A lot of effort is expended in trying to work out whether there is in fact a homosexuality gene. The efforts come from all sides of the question too. There are those who want to insist that it is purely learnt behaviour (ie, that there’s nothing “natural” about it) so that it’s OK to discriminate against gays (or even attempt to “cure” them). Then there’s the other side of that argument actually looking for it, so that they can say that it is natural, just part of human variation, so the former group should simply shut up and allow people to get on with it.

There’s also the more esoteric puzzle, for if such a gene exists why is it still present in the human population? It’s not as if being attracted to people you can’t have children with is likely to encourage the passing on of your own genes now, is it?

This looks like an interesting answer to the whole question:

Bisexual men might have their “hyper-heterosexual” female relatives to thank for their orientation.

Previous work has suggested that genes influencing sexual orientation in men also make women more likely to reproduce. Andrea Camperio Ciani and colleagues at the University of Padua, Italy, showed that the female relatives of homosexual men tend to have more children, suggesting that genes on the X chromosome are responsible. Now the team have shown that the same is true for bisexuality.

“It helps to answer a perplexing question - how can there be ‘gay genes’ given that gay sex doesn’t lead to procreation?” says Dean Hamer of the National Institutes of Health in Bethesda, Maryland, who was not involved in the work. “The answer is remarkably simple: the same gene that causes men to like men also causes women to like men, and as a result to have more children.”

Sexual attraction

The researchers asked 239 men to fill out questionnaires about their families and their past sexual experiences. On the basis of their answers, the men were classified as heterosexual, bisexual or homosexual. The results showed that the maternal aunts, grandmothers and mothers of both bisexual men and homosexuals had more children than those of heterosexual men.

Camperio Ciani emphasises that, rather than being a “gay gene”, this unidentified genetic factor is likely to promote sexual attraction to men in both men and women. This would influence a woman’s attitude rather than actually increasing her fertility, making her likely to have more children.

Simon LeVay, a neuroscientist and writer based in West Hollywood, California, describes this as a sort of “hyper-heterosexuality” and explains how it would help to ensure that homosexual behaviour was passed on through the generations. “The positive effect of an X-linked gene on female fecundity tends to outweigh the negative effect of the gene on male fecundity.”

Now I’m hesitant to say that something is correct when it’s based on a sample of fewer than 300 people but given that it is an appealing explanation. For genetics and evolution are more complicated than most of us think. It’s entirely possible for there to be some genetic variation which is of benefit to most who receive it and yet lethal to a minority’s chances of passing on their own genes. As long as, at the population level, that gene succeeds, then it will continue to propagate.

The best example I know of to explain this is sickle cell anaemia, something prevalent in those of West African ancestry. When both parents carry a specific mutation and pass it on to the child then sickle cell anaemia is the result. But why should a disease that kills before maturity (in most cases) presist in the population? Because possession of only one of the genes, rather than the two which cause the disease, protects against certain types of malaria. That protection against malaria vastly outweighs the rarer problems of sickle cell anaemia. (I think I’m right in saying that it’sa recessive gene and that therefore, even when both parents are carriers, only 1 in 4 of children will, on average, get the disease. One in four will also not get the malaria protection. Thus the gene itself, at the population level, leaves one child in four no worse off, two vastly better of through protection and one much worse off by disease. Of such crude calculations are genetics built.)

There’salso a certain dark humour in the point that wisespread possession of this anti-malarial gene is what led to the entire slavery experience and the cross Atlantic trade but that’s another matter.

If it is true that the homosexuality gene (or rather, the combination which leads to it) is beneficial to the females of the line in their fecundity and that benefit is greater than, at the population level, the narrowing of the fecundity of the men than yes, it will indeed continue down through the generations.

There’s also agoodreason to think of why the effect works too. For far fewer men than women have children anyway. Almost all women (until recently that is) would have children while it’s thought that only 40% of men that ever lived did. The constraint upon population size was always therefore female fecundity, not male.

If we want to go off into the realms of purest speculation we might also think about how this would work the other way around. If there were a similar and opposite set of genes for lesbianism then we would expect this to be rarer. For if female fecundity, rather than male, was the limiting factor, then something which limited female fecundity is more likely to drop out of the population than something which limits male.

And yes, lesbianism is indeed thought to be less prevalent in modern societies than male homosexuality.