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

• 该研究用一种标记某些神经纤维的染色剂，测量了死后捐献的大脑中一个称为BSTc的微小脑区的大小。
• 在对照大脑中，该区域男性比女性大约44%。
• 异性恋和同性恋男性在其大小上没有差异，作者认为这意味着这一差异与性取向无关。
• 在六名跨性别女性中，无论她们的性别感受是早期开始还是后来开始，其大小都是女性的大小。
• 作者认为，几名具有异常成年期激素水平的非跨性别患者表明，成年期激素并不能解释该区域的大小。
• 这六名跨性别女性都接受了雌激素和阻断睾酮的药物醋酸环丙孕酮的治疗，除一人外都切除了睾丸（Breedlove，Nature 1995年）。
• 同一团队后来的一项研究发现，该区域的男女差异只在成年期才出现（Chung、De Vries和Swaab，J Neurosci 2002年），这削弱了它导致始于童年的性别感受这一观点。

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.