This article found that trans women’s brains are more similar to men’s brains than cis women’s brains, at least in terms of the pattern of gray matter variation.
They also found that trans women’s brains had more gray matter in the putamen than both cis men and cis women, although the difference was only significant for cis men.*
The authors found 20 areas of the brain where women had more gray matter than men. The male-to-female transsexuals (trans women) had the smallest volume of gray matter in these areas, but their data spectrum mostly overlapped with the men’s.
In two areas of the brain, the left and right putamen, male-to-female transsexuals had the largest volume of gray matter.**
“…the gray matter volume of this particular structure in the MTF transsexual group was both larger than in males and within the average range of females.”
Gray matter, in case you’re wondering, contains more neuronal cell bodies. White matter is mostly made up of axons that transmit signals within the brain. Previous studies have found that there are sex differences in the distribution of gray matter in the brain.
The authors describe the putamen as being “feminized” in MTF transsexuals. That might be, but it might also be that their putamens are simply different from cis people’s for some other reason.
In addition, the putamen has more gray matter in women than in men, but the trans women’s putamens had more gray matter than either, although the difference between trans women and cis women was probably not significant.**
The authors conclude:
“Overall, our study provides evidence that MTF transsexuals possess regional gray matter volumes mostly consistent with control males. However, the putamen was found to be “feminized” in MTF transsexuals….”
“Taken together, these findings lend support to the hypothesis that specific neuroanatomical features are associated with transsexual identity, where the particular role of the putamen requires investigation in future studies.”
The study results also support the idea that trans women’s brains are more similar to men’s brains than to cis women’s brains. Most of the time when men’s and women’s brains differ, the trans women’s brains were like men’s.
In addition, the authors briefly mention a few areas where women’s and men’s brains were similar, but trans women’s brains were different from cis women’s (see details of study below).
On the other hand, we definitely need more studies of the putamen and gender dysphoria.
As the authors point out, we do not know if the differences in the putamen are the cause or result of gender dysphoria – or if the differences are caused by another factor that also causes gender dysphoria.
It is also possible that the observed differences are caused by sexual orientation, not gender identity. The authors explain that their sample included 6 male-oriented people (25%) and 18 female-oriented people (75%). They did not know the sexual orientation of their control groups, but it is likely that 95% of the males were attracted to females and 95% of the females were attracted to males.
Their sample also included more left-handed people than the control groups. It is possible that handedness affects the size of the putamen in some way.
Another huge issue is that we have no flipping idea what the results mean. The putamen is an area of the brain that is believed to be involved in many different functions including motor skills, memory, and processing sensory information. If it is involved in gender dysphoria, we need studies to figure out how.
Hopefully we will see some studies confirming this result, this time with a control group that includes gay men and lesbians. The study should also control for handedness.
Then we will need more studies looking at exactly what is going on.
Some details of the study:
The authors looked at 24 trans women recruited through the community organizations and professionals who serve trans people. Their average age was 43 (range 23-72). They were genetic males (they had the SRY gene),*** they were free of psychoses, and they passed a physical and neurological exam. 76% of them were right-handed, compared to 90% or more of the controls.
None of them were on hormones, although they all intended to take them.
More about the results:
“females had more gray matter than males in large portions of the brain… Similarly, females had more gray matter than MTF transsexuals… Although the differences between females and MTF transsexuals did partly overlap with the difference between females and males…, they were spatially more extended, and also evident in a few regions where females and males did not differ… There was no region where females had significantly less gray matter than males… or MTF transsexuals… Similarly, there was no region where MTF transsexuals had significantly less gray matter than males… MTF transsexuals, however, showed significantly more gray matter than males in the right putamen… MTF transsexuals also showed significantly more gray matter than males in the left putamen when findings were not corrected for multiple comparisons (p<0.001, maps not shown).”
I am intrigued by the mention of a few regions where females and males did not differ but females had more gray matter than MTF transsexuals. I wish the authors had discussed these areas. Perhaps they would shed some additional light on gender dysphoria.
*The box plot data on the trans women’s putamen looks pretty different from the cis women’s data. Their median value is higher and their range seems to be much bigger and go up higher. However, the difference is probably not significant since the authors say elsewhere that there was no area where females had significantly less gray matter than trans women.
** It may be that the difference was only statistically significant in the right putamen. Elsewhere in the study the authors say that trans women had significantly more gray matter than males in the left putamen only when the findings were not corrected for multiple comparisons. Because brain scans involve collecting so many data points, the chances of finding correlations by chance are much greater. Thus you have to make corrections. On the other hand, in this case, the other half of the putamen was different at a stastically significant level.
*** Zoe Brain has pointed out that someone could have the SRY gene, but still have unusual chromosomes, if they had Kleinfelter’s syndrome (47xxy) or mosaicism (46xx/46xy). Her interpretation of this study is quite different from mine, but well worth reading.