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Who do we think we are? The brain and gender identity.

No-one, wrote Frank Beach, a notable contributor to the experimental study of hormones and sexual behaviour, ever died from lack of sex. But the personal, social and legal aspects of sexual behaviour are a pervasive pre-occupation in all humans. The variety and vagaries of sex can have severe implications, and the existence of homosexuality and disorders of gender identity demand some sort of explanation (Bancroft, 2008). Neuroscience can ask itself, therefore, why it has contributed so little to understanding human sexuality. One reason is our overall ignorance about the brain, which hinders attempts to relate particular patterns of brain activity to an observable behaviour in a way that contributes to understanding. Another is the effect of sexual mores on the study of sexuality itself: studying sex is still considered a slightly risque career, and made difficult by the politics, constraints and prejudices of human societies. It took the AIDS epidemic to convince many governments and funding bodies that studying sex was important and respectable. Most of our information on the neurobiology of sex comes from animal studies (Becker et al. , 2005), but nearly all of what we know about variations in human sexuality, including hetero- and homo-sexuality, and disorders of gender identity (transsexualism), comes from clinical material, anecdotes or even fiction (the three overlap).

It is now well-known that sex-determination is heavily reliant upon the sry gene encoded on the Y chromosome. But genes, of course, do nothing themselves: they activate molecules that are the mechanisms, and prominent amongst these is testosterone. During development, the male fetus secretes testosterone—and production may continue into early post-natal life. This has dramatic effects on the internal reproductive organs, promoting the masculine arrangement. But, here we are …