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. LeVay himself made no secret of the fact that he was overjoyed to discover a biological basis of homosexuality. If gays were "born that way" it could fatally undermine the legality and morality of homosexual discrimination. He believed, like many gay activists in the US, that a lifestyle based on an innate propensity rather than a conscious choice is far more difficult to condemn.
A similar view was taken by Dean Hamer, the American gay geneticist who in 1993 announced the discovery of a genetic trait on the X-chromosome of men that confers a susceptibility to homosexuality. Hamer had been studying the family histories of more than 100 gay men and came to the conclusion that homosexuality tended to be inherited. For instance, having a gay identical twin (who shares all your genes) increases your own chances of being gay by between 50 and 65 per cent, whereas for non-identical twins (when you share just half of your genes), the chance of being gay is raised by only 25 to 30 per cent.
Finding a so-called "gay gene" on the X-chromosome of men, which they always inherit from their mothers, gave hidden meaning to the old gay joke: "My mother made me gay". However, the findings - published in the journal Science - did not live up to expectations. For a start, an actual "gay gene" has never been found and the research has since been found to be seriously overhyped.
Nevertheless, the idea that sexual orientation was a biological rather than a psychological condition was now firmly entrenched. It became even more so following the publication of a study by Ray Blanchard in 1997 which found that the chances of you being a homosexual rise by about a third for each older brother you have.
This research, perhaps more than any other in this controversial area, has been critically appraised. Blanchard, a psychiatrist at the University of Toronto, found that the conclusions were the same in 14 different studies. The probability of this being the result of chance alone is about 1 in 10,000. "These data therefore establish beyond much doubt that homosexual males do, on average, have higher birth orders [more elder brothers] than comparable heterosexuals," Blanchard wrote in 2001.
"Because the sexual orientation of a newborn boy cannot operate backward in time to affect his older siblings, this finding implies that the number of older siblings, or some factor associated with that, must affect the newborn boy's sexual orientation," he says. What was intriguing about this study was that the effect was not seen in lesbians, and neither did it matter how many older sisters were involved. It purely came down to the number of elder brothers.
Blanchard came up with a possible explanation, articulated in his "maternal immune hypothesis". This states that there is something genetic in a male foetus that triggers an immune reaction in the pregnant mother which gets stronger with each subsequent male foetus she carries. This immune response counteracts something in the development of the foetal brain that turns it male - remembering that all foetuses will develop into females (the default sex) unless they carry sex-determining genes on the male's Y-chromosome.
Blanchard believes that the hypothesis could also explain why heterosexual males with older brothers tend to weigh less at birth than heterosexual males with older sisters. Homosexual males with older brothers weigh even less than heterosexual males with older brothers, indicating that there is some sort of developmental battle taking place in a womb carrying male foetuses for the second, third or fourth time. Marc Breedlove, professor of psychology at the University of California, Berkeley, has also found that gay men are more likely to have older brothers than straight men - gay men had a ratio of 140 brothers to 100 sisters among their older siblings compared to a general population ratio of 106 brothers to 100 sisters. But Breedlove suggests that exposure to male hormones in the womb could be the cause.
"I believe there are many social and psychological, as well as biological factors that make up sexual preference. Having said that, these data do suggest that there are some people in the world who are gay because of foetal androgen [male sex hormone] levels," says Breedlove. The researchers looked at the relative lengths of a person's fingers on the right hand - which is known to be influenced by exposure to androgens in the womb. Most heterosexual men have slightly shorter index fingers than ring fingers, whereas in women the two fingers are of similar length. Breedlove found that lesbians typically had a more masculine finger length, suggesting they were exposed to greater foetal androgens than heterosexual women.
Gay men had a more complicated pattern, and some appeared to have been exposed to greater-than-normal levels of foetal androgens. Breedlove also found that men with older brothers had a more masculinised finger length than men without older brothers, leading him to suggest that the mother's body remembers how many sons she has had and exposes each successive male foetus to more androgens, leading most of them to be more masculinised but increasing the risk that some may turn out gay.
The results contradict the findings that gay men have more female-oriented brains. "This calls into question all of our cultural assumptions that gay men are feminine," says Breedlove.
"It is just mindboggling to think that some men are gay because of the number of boys their mothers had before their birth," he adds.
But he warns about doing this sort of research at home: "There are plenty of gay men who are firstborns, many straight men with older brothers and many women whose fingers give no clues to their sexual orientation. This is not a test to be used on your friends and neighbours."