The Scientist discusses the biological basis for gender.
On the one hand, there are people who believe that gender identity begins and ends with chromosomes, while there are others who believe that people are completely defined by the state of their genitals at birth...all in accordance with the Christian Bible, of course, that most venerable of scientific tomes.
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.
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. 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.
--Baudewijntje Kreukels, VU University Medical Center, Amsterdam
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. 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.
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.
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. 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.
--Sex Differences in the Brain (2015)
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. 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. 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.
Postmortem examination isn’t a terrific means of identifying transgender people much during our lifetimes, of course.
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.
Georg Kranz, a neuroscientist at the Medical University of Vienna, used diffusion MRI data to investigate differences in white matter microstructure among trans- and cisgender subjects. Cisgender women had the highest levels of a measure of a neural property known as mean diffusivity, cisgender men the lowest, and both transgender men and women fell in between—though it’s not fully understood what mean diffusivity may represent physiologically.
It seems that these transgender groups were at an intermediate stage.
--Kranz
Controlling for individuals’ hormone levels did not alter the differences between groups, leading the authors to suggest that white matter microstructure had instead been shaped by the hormonal environment before and soon after birth—though the possibility that later life experiences also play a role cannot be ruled out.
All available evidence points towards a biologically determined identity. In [transgender] people you would say there was a mismatch in the testosterone milieu during the development of the body and then during development of the brain, so that the body was masculinized and the brain was feminized, or the other way around.
--Kranz
"What we found is that, in several regions, cis women, male-to-female trans, and female-to-male trans have thicker cortex than cis males, but not in the same regions," says Antonio Guillamon, who hypothesized in a 2016 review article that the brains of cisgender women, transgender women, transgender men, and cisgender men may each have a distinct phenotype. “The cortex is vital for gender."
Ivanka Savic, a neuroscientist at the Karolinska Institute in Sweden, also doubts the explanatory power of the developmental mismatch hypothesis. “It is not that simple that transgenderism is due to this disparity between the sex of the brain and the sex of the body,” she says. In 2011, for example, Savic and a colleague found that two brain regions, the thalamus and putamen, were smaller in transgender women than in cisgender controls, but overall gray matter volume was greater. These brain regions had been shown in previous studies to “mediate perception of the body,” Savic notes—for example, in fMRI studies where people were shown photographs of themselves and others.
The dysphoria is being unhappy with [one’s] own body, feeling every morning that ‘This body is mine, but it’s not me,’”
--Savic
In follow-up work, Savic’s group began exploring the brain’s neural networks, as revealed by fMRI, and found that “the connections between the networks mediating self and the networks mediating own body—my body—were weaker in transgender people,” she explains. Specifically, compared with cisgender individuals of both sexes, transgender men showed less connectivity among regions known as the anterior cingulate, posterior cingulate, and precuneus when they viewed images of themselves. But when the images were morphed to appear more male, connectivity between the anterior cingulate and the other two regions increased.
Additionally, logistical challenges confront scientists searching for a biological understanding of gender dysphoria. It is typically difficult to recruit enough transgender subjects to conduct studies with high statistical power. But some researchers are working to remedy that problem. In 2017, for example, the ENIGMA Consortium, which promotes networking and information-sharing among researchers working to detect modest gene effects on brain structure and function, launched a new, transgender-focused working group. And geneticist Lea Davis of Vanderbilt University is organizing a yet-to-be-funded effort to sequence and analyze the genomes of thousands of trans- and cisgender people in search of variations linked to gender dysphoria.
Apart from the big mystery regarding the roots of gender identity, researchers in the field have a number of lingering questions.
For people who transition to identifying as a binary gender different from that assigned at birth, we still also don’t know whether male-to-female and female-to-male transsexualism is actually the same phenomenon, or . . . [whether] you have an analogous outcome in both sexes but you have different mechanisms behind it.
--Elke Smith, RWTH Aachen University
Other outstanding questions include what, if any, differences there are in the brains of transgender people with different sexual orientations, and between those whose gender dysphoria manifests very early in life and those who begin to feel dysphoric during adolescence or adulthood, says Kreukels. Also still to be determined, adds Savic, is whether the brain differences that have been identified between cis and trans people persist after hormone treatment.
If we potentially provide treatment with sex hormones, which we should do for persons who need that, it is very important to know what sex hormones do to the brain.
--Savic
Phylogenetically, and with respect to evolution . . . it is important to know whether one is a male or a female, with whom to copulate. It is one of the pivotal points in biology, and the biology of humans.
--Guillamon, National Distance Education University, Spain
Savic says she hopes the results of studies on transgender people will help make gender identity a less-charged issue.
It’s sort of difficult to change the opinions of conservative Christians with science, which they are historically opposed to.
This is just part of the biology, the same way as I have black hair and somebody has red hair.
--Savic
Like I just said…
But after all it’s not like it’s brain science… .
Oh...wait!