This is an interesting study that found that taking cross-sex hormones changed the thickness of the cortex in the brain.
I have only been able to see the abstract; the study was published in May 2014 and I do not have access to it.
The study looked at 15 trans men (born female) before and after they took testosterone for at least six months. They also looked at 14 trans women (born male) before and after they took androgen blockers and estrogens for at least six months.
They found that :
“After testosterone treatment, FtMs (trans men) showed increases of CTh bilaterally in the postcentral gyrus and unilaterally in the inferior parietal, lingual, pericalcarine, and supramarginal areas of the left hemisphere and the rostral middle frontal and the cuneus region of the right hemisphere. There was a significant positive correlation between the serum testosterone and free testosterone index changes and CTh changes in parieto-temporo-occipital regions. In contrast, MtFs (trans women), after estrogens and antiandrogens treatment, showed a general decrease in CTh and subcortical volumetric measures and an increase in the volume of the ventricles.”
In other words, taking testosterone makes certain areas of your brain thicker and more testosterone changes your brain more.
Blocking testosterone and taking estrogens makes certain areas of your brain shrink. According to the abstract, this makes the ventricles get bigger – the ventricles are a network of cavities in the brain that contain cerebrospinal fluid.
We already know that there are sex differences in the thickness of the brain’s cortex, although we don’t know exactly what they mean. (You can read more about cortical thickness and what it might mean here.)
Thus study suggests that some of the sex differences we observe in the brain are related to the hormones in our bodies. Our brains are not set in stone by pre-natal exposure to hormones.
For transgender people this study shows that hormone therapy will change your brain.
It does not tell us what that will means in terms of changes in thoughts, feelings, or behaviors.
It’s also not clear if the changes in the trans women’s brains are caused by reducing the testosterone level or adding estrogen or both.
The abstract does not discuss whether the changes caused by the cross-sex hormones make the brain more “masculine” or “feminine” or neither.
It looks like this study is a follow-up to an earlier study, Cortical Thickness in Untreated Transsexuals. The earlier study found that before hormone therapy there were differences between transsexuals and control groups.
The differences the authors found in their earlier study were fairly complicated:
“We would suggest that transsexuals do not show a simple masculinization (FtMs) or feminization (MtFs) of their brains—rather, they present a complex picture in their process of sexual differentiation depending on the brain region studied and the kind of measurements taken.”
In other words, there were some ways in which trans men have brains like cis men’s and some ways in which their brains are like cis women’s while trans women have brains that are like cis women’s in some ways and like cis men’s in others.
One caveat to the pre-hormone part of the study – the authors only included people who were “erotically attracted to subjects with the same anatomical sex.” Thus, it is possible that the brain differences they observed were caused by sexual orientation, not gender identity.
Many studies of gender identity and the brain make this mistake. For example, they will compare a group of trans men who are attracted to women to a group of cis men who are attracted to women and a group of cis women who are attracted to men. It makes it impossible to be sure if any differences between the brains of trans men and cis women are due to gender identity or sexual orientation.
Studies of gender identity and the brain should include control groups of lesbians and gay men as well as straight people.
In any case, the current study shows that taking cross-sex hormones will further change the brain.
Original Article (Abstract):