Xenogenders

The Transgender Brain

Are The Brains Of Transgender People Different From Those Of Cisgender People?

Are the Brains of Transgender People Different from Those of Cisgender People?

Research into the biological basis of gender identity is in its infancy, but clues are beginning to emerge.

Shawna Williams

Feb 28, 2018

© ANA YAEL
In recent years, US society has seen a sea change in the perception of transgender people, with celebrities such as Caitlyn Jenner and Laverne Cox becoming the recognizable faces of a marginalized population. Transgender rights have also become a mainstream political issue, and the idea that people should be referred to by the names and pronouns they find most fitting—whether or not these designations match those on their birth certificates, or align with the categories of male and female—is gaining acceptance.
Yet a biological understanding of the contrast between the natal sex and the gender identity of transgender people remains elusive. In recent years, techniques such as functional magnetic resonance imaging (fMRI) have begun to yield clues to possible biological underpinnings of the condition known as gender dysphoria. In particular, researchers are identifying similarities and differences between aspects of the structure and function of the brains of trans- and cisgender individuals that could help explain the conviction that one’s gender and natal sex don’t match.

Techniques such as functional MRI have begun to yield clues to possible biological underpinnings of gender.

The results may not have much effect on how gender dysphoria is diagnosed and treated, notes Baudewijntje Kreukels, who studies gender incongruence at VU University Medical Center in Amsterdam. “It’s really important that it will not be seen as, ‘When you see [gender dysphoria] in the brain, then it’s true.’” But the insights from such research could go a long way toward satisfying the desire of some transgender people to understand the roots of their condition, she adds. “In that way, it is good to find out if these differences between them and their sex assigned at birth are reflected by measures in the brain.”

A developmental mismatch between sex and gender?

One prominent hypothesis on the basis of gender dysphoria is that sexual differentiation of the genitals occurs separately from sexual differentiation of the brain in utero, making it possible that the body can veer in one direction and the mind in another.1 At the root of this idea is the notion that gender itself—the sense of which category one belongs in, as opposed to biological sex—is determined in the womb for humans. This hasn’t always been the scientific consensus. As recently as the 1980s, many researchers argued that social norms in how we raised our children solely dictated the behavioral differences that developed between girls and boys.
Perhaps the most famous proponent of this line of thinking was psychologist John Money, who went so far as to posit that a male baby with a congenital abnormality of the penis, or who had lost his penis in a surgical accident, could successfully be raised as a female following treatment with surgery and hormones. In at least one of Money’s cases, however, this course of action backfired dramatically: the subject reverted to living as a man during his teen years, and later committed suicide. Sex differences in the brain are now well documented, although the extent to which these arise from biological versus social factors is still hotly debated.

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The developmental mismatch idea draws support from two sets of findings. Animal studies demonstrated that the genitals and the brain acquire masculine or feminine traits at different stages of development in utero, setting up the potential for hormone fluctuations or other factors to put those organs on different tracks. (See “Sex Differences in the Brain,” The Scientist, October 2015.) And human studies have found that, in several regions, the brains of trans people bear a greater resemblance to those of cis people who share the trans subjects’ gender than to those of the same natal sex.
Dick Swaab of the Netherlands Institute for Neuroscience is a pioneer in the neuroscience underlying gender identity. In the mid-1990s, his group examined the postmortem brains of six transgender women and reported that the size of the central subdivision of the bed nucleus of the stria terminalis (BSTc or BNSTc), a sexually dimorphic area in the forebrain known to be important to sexual behavior, was closer to that of cisgender women than cisgender men.2 A follow-up study of autopsied brains also found similarities in the number of a certain class of neurons in the BSTc between transgender women and their cisgender counterparts—and between a transgender man and cisgender men.3 These differences did not appear to be attributable to the influence of endogenous sex hormone fluctuations or hormone treatment in adulthood. In another study published in 2008, Swaab and a coauthor examined the postmortem volume of the INAH3 subnucleus, an area of the hypothalamus previously linked to sexual orientation. The researchers found that this region was about twice as big in cisgender men as in women, whether trans- or cisgender.4
And it’s not just brain structure that appears to link transgender individuals more closely to people of their experienced gender than those of their natal sex. Functional similarities between transgender people and their cisgender counterparts were apparent in a study led by Julie Bakker of VU University Medical Center and the Netherlands Institute for Neuroscience in Amsterdam that examined neural activity during a spatial-reasoning task. Previous studies had indicated that the exercise engaged different brain areas in men and women. Bakker and colleagues found that trans boys (who had not been exposed to testosterone, but had had female pubertal hormones suppressed) as well as cisgender boys, displayed less activation than cisgender girls in frontal brain areas when they performed the task.5