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No brain marker for gender identity

"A female brain in a male body" is an empty metaphor. Not a single brain study provides a marker that allows for an individual diagnosis. Yet this claim is sold as scientifically established — a creed disguised as neuroscience.

The Zhou Study (1995): Foundation Under a Myth

Jiang-Ning Zhou, Michel Hofman, Louis Gooren and Dick Swaab published a study in Nature in 1995 on the BSTc nucleus in six post-mortem brains of trans women (source). Conclusion: BSTc volume was comparable to that of women. This small study is still cited three decades later, although the Cass Review points to the limits of this kind of brain research (source). More detail: Zhou's brain research .

Four important limitations

1. Sample size n=6, which greatly limits statistical power. 2. On a possible effect of hormone treatment: the authors conclude that BSTc size was not influenced by sex hormones in adulthood (source), but with six brains this conclusion rests on very little data. 3. BSTc dimorphism only appears in adulthood (Chung et al., 2002 (source)), long after the onset of gender dysphoria in childhood. 4. The follow-up studies come from the same research group: Kruijver et al. (2000, 42 brains (source)) and Garcia-Falgueras and Swaab (2008, eleven trans women and one trans man (source)).

What reviews and larger studies show

Guillamon et al. (2016, Archives of Sexual Behavior ) reviewed neuroimaging studies of homosexual trans women and trans men and concluded that their brains show a mix of masculine, feminine and demasculinised or defeminised areas, a phenotype that differs from that of heterosexual men and women (source). Joel et al. (2015, PNAS ) showed that brains do not show a uniform 'male' or 'female' pattern in any case; they are mosaics. Sex differences, where they exist, overlap widely. These are average group differences; according to the Cass Review, research has not reliably identified brain changes directly linked to gender incongruence (para. 8.20) (source).

The ad-hoc saving of a dogma

When brain studies show no marker, the affirmative argument falls back on: 'gender identity need not be biological'. A classic unfalsifiable retreat: the claim is adjusted to every negative result. See also no measurable marker and no genetic marker for this pattern. The Cass Review calls care for young people with gender dysphoria 'an area of remarkably weak evidence' (source).

What remains: self-reporting

No gene, no brain scan, no blood test. In the absence of a marker, assessment relies on self-report within a clinical assessment; according to the Cass Review, few guidelines recommend formal measurement tools and the existing ones are poorly validated (para. 10.8) (source). Whoever lets go of the idea of a 'trans brain' sees what remains: a metaphysical claim disguised as a diagnosis.