Summary

Dutch scientists led by Dick Swaab measured a tiny area deep in the brain called the BSTc. It was larger in men than in women, and in six trans women who had died it was the size usually seen in women. Published in Nature, the study became the best-known evidence for the idea of a 'female brain in a male body'. The evidence is thin. There were only six trans brains, all from people who had taken estrogen and testosterone-blocking drugs for years, and most had had their testes removed. So hormone treatment, not something present from birth, could explain it. A follow-up by the same group (Chung et al., 2002) found the male-female difference in this area does not develop until adulthood, long after most gender distress begins. No independent team has repeated it. It is an unconfirmed observation, not proof of an inborn gender identity.

Key findings

• Studiet målte størrelsen af et lille hjerneområde kaldet BSTc i donerede hjerner efter døden ved hjælp af en farve, der markerer visse nervefibre.
• I sammenligningshjerner var dette område omkring 44 % større hos mænd end hos kvinder.
• Heteroseksuelle og homoseksuelle mænd adskilte sig ikke i dets størrelse, hvilket forfatterne tog som tegn på, at forskellen ikke er forbundet med seksuel orientering.
• Hos seks transkvinder havde det kvindelig størrelse, uanset om deres kønsfølelser begyndte tidligt eller senere i livet.
• Forfatterne argumenterede for, at nogle få ikke-transkønnede patienter med usædvanlige hormonniveauer som voksne viste, at hormoner som voksen ikke forklarer dette områdes størrelse.
• Alle seks transkvinder var blevet behandlet med østrogen og det testosteronblokerende lægemiddel cyproteronacetat, og alle på nær én havde fået fjernet deres testikler (Breedlove, Nature 1995).
• Et senere studie af den samme gruppe fandt, at forskellen mellem mænd og kvinder i dette område først viser sig i voksenalderen (Chung, De Vries & Swaab, J Neurosci 2002), hvilket svækker idéen om, at det forårsager kønsfølelser, der begynder i barndommen.

Abstract

Transsexuals have the strong feeling, often from childhood onwards, of having been born the wrong sex. The possible psychogenic or biological aetiology of transsexuality has been the subject of debate for many years. Here we show that the volume of the central subdivision of the bed nucleus of the stria terminals (BSTc), a brain area that is essential for sexual behaviour, is larger in men than in women. A female-sized BSTc was found in male-to-female transsexuals. The size of the BSTc was not influenced by sex hormones in adulthood and was independent of sexual orientation. Our study is the first to show a female brain structure in genetically male transsexuals and supports the hypothesis that gender identity develops as a result of an interaction between the developing brain and sex hormones.

Conclusion

The authors concluded that the BSTc is sexually dimorphic in humans (larger in men), that male-to-female transsexuals have a female-sized BSTc, and that this difference was not explained by adult sex-hormone levels or by sexual orientation. They presented it as the first evidence of a female brain structure in genetically male transsexuals, supporting the hypothesis that gender identity develops from an interaction between the developing brain and sex hormones.