In Scientific American Mind, journalist Francine Russo takes on a fascinating research question: “Is there something unique about the transgender brain?” She reviews some of the relevant brain research on transsexuals and concludes that transgenderism is indeed a phenomenon of the brain. Although I agree with Russo that transgenderism is a phenomenon of the brain, I believe Russo over-focussed on gender identity, which led her away from the better explanation of the data:
These brain scans don’t reflect gender identity, they reflect sexual orientation.
The research that Russo examined she explained correctly: Biological males and females differ from each other on certain subtle, non-learned features of the brain and body, and some samples of transsexuals are in-between on these features. That is, they were shifted away from what is typical for their sex-at-birth and towards the other sex. Taken by itself, this would indeed seem like strong evidence that transsexuality in biological males is caused by having “feminine” or “feminized” features of the brain, and ditto for females/masculine/masculinized.
Unfortunately, Russo’s analysis excludes other, closely related research that would have led her to a different conclusion: Namely, gay men and lesbians show the same shifts! That is, the differences are not specific to transgenderism. For example, Russo pointed out (correctly) that males and females differ in “echolike sounds produced by the inner ear in response to a clicking noise” and that transgendered folks are shifted towards what the opposite sex usually shows. Missing, however, was the research showing that the same shift is present in cis-gendered homosexuality (e.g., McFadden & Pasanen, 1998). [For those interested, these echo-like responses are called “click-evoked otoacoustic emissions,” and the shift among transgendered folks was reported by Burke et al. (2014). The full reference appears below.]
The reason this matters is that, in the MRI study Russo describes (Zubiaurre-Elorza et al., 2013), all of the male-to-female transsexuals were attracted to men, but all of the control males were attracted to women, and all of the female-to-male transsexuals were attracted to women, but all the control females were attracted to men. That is, although the subject groups differed from the control groups in their gender identity (as they must), they also differed in their sexual orientation. This is what we call a confound: We can’t know whether the brains of the male-to-female transsexuals differed from the controls because they wanted to be women while the controls wanted to be men, or because they were attracted to men while the controls were attracted to women (and ditto for female-to-male transsexuals with regard to men/women/women/men).
Although Russo attributes Zubiaurre-Elorza’s MRI findings to gender identity, those findings are better attributed to sexual orientation. This would have shown by other research that Russo excluded. Specifically, Russo excluded other MRI studies of transsexuals, namely Rametti et al. (2010) and Savic and Arver (2010). Rametti used a set-up very much like Zubiaurre-Elorza. (In fact, Rametti and Zubiaurre-Elorza are from the same research team, led by Antonio Guillamon.) Rametti compared 18 male-to-female transsexuals who were attracted to men with: 19 cis-gendered control males who were attracted to women, and with 19 cis-gendered control females who were attracted to men. Guillamon contrasted the male controls with the female controls to find the features of the brain where males and females differ (there were six such features), and they found the transsexual group to be intermediate on all of them. That is, male-to-female transsexuals who were attracted to men were in between cis-gendered males who were attracted to women and cis-gendered females who were attracted to males.
Now, let us compare that with the Savic and Arver study: They also analyzed male-to-female transsexuals (24 in total), but these were male-to-female transsexuals who were attracted to women. Savic and Arver contrasted them with 24 control males who were attracted to women, and with 24 control females who were attracted to men. Like Rametti, Savic and Arver contrasted the male controls with the female controls to find the brain features where males and females differ (there were eight), but found the transsexual group to differ from the male controls on none (Savic & Arver, 2010, Table 3). In contrast with Rametti and with Savic and Arver, the transgendered bio males who were attracted to women did not differ from cis-gendered bio males who were attracted to women. That is, what we’ve observed in the brain depends on whether the groups differ in sexual orientation, not gender identity.
Just as interestingly (if not more so!) the male-to-female transsexuals who were attracted to females did show other brain differences from the control males, but they were not features in which females differ from males. To me, this brings us to the next fascinating and unanswered question for us sexual neuroscientists: What is it about male-to-female transsexuals who are attracted to females that makes their brains different from other people’s, and is not just in between biological males and females?
References
Burke, S. M., Menks, W. M., Cohen-Kettenis, P. T., Klink, D. T. & Bakker, J. (2014). Click-evoked otoacoustic emissions in children and adolescents with gender identity disorder. Archives of Sexual Behavior, 43, 1515−1523. doi: 10.1007/s10508-014-0278-2.
McFadden, D., & Pasanen, E. G. (1998). Comparison of the auditory systems of heterosexuals and homosexuals: Click-evoked otoacoustic emissions. Proceedings of the National Academy of Sciences, 95, 2709–2713.
Rametti, G., Carrillo, B., Gómez-Gil, E., Junque, C., Zubiarre-Elorza, L., Segovia, S.,... Guillamon, A. (2010). The microstructure of white matter in male to female transsexuals before cross-sex hormonal treatment: A DTI study. Journal of Psychiatric Research. doi:10.1016/ j.jpsychires.2010.11.007.
Savic, I., & Arver, S. (2010). Sex dimorphism of the brain in male-to- female transsexuals. Cerebral Cortex. doi:10.1093/cercor/bhr032.
Zubiaurre-Elorza, L., Junque, C., Gómez-Gil, E., Segovia, S., Carrillo, B., Rametti, G., Guillamon, A. (2013). Cortical thickness in untreated transsexuals. Cerebral Cortex, 23, 2855−2862.
For professional researchers, see also:
Cantor, J. M. (2011). New MRI studies support the Blanchard typology of male-to-female transsexualism. Archives of Sexual Behavior, 40, 863–864. doi: 10.1007/s10508-011-9805-6
