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

• قاست الدراسة حجم منطقة دماغية صغيرة تسمى BSTc في أدمغة متبرَّع بها بعد الوفاة، باستخدام صبغة تعلّم ألياف عصبية معينة.
• في أدمغة المقارنة، كانت هذه المنطقة أكبر بنحو 44% لدى الرجال منها لدى النساء.
• لم يختلف الرجال الغيريون والمثليون في حجمها، وهو ما عدّه المؤلفون يعني أن الفرق غير مرتبط بالتوجه الجنسي.
• في ست نساء متحولات كانت بحجم الإناث، سواء بدأت مشاعرهن الجندرية مبكراً أو لاحقاً في الحياة.
• رأى المؤلفون أن بضعة مرضى غير متحولين لديهم مستويات هرمونات بالغة غير معتادة أظهروا أن هرمونات البالغين لا تفسر حجم هذه المنطقة.
• عولجت جميع النساء المتحولات الست بالإستروجين والدواء الحاصر للتستوستيرون أسيتات السيبروتيرون، وأُزيلت خصى جميعهن عدا واحدة (بريدلوف، Nature 1995).
• وجدت دراسة لاحقة للمجموعة نفسها أن الفرق بين الذكر والأنثى في هذه المنطقة لا يظهر إلا في سن البلوغ (تشونغ ودي فريس وسواب، J Neurosci 2002)، مما يضعف فكرة أنها تسبب مشاعر جندرية تبدأ في الطفولة.

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.