Homosexuality has always posed problems for Darwinists. But Jim McKnight thinks he has found the advantage that keeps a putative homosexual gene alive and well and fitting in very nicely with Darwinist theory

Darwinian explanations of human affairs have been growing in popularity of late with the discovery of genes predisposing to a range of activities previously exclusively thought of as social behaviour. Attempting to explain why we have the genes we do has become a popular scientific pastime.

Nevertheless, Darwinists do not have it all their own way. There are some perennial difficulties with evolutionary theory - chief among which is the problem of homosexuality. This seems a tailor-made rebuttal of the great evolutionary credo, survival of the fittest. Critics delight in pointing out that homosexuals have fewer children, so if there is a gene for homosexuality, as recent American research suggests, then how do you explain a preference that clearly leads to reduced fitness? This conundrum is beloved of social constructivists who love pointing out that homosexuality is sexual behaviour, and that sexual success is the mainspring of evolutionary theorising. How then do we explain a sexual preference that leads to reduced reproductive success, given homosexuality's survival over the millennia?

There have been several attempts to deal with this difficulty over the years, but none has really tackled the problem adequately. My research group at the University of Western Sydney, Macarthur, has been investigating the extent of gayness within families of homosexuals with the support of the Australian Research Council, and our results so far suggest there is evidence for a gay gene, at least enough for this evolutionary paradox to be a real difficulty for Darwinists such as myself.

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Based on our research, it seems that male homosexuals are essentially evolutionary byproducts, whose reduced reproduction sidelines them to the status of "also-rans" in the great evolutionary game of maximising one's reproductive success. While this is a controversial position, it is a common one with Darwinists who have grasped the nettle of trying to explain the survival of a homosexual gene in the face of its diminished reproduction. Virtually all theories about the possible mechanisms for its survival accept that a gay gene enhances the reproduction of straight relatives who carry a weaker dose of homosexuality than their gay kin. Somehow, the gay gene ensures that they have enough extra children to balance the diminished reproduction of their gay relatives.

The nature of this advantage is even more controversial than assigning homosexuals to the evolutionary scrap heap. Dean Hamer, an American molecular biologist at the National Institutes of Health, announced in 1993 that he had discovered the markers for the gay gene. He speculated that the gene gave women an enhanced desire to love men and that, while this enhanced women's reproduction, when they passed this desire on to their sons it became a disadvantage. He speculated that this led to a balancing act in which the heightened reproduction of mothers offset their sons' disinclination to have sex with women. The reproductive advantage needs to be at only a low level to keep the gene in the gene pool, about 2 to 3 per cent.

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Our preliminary findings, however, do not support Hamer's belief that the advantage is in the maternal line. Other researchers are reaching similar conclusions. We did find that gay men had many more homosexual relatives than straight men in our sample (an uncontroversial finding of many similar studies) but our results did not lie in the maternal line. We found that gay men come from larger families overall and that this was not a result of religious factors or other variables. Women in our sample were producing more children than their brothers, sons, and uncles, but there were no differences in reproductive rates between the maternal and paternal lines. This led us to speculate that Hamer and other researchers were measuring a general fecundity effect rather than an enhanced desire to love men.

This is an interesting enough finding in its own right, but we are still left with the question: what is the advantage that keeps a putative homosexual gene alive and well? Many studies have noted that gay men have several traits that are attractive to women, including their interest in clothes and the arts as well as their enhanced communicative and social skills. These would prove attractive to women who encounter them in men who have a slight dose of homosexuality, but not enough to disqualify them as prospective mates. A virile man who is also charming, sensitive, warm, caring and a good communicator is an appealing prospect for a woman. A homosexually enhanced straight male with a measure of this charm then has an evolutionary edge in winning mates ahead of his less fortunate peers.

Enough of an advantage perhaps to keep a gay gene within the gene pool.

My scenario has met with a measure of cautious approval from some sectors of the gay community who acknowledge the increasing biological evidence for at least some forms of homosexuality. Others have been less enthusiastic, particularly those whose agenda is to gain acceptance for gay identity based on the range of sexual choices we are all capable of making. For these critics, biological explanations threaten to return homosexuality to the realm of sexual inversions and legitimate homophobic prejudice. But it seems that all genetic variation is potentially precious.

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Jim McKnight is chair of the department of psychology, University of Western Sydney. Straight Science? Homosexuality, Evolution and Adaptation is published by Routledge, Pounds 14.99.