Brains and Gender

Q: Do brain studies prove that gender identity is innate?

No. The findings come from small, often unreproduced studies in which participants had usually already been using hormones. Cause and effect cannot be distinguished. Cautious scientists conclude that there are indications of neurobiological correlates, not evidence of an innate trans brain. The samples often included fewer than ten participants.

Q: Can hormones change the brain?

Yes, demonstrably. Cross-sex hormones cause structural and functional changes in the adult brain. This makes many “trans brain” studies scientifically problematic. For transgender people who had used estrogen or testosterone for years before being scanned, it is impossible to determine whether observed differences are a cause or a consequence of hormonal treatment. This is the central methodological problem.

Q: Is there a “trans gene”?

No. No single gene has been found that determines gender identity. Twin studies point to a modest hereditary component, but environmental factors play a major role. Concordance among identical twins is far from 100 percent. Prenatal androgen exposure does influence gender-related behavior, but that is different from a specific gender identity.

Q: What does Joel et al. (2015) say about brain sex?

Joel et al. analyzed more than 1,400 brain scans and concluded that human brains do not exhibit binary dimorphism, but are instead mosaics of various characteristics. This makes the concept of “the brain of the opposite sex” conceptually problematic. According to this study, there is no clearly defined male brain or female brain.

Two MRI cross-sections of the brain — Photo: JS 95, CC BY-SA 4.0, via Wikimedia Commons

The claim that transgender people have “a brain in the wrong 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. However, the available neuroscientific evidence is far weaker and more ambiguous than this certainty suggests. The current state of knowledge: there is no conclusive neuroscientific evidence for an innate “trans brain.”

The classic studies and their limitations

Frequently cited postmortem brain studies (Zhou et al. 1995, Kruijver et al. 2000) reported that the bed nucleus of the stria terminalis (BSTc) in trans women appeared more similar to that of cisgender women. However, the samples were small (often fewer than ten participants), most participants had used cross-sex hormones for years, and the brain regions examined do not fully mature until adulthood — undermining direct conclusions about “innate” differences. Attempts at replication have produced inconsistent results. What brain scans of transgender people do and do not show is explained at transgenderidentiteit.nl.

Hormones change the brain

This is the central methodological problem. Cross-sex hormones cause demonstrable structural and functional changes in the adult brain. For transgender people who have used estrogen or testosterone for years before being scanned, it is effectively impossible to determine whether observed differences are a cause or a consequence of hormonal treatment. Studies of hormone-naive participants are rare, small in scale, and methodologically weak.

No coherent “sexed brain” pattern

The broader 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 there is no clearly dimorphic “male brain” and “female brain.” If binary brain sexes do not exist in the first place, the claim that transgender people have “the brain of the opposite sex” becomes conceptually problematic.

Genetic and prenatal indications — limited

Twin studies point to a modest hereditary component in gender dysphoria, but concordance among identical twins is far from 100% — meaning that environmental factors play a major role. Studies of prenatal androgen exposure (such as in congenital adrenal hyperplasia) show an influence on gender-related behavior, but this is different from a specific “gender identity”: the majority of girls with CAH do not develop a trans identity. See also genetics and gender.

The sudden demographic shift rules out biology alone

A conclusive biological explanation for gender identity would also have to explain why the number of young people identifying as trans or non-binary has increased ten- to twentyfold since 2010, accompanied by a reversal in the sex ratio. Brain structures do not change on this timescale. The sociocultural and media-driven factors identified as explanations for this increase are at odds with a strong biological theory.

Scientific caution is warranted

Conclusion: there are intriguing neurobiological indications, but no conclusive evidence for an innate “trans brain.” The categorical way this is communicated in patient information leaflets, on activist websites, and in public media is not consistent with the actual state of the research. Neuroscience should not be used to legitimize irreversible medical interventions for minors while the evidence remains weak and methodologically problematic.

Do brain studies prove that gender identity is innate?

Can hormones change the brain?

Is there a “trans gene”?

What does Joel et al. (2015) say about brain sex?