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Brain and gender

The claim that trans people have "a brain in the other body" — a female brain in a male body or vice versa — is regularly used in public communication to present trans identification as innate and biological. The available neuroscientific evidence is, however, much weaker and more ambiguous than this certainty suggests. The state of affairs: there is no conclusive brain-scientific evidence for an innate "trans brain".

The classical studies and their limitations

Frequently cited MRI studies (Zhou et al. 1995, Kruijver et al. 2000) reported that the bed nucleus of the stria terminalis (BSTc) in trans women would resemble that of cisgender women more closely. The samples were, however, small (often fewer than ten participants), most participants had used cross-sex hormones for years, and the brain regions found only fully mature in adulthood — which undermines direct conclusions about "innate" differences. Replication attempts yield mixed results.

Hormones change the brain

This is the methodological core problem. Cross-sex hormones cause demonstrable structural and functional changes in adult brains. In trans people who have used oestrogen or testosterone for years before being scanned, it is effectively impossible to determine whether observed differences are cause or consequence of the hormonal treatment. Studies in hormone-naive participants are rare, small in scope and methodologically weak.

No coherent "brain-sex" picture

The wider field of sex differences in the brain is itself controversial. Joel et al. (2015) argued that most human brains are a "mosaic" of so-called male and female characteristics, and that an unambiguously dimorphic "male brain" and "female brain" does not exist. If binary brain sexes do not exist in the first place, the claim that trans people have "the brain of the other sex" becomes conceptually problematic.

Genetic and prenatal clues — limited

Twin studies point to a modest hereditary component for gender dysphoria, but concordance in identical twins is far from 100% — meaning environmental factors weigh heavily. Studies on prenatal androgen exposure (as in congenital adrenal hyperplasia) show influences on gender-related behaviour, but this is something different from a specific "gender identity": the majority of CAH girls do not develop a trans identity. See also genetics and gender.

The sudden demographic shift rules out pure biology

A conclusive biological explanation for gender identity would also need to explain why the number of young people identifying as trans or non-binary has multiplied tenfold to twentyfold since 2010, with an inversion of the sex ratio. Brain structures do not change on this time scale. The socio-cultural and media-driven factors invoked for this rise are at odds with a strong biological theory.

Scientific caution is in order

Conclusion: there are intriguing neurobiological clues, but no conclusive evidence for an innate "trans brain". The categorical communication about this in patient leaflets, on activist sites and in mainstream media is not in line with the actual state of the research. Brain science should not be used as a legitimation for irreversible medical interventions in minors as long as the evidence is weak and methodologically problematic.