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

• この研究は、特定の神経線維を染める染色法を使って、死後に提供された脳のBSTcと呼ばれる脳のごく小さな領域の大きさを測定しました。
• 比較用の脳では、この領域は男性が女性より約44%大きくなっていました。
• 異性愛の男性とゲイの男性では大きさに違いがなく、著者らはこれを、その差が性的指向とは関連しないことを意味すると考えました。
• トランス女性6人では、性別への感覚が早くから始まったか、人生の後半に始まったかにかかわらず、女性の大きさでした。
• 著者らは、成人のホルモン値が珍しい、トランスではない少数の患者から、成人のホルモンはこの領域の大きさを説明しないことが示されたと主張しました。
• トランス女性6人全員が、エストロゲンとテストステロンを遮断する薬である酢酸シプロテロンで治療されており、1人を除く全員が精巣を摘出していました（Breedlove、Nature 1995）。
• 同じグループによる後の研究は、この領域の男女差が成人になって初めて現れることを見つけており（Chung、De Vries、Swaab、J Neurosci 2002）、これは、それが子ども時代に始まる性別への感覚を引き起こすという考えを弱めます。

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.