SCIENCE&TECHNOLOGY: Born to Be Gay ; New Research Suggests That Homosexuality Could Be `Hard-Wired' into the Brain in Utero. and the Evidence Was Revealed in the Blink of an Eye. STEVE CONNOR Assesses the Latest Claims and Counterclaims

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It is 30 years since homosexuality was removed from the list of recognised mental disorders drawn up by the American Psychiatric Association. Although few in the medical establishment still view it as either an illness or a sexual deviation, being gay still attracts the interests of researchers keen to understand why some people feel an overwhelming attraction for the same sex - a biological conundrum given that such relationships would naturally be childless and so serve no apparent advantage in terms of human evolution.

The latest study, by a London research team, suggests that gay men and lesbians have acquired their sexual orientation very early in life, perhaps even in the womb. In effect the findings suggest that homosexuality is "hard-wired" into the brain long before the onset of adolescence.

The researchers investigated the "startle response", when the eye blinks involuntarily after a sudden, loud noise. If the loud noise is preceded by a quieter noise the warning it provides results in significantly lower startle response, a phenomenon known as prepulse inhibition (PPI). The differences in PPI between heterosexual men and women is statistically significant and because it is beyond the voluntary control of someone's brain, it is believed to be an innate, deep-seated characteristic. In heterosexual women, for instance, PPI causes a 13 per cent weaker startle response. In heterosexual men, meanwhile, PPI causes the startle response to be 40 per cent weaker. Lesbians have a PPI of 33 per cent - so are more like heterosexual men - and gay men have an average PPI of 32 per cent, making them more intermediate between straight men and heterosexual women.

Glen Wilson of the Institute of Psychiatry in London says that the findings show a fundamental differences between the behaviour of the brain of men and women, and between homosexuals and heterosexuals. "The PPI test is a powerful measure of the brain's ability to filter and process information. Information processing is fundamental to the way the brain works and these results suggest evolutionary divergences between male- and female-oriented brains," he says.

The discovery that gays and non-gays differ in their PPI suggests that sexual orientation has a fundamental, biological root, which has serious implications for the mental well-being of homosexuals, says Qazi Rahman of the University of East London. "These findings may well affect the way we as a society deal with sexuality and the issues surrounding sexual orientation," Dr Rahman says.

"They may also have far-reaching health implications, offering clues as to why men and women, and gay men and lesbians, sometimes suffer from different types of mental-health disorders. If we know that certain groups differ from each other in brain function or a biological marker then we are in a position to provide better treatments which are tailored to suit particular groups," he says.

Critics of such research may argue that studying differences between homosexuals and heterosexuals can only exacerbate the sort of prejudice that led to homosexuality being treated as a mental disorder 30 years ago. Others like Dr Rahman argue that understanding why something is the way it is makes it easier to help people when things go wrong.

The recent history of research into the biological basis of homosexuality goes back to a study carried out by Simon LeVay, a gay neuroscientist at the Salk Institute in San Diego, California, who claimed to have found structural differences in the brains of homosexual and heterosexual men. Post-mortem examinations studied by LeVay revealed that a region of the brain called the interstitial nuclei of the anterior hypothalamus is on average two or three times bigger in heterosexual men than it is in women. In gay men, however, this region is about the same size as in women.

This supported the widespread notion that the brains of gay men were in some ways a bit like women, at least in the way they thought about men. …