BACK in 1993, when Dean Hamer discovered what appeared to be a gene that predisposed men to homosexuality, he created a paradox. According to Darwinian thinking, if homosexual men have fewer children than straight men the 鈥済ay gene鈥� should quickly disappear from the population. Yet we know that homosexuality persists.
So what is the mechanism that keeps the gene in circulation? Perhaps gay men are good at looking after their straight brothers, who share a proportion of their genes. That way, the gay gene may be passed on by the brothers of gay men, even if they themselves have no children. Unfortunately for this idea, studies so far have found that gay men do not help out their brothers financially or emotionally any more than straight men do.
Another idea espoused by Hamer and others is that a gene which predisposes men to homosexuality might also increase a woman鈥檚 chances of having more children. In this way, the reduced likelihood of gay men passing on the gene would be more than compensated for by women carrying the same gene. This is precisely what a group of researchers from Italy have found. The mothers and maternal aunts of gay men had more children than those of straight men (see 鈥淕ay genetics鈥�). And this difference does not appear in the father鈥檚 family.
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That the extra children should appear in the mother鈥檚 side of the family fits with the idea of a gay gene passing from mother to son on the X chromosome. However, no simple pattern of inheritance can account for the Italian findings: the only model that fits is one where several genes, at least one of which is on the X chromosome, combine in some way to influence sexual orientation. So, as has been suggested before, it may not be one gene we are searching for but several.
While the Italian team supports the idea that genes influence homosexuality, they also caution that genetics is not the whole story. Their findings confirm that gay men are less likely to be firstborn children than later-born, and are more likely to have older brothers than sisters. They also back up previous results that gay men have more male homosexual relatives on their mother鈥檚 side of the family. Yet these factors account for only a fifth of the variance seen in male sexual orientation. So a boy鈥檚 experiences are still critical in deciding whether on not he becomes gay.
These findings, assuming they can be repeated, will not satisfy gay groups who want homosexuality to be seen as purely a genetic trait. Nor will they please commentators who see homosexuality as a choice made by 鈥渟inful鈥� people. What they do show is human sexual behaviour in all its splendid complexity.
