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Brain development: The most important sexual organ Carina Dennis1 Top of pageAbstract New evidence suggests that the brain begins to develop differently in males and females much earlier than was thought — before sex hormones come into play. Carina Dennis considers the implications. Susan was whisked away as soon as she was born. Her mother wasn't allowed to bathe or undress her for several days; the doctors simply said that there was something wrong. Susan had an 'intersex' condition — her genitalia offered no clear indication whether she was a girl or a boy. Although Susan is genetically male, doctors convinced her parents that she should undergo surgery to turn her into a girl — the operation is easier to perform, and medical opinion was that, with appropriate hormone treatment, she would develop normally in her assigned gender. And so it turned out: Susan is now in her twenties, married and happy as a woman. But for a small proportion of the one in about 4,000 children born with 'ambiguous' genitalia, things don't work out so well. Tony was also born genetically male and had similar surgery. But he never felt comfortable as a girl. After many years of unhappiness, he began testosterone treatment and now lives as a man. Susan's and Tony's doctors believed that the brain is a sexual blank slate until it comes under the influence of sex hormones. But biologists are now starting to realize that hormones aren't the only significant determinant of the brain's sexual destiny. Indeed, male and female brains may even start moving down different developmental paths before sex hormones are produced in significant quantities. "There is plenty of evidence that hormones organize the brain sexually, but it's not the whole story," says Eric Vilain, a geneticist at the University of California, Los Angeles (UCLA). New findings about the genetic and other factors that influence the brain's sexual development could do more than simply rewrite the textbooks. They might provide insights into conditions such as transsexualism — and perhaps eventually lead to tests that could determine whether a baby with an intersex condition is more likely to grow up thinking, feeling and behaving like a man or a woman. Sex discrimination
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