Debunking the “Brain Sex Hypothesis” — Excerpt from Binary
Activists claim that trans-identified people have opposite sex brains. But what does the science actually say? The following is an excerpt from Chapter 10 of my book, Binary.
The traditional “brain sex” hypothesis is that, because the brain develops later than the genitals, the brain and genitalia may develop in opposite directions, whether due to expression of certain genes or exposure to atypical levels of sex hormones like androgen or estrogen.[1] The result would be, according to sex spectrum proponents, a “female” brain in a male body or vice versa, placing someone on the spectrum between male and female. This hypothesis, however, is no longer tenable in neuroscience.
Males and females have, on average, differences in the brain that vary by sex, even after controlling for height and weight.[2] These regions are called sexually dimorphic. Though they are relatively small to moderate differences, they do exist. Like all sex differences, some males can have some traits similar to typical females, and some females can have some traits more similar to males, such as a short male or a tall female. The same concept applies to sex differences in the brain. Some trans individuals have been shown to have sexually dimorphic regions more like the opposite sex, providing evidence for sex spectrum proponents that trans individuals are the sex they claim to be. However, these claims no longer hold up, because it has been repeatedly shown that when you account for sexual orientation, the differences disappear. In other words, the differences in sexually dimorphic brain regions found in some trans individuals are not due the person’s trans identity, but rather due to the cohort’s homosexuality.[3]
Of the studies that showed differences in sexually dimorphic regions of the brain in trans individuals, not only did most studies have zero homosexual controls, but most of the trans subjects were homosexual or mixed.[4] This confounds the results, because sexuality was not controlled for.[5] [6] [7] Some authors even noted this in their own studies, saying that because they did not account for sexual orientation, their results could be due to sexual orientation or gender identity.[8] However, of those that did control for sexual orientation, the differences in sexually dimorphic regions in the homosexual trans subjects aligned closely with those of the homosexual controls. And these differences did not show up in trans subjects who were heterosexual.[9] [10] For example, one study of heterosexual male-to-female transsexuals revealed their brains aligned more closely with that of heterosexual male controls, showing no signs of feminization.[11] Another showed the sex-atypical findings in trans individuals became sex-typical once sexual orientation was accounted for.[12] Controlling for sexual orientation seems to eliminate the relationship between atypical sexually dimorphic regions and being trans. Sexually dimorphic differences in some trans individuals, once thought to be evidence of the “opposite sex brain” hypothesis, seem to be evidence of the subject’s homosexuality.
Thus, studying both homosexual and heterosexual transsexuals in comparison with homosexual and heterosexual controls reveals that sexually dimorphic brain differences are not due to trans identity. Trans individuals who are heterosexual tend to have brains which align with heterosexual controls, and trans individuals who are homosexual tend to have brains which align with homosexual controls. The authors of one study exploring brain network connectivity differences concluded:
The present data do not support the hypothesis that sexual differentiation of the brain in individuals with [gender dysphoria] is in the opposite direction as their sex assigned at birth.[13]
Knowing that differences in sexually dimorphic regions of the brain seem unrelated to having gender dysphoria or having a trans identity, we are left with a question: Can any of the experiences of gender dysphoric trans people be explained through neuroscience? Tentatively, it seems like they can. Some areas of the brain that are non-sexually dimorphic do show differences that are unique to having gender dysphoria and being trans, such as the default mode network.[14] This extensive and interconnected group of brain structures is involved in the perception of self and the body.[15] Differences in these areas can be neurological markers for conditions involving self-body perception, such as eating disorders, body integrity disorders, and even gender dysphoria.[16] [17]
One 2019 study by neuroscientists Manzouri and Savic showed that in both FtM and MtF transsexuals with gender dysphoria, the functional connections within the default mode network were less pronounced than in both male and female controls. And these differences persisted even after controlling for sexual orientation. In her piece, Legato even admits that neurobiological indicators for gender dysphoria and trans identity are more linked to self-referential networks. Quoting a neurobiologist who studies the brains of both trans and control subjects, she writes:
Gender dysphoria is related to the cerebral networks mediating self-body perception which are formed early in development and finally become permanent.
It is unclear why Legato thinks that gender identity misalignment, believing one is the opposite sex, somehow makes the categories of male and female unreliable. We can only know whether someone’s “gender identity” is misaligned if we first know what sex they are. Being able to clearly divide humans into males or females is an important prerequisite to treating and understanding males and females who experience gender dysphoria. It is absurd to use one’s sense of self, whether congruent or incongruent, as a marker for one’s sex, and further, to claim that incongruence itself is evidence of male and female being undefinable categories. The incongruence can only exist if the two sexes exist.
This excerpt is from the book, Binary. You can order the book on Amazon in print and eBook formats.
[1] Bao, A., Swaab, D. (2011). Sexual differentiation of the human brain, gender identity, sexual orientation, and neuropsychiatric disorders. Frontiers in Neuroendocrinology, 32, 214-226.
[2] DeCasien, A., et al. (2022). Sex differences in the human brain: a roadmap for more careful analysis and interpretation of a biological reality. Biology of Sex Differences, 13(43).
[3] Manzouri, A., Savic, I. (2019). Possible neurobiological underpinnings of homosexuality and gender dysphoria. Cerebral Cortex, 29.
[4] Kurth, F., et al. (2022). Brain sex in transgender women is shifted towards gender identity. Journal of Clinical Medicine, 11(6).
[5] Luders, E., et al. (2009). Regional gray matter variation in male-to-female transsexualism. NeuroImage, 46(4), 904-907.
[6] Nawata, H., et al. (2010). Regional cerebral blood flow changes in female to male gender identity disorder. Psychiatry and Clinical Neurosciences, 64.
[7] Uribe, C., et al. (2020). Brain network interactions in transgender individuals with gender incongruence. NeuroImage, 211.
[8] Kranz, G., et al. (2012). Cerebral serotonin transporter asymmetry in females, males, and male-to-female transsexuals measured by PET in vivo. Brain Struct Funct, 219(1).
[9] Manzouri, A., Savic, I. (2019). Possible neurobiological underpinnings of homosexuality and gender dysphoria. Cerebral Cortex, 29.
[10] Savic, I., Arver, S. (2011). Sex dimorphism of the brain in male-to-female transsexuals. Cerebral Cortex, 21.
[11] Savic, I., Arver, S. (2011). Sex dimorphism of the brain in male-to-female transsexuals. Cerebral Cortex, 21.
[12] Burke, S., et al. (2017). Structural connections in the brain in relation to gender identity and sexual orientation. Scientific Reports, 7.
[13] Feusner, J., et al. (2017). Intrinsic network connectivity and own body perception in gender dysphoria. Brain Imaging Behavior, 11(4).
[14] Khorashad, B., et al. (2021). Cross-sex hormone treatment and own-body perception: behavioral and brain connectivity profiles. Scientific Reports, 11, 2799.
[15] Simon, R., Engström, M. (2015). The default mode network as a biomarker for monitoring the therapeutic effects of meditation. Frontiers in Psychology, 6.
[16] McFadden, K., et al. (2014). Reduced salience and default mode network activity in women with anorexia nervosa. Journal of Psychiatry & Neuroscience, 39(3), 178-188.
[17] Fang, A., et al. (2022). Maladaptive self-focused attention and default mode network connectivity: a transdiagnostic investigation across social anxiety and body dysmorphic disorders. Social Cognitive and Affective Neuroscience, 17(7), 645-654.
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