One of the most common things I’ve read in various transition stories is how, after starting hormone replacement therapy [HRT], the man or woman in question began to think differently. “It rewired my mind”; “It made me think like a woman”; “It brought me clarity I didn’t have before.” These are not unexpected experiences, since hormones essentially control how cells talk to one another, and the brain is a bundle of cells soaking in a chemical bath. Alter the chemicals in that bath and of course you’ll alter how the brain functions. But what, empirically, has science discovered about the way HRT affects the brain?
I was prepared to do a ton of PubMed surfing on this subject. In fact, I had already started before discovering this great page from AE Brain, a transwoman and scientist who has been collecting a list of scientific papers on transgender and intersex for some years now. Not only does she have a much more thorough listing of papers relating to brain differences than I was able to uncover in my last Science! article and ten some, but she had also listed the studies I had found about HRT and the brain.
Darn it, all the good ideas are already taken!
One interesting study listed is “Changing your sex changes your brain: influences of testosterone and estrogen on adult human brain structure” by Pol, et al (European Journal of Endocrinology, 2006). They recruited both MtF and FtM transsexual volunteers and measured their brain volumes in several ways both before and after 4 months of HRT. The result?
The ﬁndings suggest that treatment of MFs with estrogens and anti-androgens decreases the male brain size towards female proportions, whereas treatment of FMs with androgens (not substantially affecting circulating estrogen levels) increases the female brain size towards male proportions. The magnitude of this change (i.e. 31 ml over a 4-month period) is striking, since it signiﬁes a decrease in brain volume, which is at least ten times the average decrease of around 2.5 ml per year in healthy adults (16).
Given what prior studies suggest about the ways in which some structures in a transsexual brain already resemble their gender-identifying counterparts, this is an interesting find. Estrogen is, in a sense, rescaling the MtF brain, thus making it a better match for the parts already scaled to perceived gender.
But really, when transfolk say that hormones affected their mind, they’re not talking about brain size; they’re talking about the way they think. What does science have to say?
The honest answer is, I can’t really find anything out there. As far as I have been able to determine, no one has performed a good scientific study on how thinking is effected by HRT. There’s an occasional study, like the one by Schoning, et al. (2010) that will study brain function both pre- and post-HRT. But Schoning, et al., found that even before HRT, MtF brains activated differently, and in fact its conclusion was that HRT did nothing to change that.
So where is the evidence that HRT changes the way we think? It may simply be a preponderance of anecdotes, the accumulated and shared experiences of transgender persons who undertake HRT.
The resource page maintained by A. E. Brain really is worth a perusal. There are a lot of studies out there, more than I realized when I wrote my last Science! piece. But they are almost exclusively dedicated to the ways in which transgender brains are a priori different from cicgender … and the evidence seems to be building that they are different. Perhps HRT simply takes these different brains and “tunes them up,” like putting the proper fuel in a car engine.
Someday, I will find out what HRT does myself. And when I do, I will be sure to add my experience to the pile.