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

• Изследването измерва размера на малка мозъчна област, наречена BSTc, в дарени мозъци след смъртта, с оцветител, който маркира определени нервни влакна.
• В сравнителните мозъци тази област е около 44% по-голяма при мъжете, отколкото при жените.
• Хетеросексуалните и гей мъжете не се различават по размера ѝ, което авторите приемат за означаващо, че разликата не е свързана със сексуалната ориентация.
• При шест транс жени тя е с женски размер, независимо дали чувствата им към пола са започнали рано или по-късно в живота.
• Авторите твърдят, че няколко нетранс пациенти с необичайни хормонални нива като възрастни показват, че хормоните в зряла възраст не обясняват размера на тази област.
• И шестте транс жени са били лекувани с естроген и лекарството, блокиращо тестостерона, cyproterone acetate, а всички освен една са имали отстранени тестиси (Бридлав, Nature 1995 г.).
• По-късно изследване на същата група установява, че разликата между мъже и жени в тази област се появява едва в зряла възраст (Чунг, Де Врис и Сваб, J Neurosci 2002 г.), което отслабва идеята, че тя причинява чувства към пола, започващи в детството.

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.