SCIENCE / Nature, not nurture?: New studies suggest that homosexuality has a biological basis, determined more by genes and hormones than social factors or psychology, says Sharon Kingman

ARE homosexuals born or made? The role of domineering mothers and ineffectual fathers used to figure large in this debate, once the sole preserve of psychiatrists, psychologists and sociologists. But recent discoveries have moved the focus firmly into the arena of medical research. Science may, it seems, be about to furnish proof that homosexuality has a biological basis - that it is part of the spectrum of normal human behaviour, as common or garden as being extrovert or left-handed.

Several groups of researchers around the world are hard at work unravelling the threads of how homosexuality may be biologically determined. Some are looking for genes associated with homosexuality in families that have many homosexual members. Others are studying the brains of homosexuals to identify how they are different from those of heterosexuals.

These may sound like very different lines of study. But they could ultimately link up to provide a unified theory to explain, in part at least, why some people are homosexuals and recognise themselves as such from a very early age. The implications of such work are enormous. If homosexuality were proved to be a biological phenomenon, the result could be a more lenient attitude by society towards homosexuals - even to the extent, in some countries, of introducing new legislation outlawing discrimination against them.

Conversely, some homosexuals fear that identification of a gene that predisposes towards homosexuality could raise the spectre of people trying to 'diagnose' homosexuality. Conducting prenatal screening and terminating affected pregnancies might become common practice, with people developing 'treatments' to 'cure' the perceived abnormality.

Scientists point out, however, that even if such a gene were to be found, it is unlikely exclusively to determine someone's sexual orientation. This is still more likely to be the result of many different influences, biological or otherwise.

The present surge of interest in the biology of homosexuality began with several observations comparing the brains of homosexual men with those of heterosexuals. Such studies have largely been made possible because of the Aids epidemic. Before Aids, it was rare for doctors to record people's sexual orientation on their medical notes.

Now, because so many young homosexuals are dying of Aids, researchers can for the first time study the brains of a large number of men they know to be homosexual, and compare them with the brains of those known or presumed to be heterosexual. This also explains why studies have so far concentrated on homosexual men and have not included comparisons of lesbians with heterosexual women.

The first breakthrough came in 1990 when Professor Dick Swaab, director of the Netherlands Institute for Brain Research in Amsterdam, and his colleagues found a structural difference between the brains of homosexual men and heterosexual men. The difference was in an area of the brain called the suprachiasmatic nucleus, which is involved in regulating the 'biological clock' - the mechanism that controls daily cycles of sleep - and may have a role in sexual activity. Swaab found that the volume of the suprachiasmatic nucleus was almost twice as large in homosexual men as in heterosexual men, and had double the number of cells.

About a year later, Simon LeVay, a researcher at the Salk Institute for Biological Studies in San Diego, made a similar discovery. He found that the volume of an area of the brain called INAH-3 was more than twice as large in heterosexual men as in homosexual men. INAH-3 is part of the anterior hypothalamus, a region known to regulate sexual behaviour in male monkeys. This was the first time that such a difference had been reported in a part of the brain involved in sexual behaviour.

Most recently, Laura Allen and Roger Gorski, anatomists at the University of California at Los Angeles, reported that a structure in the brain called the anterior commissure, which contains nerve fibres connecting the halves of the brain, was larger in homosexual men than in heterosexual men. Intriguingly it has no known role in sexual behaviour, Allen says.

No one can say to what extent, if any, these findings can be linked to the sexual orientation of those whose brains were studied. Even in the case of Simon LeVay's finding, it is unlikely that the differences observed could actually be the cause of the homosexuality. In each case, the researchers do not believe that Aids is a possible cause for the differences in size. The most likely theory, most of them agree, is that the differences in size result from the way the brain responds to the sex hormones that influence its development very early on in life.

Experiments on laboratory animals suggest this is true. A male rat that has been castrated on the first day of life, removing the testes which produce the male sex hormone, testosterone, will exhibit female sexual behaviour. The structure of its brain will be like that of a female rat. So it seems possible that the levels of sex hormones individuals are exposed to in early life, even while in their mother's womb, could influence the development of their brain. If the differences between men and women are determined by the combined influences of genes and hormones, it is not such a huge leap of the imagination to suppose that similar factors determine someone's sexual orientation.

If homosexuality does have biological roots of this kind, researchers predict, many more anatomical differences between the brains of homosexuals and heterosexuals will be found. 'Any structure that differs in size between men and women,' says Laura Allen, 'is likely to vary with sexual orientation.' She adds that the differences in anatomy that have been observed may explain variations in some brain functions which have been shown to exist between heterosexual men, homosexual men and women.

It is well known that men are better at tests of spatial ability (such as putting blocks together) while women are likely to score higher on tests of verbal ability. Recently, Sandra Witelson, a neuroscientist at McMaster University in Hamilton, Ontario, and Cheryl McCormick, a psychologist at McGill University in Montreal, have extended such tests to see how homosexual men perform.

They found that homosexual men fell midway between those of heterosexual men and women. There was a statistically significant difference between the scores of the homosexual men and the heterosexual men, but not between the homosexual men and the women. Witelson says: 'It is as though, in a neurological sense, homosexual men are a third sex.'

In parallel with these studies of the brains and the mental abilities of homosexuals, other researchers have been investigating what role genes could play in determining sexual orientation. Michael Bailey, assistant professor of psychology at North Western University in Chicago, advertised in gay publications for homosexuals to contact him if they were either one of a pair of identical twins (genetically identical); or one of a pair of non-identical twins (genetically similar but not identical); or one of two adoptively related brothers (genetically unrelated).

With about 50 homosexual or bisexual men in each of the three groups, Bailey found that 52 per cent of the identical twins he contacted were also homosexual, compared with 22 per cent of the fraternal twins and only 11 per cent of the adoptive brothers. This shows, he says, that the likelihood of a homosexual man having a homosexual brother increases with the degree of genetic relatedness. It suggests that there is a genetic contribution to homosexuality but that the cause is not entirely due to genes.

'It seems fairly compelling evidence that there are genes that affect male sexual orientation,' Michael Bailey says. He has also conducted a similar study of lesbian women and their sisters. Until the results of that are published, all he will say is that he has seen 'very similar results'.

He is enthusiastic about the prospects of finding a gene that is associated with sexual orientation. 'That would be a tremendous breakthrough. We could find out what the gene does. Not only would that information tell us about sexual orientation, but it would tell us about sexual differentiation of the brain generally and what makes men different from women.'

Current research is centring on families in which there are several homosexual members - some with homosexual men and homosexual women in the same family, others with just homosexual men or just homosexual women.

Dean Hamer, a senior researcher at the National Cancer Institute in Washington DC, who is conducting one such study, says it could take between four months and 10 years to identify relevant genes, depending on luck and whether there is just one gene or 100.

The big question is what the function of the gene will be, if and when it is found. Researchers expect that it may somehow affect the way in which sex hormones influence the developing brain, perhaps by having some kind of effect on the synthesis or breakdown of sex hormones.

If homosexuality does turn out to be a natural, biologically determined phenomenon, the implications for homosexuals and society could be wide-ranging and might well affect anti-discrimination legislation. UCLA's Laura Allen says: 'In the United States there is a law that protects people who have immutable characteristics, such as race, from discrimination. So if homosexuality were proved also to be an immutable characteristic, then the law would have to be changed.'

If homosexuality ultimately loses its social stigma, people might also realise that it has its advantages: although there have been no scientific studies so far to prove that there is a higher incidence of genius among homosexuals, Laura Allen points out that many writers, artists and musicians are homosexual. 'The nicest thing that could come out of it,' she says, 'is that people with homosexual children could become more accepting of them if they knew it was just natural.'-