Photo source unknown, presumed public domain.

There’s some more new evidence for one of the theories as to how gene variants that make men more likely to be gay could persist in human populations in the face of their obvious selective disadvantages: the same genes could, when carried by women, lead to greater fertility.

I recently posted about a study of Samoan fa’afafine, that documented this effect; now an Italian team is reporting, in a forthcoming article in The Journal of Sexual Medecine, that they’ve found the same thing in a sample of 200-some French and Italian women [$a].

The authors interviewed women who were the biological mothers or aunts of gay men, and compared them to women who were mothers or aunts of straight men. They gave each participant a questionaire covering the key question—how many children they’d had. It also covered a sort of focused medical history, covering a slew of conditions that might have affected their fertility—anything from chlamydia infections to ovarian cysts to complicated pregnancies—and asked about their sexual behavior and history. Finally, the team gave the women in their sample a standardized personality test.

Even this relatively small sample showed the previously documented effect of shared genetics with gay men—women who had gay sons or nephews had more children than those who didn’t. Mothers and aunts of gay men also reported lower rates of medical conditions that could reduce their ability to have children. They said they’d had more partners than mothers and aunts of straight men (but this difference wasn’t statistically significant) and were also less concerned about family issues, and more likely to have been divorced. Finally, the personality test revealed that mothers and aunts of gay men were more extraverted.

That’s a big pile of factors tested, which makes me wonder about multiple testing issues with a small sample size. The study’s authors build a somewhat complicated narrative out of it all: They speculate that the same genes that make men gay make women less likely to have fertility-reducing conditions, but also more extraverted and more “relaxed” about building a family—which apparently also helps them have more children. So, okay, I guess that’s plausible given the results.

Here’s what the study doesn’t do, however: it doesn’t identify any specific genes involved in making gay men gay. It can’t actually test the hypothesis that there’s a genetic basis to same-sex attraction at all, much less the hypothesis that genes promoting same-sex attraction in men are located on the maternally-inherited X-chromosome. For those questions, you really need full pedigree data—or, better yet, lots and lots of genetic data; interviewing only female relatives isn’t remotely enough.

The text of the article doesn’t necessarily make that point as clearly as it could. The authors spend a great deal of time talking about the X-chromosome hypothesis, and though they make the requisite disclaimer in the Conclusions section—

With this type of limited data, we cannot directly derive a causal connection between the hypothetical sexually antagonistic autosomal or X-chromosome-linked genetic factors and health, behavior, and personality.

—that disclaimer elides the point that their data set can’t really test anything to do with genetics indirectly either.

The authors repeatedly describe their sample as a “pilot study,” however, so maybe something bigger, and more rigorous, is in the works.◼

Reference

Camperio Ciani, A., Fontanesi, L., Iemmola, F., Giannella, E., Ferron, C., & Lombardi, L. (2012). Factors associated with higher fecundity in female maternal relatives of homosexual men. The Journal of Sexual Medicine DOI: 10.1111/j.1743-6109.2012.02785.x