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One of the most common things I’ve read in various transition stories is how, after starting HRT, the man or woman in question began to think differently. “It rewired my mind,” said one; “It made me think like a woman,” said another; “It brought me clarity I didn’t have before,” said a third. 

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? And are these merely differencs of chemical reaction, or is the brain one of the body parts that is physically altered by HRT? 

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. She has compiled a thorough amount of support for not only the argument that trans brains start out different, but also that HRT does in fact change the physical brain. 

One interesting study 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. They’re anecdotes without a lot of solid research to back them up. As far as I have been able to determine, no one has performed a good scientific study on how cognition 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. Scientific research on the trans brain is 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. You’ve got a female brain? You need to be using the right octane! Thus, it’s not so much that HRT changes the brain or the way we think; it just allows us to think better

[An earlier version of this post appeared on Ali Finds Her Self.]


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