ConclusionSome of these criticisms have been met - at least partially - since 2011. Others remain. As a summary of the state-of-the-art back then, I could only improve on it marginally.
The amount of research that has been completed on the differences between transsexual and cissexual brains is small, but suggestive. Some areas of the brain appear to be sexually dimorphic irrespective of genetics or hormones (e.g., BSTc), whereas others appear to be more dependent upon sex hormones (e.g., brain volume). From the results of these studies, one may infer that pre-transition transsexual women’s brains are feminine in the BSTc and INAH3, partially feminine in their white matter tracts, and masculine in total brain and hypothalamic volume. Data in transsexual men is rarer because these studies are conducted in the western world, where transsexual women outnumber transsexual men (Gooren, 2006). However, studies of transsexual men appear to imply that the reverse is true for them.
These studies have numerous limitations. First, they have yet to be replicated. Replication is needed to ensure reliability and generalizability of these results. Second, these studies (especially those involving deceased brains) have small numbers of subjects, especially for the transsexual subjects. Studies involving brains from the Netherlands Brain Bank typically had fewer than 10 transsexual brains to study, and the latter two (Kruijver et al, Chung et al) only had a single transsexual male brain. Although fewer numbers of subjects are generally more acceptable in biological research than in psychological research, it is still a potential source of error.
Potentially the most glaring limitation in these studies is their conflation of sexual orientation and gender identity. Berglund et al (2008), for example, only compared heterosexual cissexual women and men with gynephilic/homosexual transsexual women. While they could not include androphilic/heterosexual transsexual women in their study because of rarity, they failed to include homosexual cissexual men and women as comparison groups. This introduces a potentially confounding variable.
Despite these limitations, evidence so far is suggestive of a biological influence in transsexuality. More research is needed to confirm and expand these preliminary findings.