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

• 研究使用標記某些神經纖維的染劑，測量了死後捐贈大腦中一個稱為 BSTc 的微小腦區的大小。
• 在對照用的大腦中，男性的這個區域比女性大約 44%。
• 異性戀與同志男性在其大小上沒有差異，作者認為這表示該差異與性傾向無關。
• 六名跨性別女性的這個區域是女性大小，不論她們的性別感受是早年還是晚年才開始。
• 作者主張，少數成人荷爾蒙濃度不尋常的非跨性別患者，顯示成人的荷爾蒙無法解釋這個區域的大小。
• 六名跨性別女性全都接受過雌激素和阻斷睪固酮的藥物 cyproterone acetate 的治療，且除一人外都已切除睪丸（Breedlove，《Nature》1995）。
• 同一團隊後來的一項研究發現，這個區域的男女差異直到成年才出現（Chung、De Vries 和 Swaab，《J Neurosci》2002），這削弱了它造成童年就開始的性別感受的說法。

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.