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

• L'étude a mesuré la taille d'une minuscule zone du cerveau appelée BSTc dans des cerveaux donnés après le décès, à l'aide d'une coloration qui marque certaines fibres nerveuses.
• Dans les cerveaux de comparaison, cette zone était environ 44 % plus grande chez les hommes que chez les femmes.
• Les hommes hétérosexuels et gays ne différaient pas par sa taille, ce que les auteurs ont pris pour signifier que la différence n'est pas liée à l'orientation sexuelle.
• Chez six femmes trans, elle avait une taille féminine, que leurs ressentis de genre aient commencé tôt ou plus tard dans la vie.
• Les auteurs ont soutenu que quelques patients non trans ayant des niveaux d'hormones adultes inhabituels montraient que les hormones adultes n'expliquent pas la taille de cette zone.
• Les six femmes trans avaient été traitées par œstrogènes et par l'acétate de cyprotérone, médicament bloquant la testostérone, et toutes sauf une avaient subi l'ablation des testicules (Breedlove, Nature 1995).
• Une étude ultérieure du même groupe a trouvé que la différence entre hommes et femmes dans cette zone n'apparaît qu'à l'âge adulte (Chung, De Vries et Swaab, J Neurosci 2002), ce qui affaiblit l'idée qu'elle cause des ressentis de genre qui commencent dans l'enfance.

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.