this data. It would be better perhaps to look at the male and female groups, and then try to work out which group each individual MtF is most similar to. You could do that using a Support Vector Machinesuch as was previously used to detect autism.
This would also have the advantage that it would integrate the results across different brain areas: maybe the important thing is not just the size of individual areas but the relative size of one area to another area.
My real problem though is with the language used to discuss the data. The authors say that the study doesn't support "atypical sex dimorphism in the brain" yet this wasn't a study of "the brain". It was a study of one specific aspect of the brain, namely the volume of different regions. There could be all kinds of chemical and microstructural differences that don't show up on these scans.
There are lots of people with severe epilepsy, for example, whose brains clearly differ in some major way from people without epilepsy, yet they look completely normal on MRI. Only using other methods, like EEG, reveals the difference. Because the difference is chemical, not structural.
I have no idea how, or if, the brains of MtF transsexuals are "feminized" but this study doesn't rule it out. Now I'm sure the authors know all this. And in fact they themselves recently published a paper showing atypical neural responses to smelling "oderous steroids" in transsexual people. But while neuroscientists will know what they meant, I worry that studies like this could be miscontrued by other people (like Wikipedia readers) as a result of overenthusiastic language in papers.
Savic I, & Arver S (2011). Sex dimorphism of the brain in male-to-female transsexuals. Cerebral cortex (New York, N.Y. : 1991), 21 (11), 2525-33 PMID: 21467211