Some have argued that homosexuality is unnatural because the biological purpose of sex is for reproduction, and gay couples can’t conceive children in the “natural” way. But as it turns out, homosexuality might actually be related to increased fertility and reproduction, though not in a way you might expect.
A team of Italian researchers led by Andrea Camperio-Ciani had been working on solving the Darwinian paradox of homosexuality—that is, if being gay is hereditary, and gay people have fewer or no children, homosexuality should have vanished from the gene pool. In 2004, the team studied Italian families and found that the female relatives of gay men were more fertile than average women. After using a series of computer models to analyze that data, the scientists released a study this week saying that homosexuality in men is genetically connected to women who have high fertility. In their model, male homosexuality has to be governed by two genetic loci—particular fixed positions on a chromosome—and at least one locus and maybe both must be on the X chromosome, meaning it’s passed down from mother to child.
What’s more, Camperio-Ciani and his team now say male homosexuality is an example of sexually antagonistic selection—meaning a trait that gives one of the sexes a reproductive advantage diminishes the reproductive advantages of the other. This shows up all the time in the animal kingdom—for instance, fruit fly seminal fluid makes females lose interest in mating with other males, and actually shortens their lives. Most of the time it’s males taking the advantage, but in this case the researchers say it’s vice versa: Homosexuality means men will probably have fewer children, but their females relatives will have more, keeping homosexuality in the gene pool.
Even if the scientists are right, there’s a lot left to be resolved. The researchers don’t know exactly how this process works; they arrived at their conclusion because it was the only explanation that could fit their data. This finding also relates only to gay men, not lesbians, leaving homosexuality in women unexplained.
And though some are loath to admit it, there’s more to sex than just reproduction. You need only look at homosexuality in animals to see that. As such, sexually antagonistic selection is probably just one part of the picture.