Ivanak Savic and Stefan Arver have scanned the brains of 24 heterosexual, pre-operative male-to-female transsexuals and compared their structure to the brains of 24 heterosexual male and 24 heterosexual female controls. Homosexual transsexuals were omitted to help avoid the complicating influence of sexuality on the results. None of the transsexual participants had taken any hormone treatments, which is another factor that could have skewed the findings.
The scans threw up several of the structural brain differences associated with biological sex that have been reported before. For example, the men's brains had more grey matter in the cerebellum (involved in motor control) and lingual gyrus (involved in vision) and less gray matter and white matter in the precentral sulcus (part of the frontal lobe), compared with the women's brains. The men also had smaller hippocampi (involved in memory) than the women. In all these respects the brains of the male-to-female transexuals resembled the brains of the male control group. Likewise, the male-to-female transsexuals, like the male controls, had more asymmetric brains than the female controls. "The present study does not support the dogma that male-to-female transsexuals have atypical sex dimorphism in the brain but confirms the previously reported sex differences in structural volumes, gray, and white matter fractions," the researchers said. In other words, the male-to-female transsexuals may have felt like women, but their brains had structural characteristics typical of men.
But that's not to say that the male-to-female transsexual participants had brains that were unremarkable. Compared with the male and female controls, they had a smaller thalamus (the brain's relay centre) and putamen (an area involved in motor control) and increased gray matter in the right insula and inferior frontal cortex (regions involved in representing the body, among other functions). Savic and Arver advised treating these differences with caution. They've never been found before so need to be replicated with a larger sample. And even if confirmed, it's not clear what these differences mean, or whether they are a cause or consequence of gender dysphoria. "One highly speculative thought is that the enlargement of the ... insular and inferior frontal cortex ... could derive from a constant rumination about one's own body," the researchers said.
More research is needed, with larger samples and including studies of homosexual transsexuals and female-to-male transsexuals. "Any interpretation must, therefore, proceed cautiously and can at this point only be highly speculative," the researchers said.
Savic, I., and Arver, S. (2011). Sex Dimorphism of the Brain in Male-to-Female Transsexuals. Cerebral Cortex, 21 (11), 2525-2533 DOI: 10.1093/cercor/bhr032
Post written by Christian Jarrett for the BPS Research Digest.