A cord of nerve fibres that connects the left to the right side of the brain is larger in homosexual men than in heterosexual men or in women, according to researchers at the University of California in Los Angeles (UCLA).
The discovery, based on studying brains obtained from autopsies, was warmly welcomed by homosexual groups in the United States as evidence that homosexuality is not merely a question of choice. They called for legislation to be introduced outlawing all forms of discrimination against homosexuals, bringing them into line with ethnic minority groups and women.
Last year, a public debate erupted after Dr Simon LeVay, a neuroscientist at the Salk Institute in La Jolla, California, published claims that a part of the brain which influences sexual behaviour was smaller in homosexual men than it was in heterosexual men.
The latest findings, reported in the Proceedings of the National Academy of Sciences, suggest that homosexuality is not caused by any single anatomical difference, but by many.
The UCLA discovery concerns the anterior commissure, a brain structure not thought to be directly linked with sexual behaviour. Research has shown that it is generally larger in women than in men. But in homosexual men it is even bigger, according to the UCLA scientists, Dr Laura Allen and Dr Roger Gorski.
Dr LeVay, who is homosexual, welcomed the findings. 'This is very significant because here we have a part of the brain that has nothing to do with sex. Something unusual is clearly happening when the brain is organising itself in foetal life. This strongly supports the idea that there is a biological basis for the determination of sexual orientation. It's one more nail in the coffin of critics who argue that homosexuality is a choice and thus immoral.'
While many experts praised the research, some sceptical voices were raised. The UCLA scientists were criticised for using the brains of Aids victims, all of whom were male. They were unable to obtain samples from known lesbians as they rarely die of sexually transmitted diseases and their sexual orientation (unlike homosexual male Aids victims) is not noted on death certificates.
Other critics claimed that it was an oversimplification to read significance into anatomical differences in the human brain, because so little was known about the subject.Reuse content