Might societal acceptance of homosexuality drive it extinct?

by Alan Cohen


The precise causes of homosexuality are not completely clear, but most scientists agree that there is a genetic component, perhaps 35% for gay men and about half that for lesbians. There is unlikely to be a single gene that makes people gay, but there may be some genetic variation that increases the probability of becoming gay, or is a necessary precondition but also requires a certain environment (e.g. intra-uterine). (I have written previously on why natural selection could produce homosexuality, counter-intuitive as that sounds.)

In any case, the simple fact of genetic variation associated with homosexuality means that natural selection can act on it, and this has a major and paradoxical implication. The more biological children of gay people, the more transmission of these genes, and the more gay people there are. The less biological children of gay people, the fewer gay people there will be in subsequent generations. But in what societies do gay people have the most biological children? Almost certainly those societies that forbid homosexuality, forcing gays to marry and have children. Modern societies with liberal social policies will likely see many gay couples adopting, but fewer having biological children. Over time, this means that societal acceptance of homosexuality might actually drive it extinct, or nearly so. Unfortunately, natural selection in this case will function such that every society will achieve, long-term, the opposite of what it wants.

Of course there are a few caveats. The precise genetic mechanisms underlying homosexuality are unknown, and are likely diverse and differ between men and women. It makes a big difference if some people become gay due to purely environmental factors and others due to purely genetic factors, or if everyone has a mix. If environment can sometimes be sufficient, homosexuality would not go extinct in modern societies. The exact proportion of genetic variance is also important: the less genetic variance, and the less directly and simply those genes act, the longer it will take for natural selection to drive down rate of homosexuality. Also, our environment (chemical, nutritional, social) is changing quickly, raising the possibility that complex gene-environment interactions might counteract the simple selection scenario I describe. But they might also exacerbate it.

Additionally, it is not yet clear how many fewer biological children gay people will have in accepting societies. Will in vitro fertilization increase sufficiently to compensate? The more biological children gay people have, the smaller the selection gradient and the more slowly natural selection will operate. But, in theory, even a small decrease in reproductive rates could be enough to drive homosexuality extinct long-term. If there are other beneficial effects of genes that favor homosexuality, as I hypothesized in my previous post, homosexuality might not go extinct, but simply become more rare.

Lay people have a tendency to think that evolution does not operate in modern human populations, but evolutionary biologists have long known this is false. People reproduce at different rates, and some genetic variants are associated with who reproduces more and younger. This is sufficient for natural selection to occur. And evolution is blind to our social values: just because we support a more inclusive society does not mean we will get it, or that evolution will help us. The world can be a cold place. Or perhaps we will find a creative solution: encouraging gay men and women to donate disproportionately to sperm and egg banks, for example. Much remains to be seen…