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Best of mates: why ‘gay genes’ are a good fit for Darwin

Does homosexuality have something to offer everyone? marlin harm/Flickr

Sexual orientation has long been cause for discussion and controversy, but just where does our sexual orientation come from? Are people “born gay” or are environmental causes at play?

Historically, many people have held strong moral views about homosexuality and such views often stem from religious beliefs or an idea that heterosexuality is “natural” and everything else is unnatural or perverse.

Science is less concerned with dictating what people should or shouldn’t do, and more with investigating what people actually do and why they do it.

Decades of research – reviewed by Qazi Rahman and Glenn Wilson of the University of London – has revealed that genetic influences play a significant role in sexual orientation.

For example, non-heterosexuality runs in families and genetically identical twin pairs are more likely to share the same sexual orientation than are genetically non-identical twin pairs.

Analysis of twin pair correlations in many studies shows that genetic factors account for around 30% to 50% of variation in sexual orientation, although we have yet to find a specific “gay gene”.

In reality, there are probably thousands of genes that contribute to the likelihood of being non-heterosexual, so any one of those genes will have only a tiny effect and so will be difficult to find.

“Gay genes” also pose an evolutionary conundrum.

Natural selection favours individuals (and their genes) that reproduce most successfully. Given homosexuality is non-reproductive, how have “gay genes” managed to survive natural selection?

Work by myself and others has shed light on this Darwinian paradox.

I have shown that psychologically feminine men and psychologically masculine women (those who see themselves as similar to the opposite gender) are more likely to be homosexual.

On the other hand, I’ve also shown that psychologically feminine men and masculine women who happen to be heterosexual are more “successful” at mating – that is, they have more lifetime sexual partners (there’s some evidence that behaviourally androgynous characteristics are attractive to the opposite-sex).

Using genetic modelling on thousands of identical and non-identical twins, I’ve shown the above is due to genetic factors with multiple effects: these predispose to homosexuality and sex-atypicality and, in heterosexuals, to having more sexual partners.

So “gay genes” actually increase mating success in heterosexual carriers of those genes.

Consistent with this, I’ve also shown that heterosexuals who have a homosexual co-twin (and who therefore may carry “gay genes”) tend to have a higher than average number of sexual partners – that is, they are more successful at acquiring mates.

So it appears that any evolutionarily detrimental effect of “gay genes” (predisposing to homosexuality) is balanced out by the evolutionarily beneficial effects (increased mating success) in heterosexual carriers of those genes. This might explain why such genes have survived natural selection.

Knowing sexual orientation is influenced by genes and can be explained in evolutionary terms may even, for those who hold such views, make homosexuality seem less a perverse or immoral choice and more just a case of being “born gay”.

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