Researchers of fraternal birth order effect argue that sexual orientation correlates with an individual’s number of older brothers. Could this help to explain the homophobia of some traditional religious societies?

Reading an excerpt from the book Sex and War has got me thinking about the fight between the Mormons and California’s Proposition 8 that took place between 2008 and 2013. Sex and War features a meditation on how biology might help to explain why and how humans kill each other. The battle between the Mormons and the LBGT civil rights movement, most famously waged over the Mormon’s support of California’s anti-gay Prop. 8, did not turn bloody, but I can’t help but think about the interesting biological background to all of this.

Discussions and debates over the origins of homosexuality have tended to focus on two possibilities: You’re either gay because you’ve got a “gay gene,” or you’re gay because of some aspect of your upbringing. (The latter option is usually imagined to involve something nasty, like a pedophilic priest.)

These two options—gene-gay and turned-gay—fit neatly in the (yawn) nature-nurture debate, and that probably explains why almost everyone seems to keep ignoring a third option, one for which there is astoundingly robust data: womb-gay.

The official name of the womb-gay idea—bestowed by Ray Blanchard, the man who articulated the phenomenon—is the fraternal birth order effect. Blanchard is head of clinical sexology services at the Centre for Addiction and Mental Health in Toronto and professor of psychiatry at the University of Toronto.

Isn’t it interesting that traditional religious societies often feature large families and polygamy? So wouldn’t a neat way to explain this be that the fraternal birth order effect co-evolves successfully with polygamy?

The upshot of the fraternal birth order effect is this: “In men, sexual orientation correlates with an individual’s number of older brothers, each additional older brother increasing the odds of homosexuality by approximately 33%.” And this isn’t because big brothers somehow socially pressure their little brothers into becoming gay. Another sex researcher, Anthony Bogaert of Brock University in Canada, has shown decisively that it isn’t due to family environment; adopted male siblings don’t show the fraternal birth order effect, and the effect holds even when biological male siblings are raised separately. It doesn’t happen in females, and female fetuses don’t add to the effect. The effect happens only among male siblings who have inhabited the same woman’s womb.

So if you are a man, the farther down the reproductive chain you were in terms of male fetuses inhabiting your biological mother’s womb, the greater the chance you are gay. Blanchard estimates this effect accounts for the sexual orientation of somewhere around 15 to 29 percent of gay men.

Why on Earth would this happen? That’s not at all clear, but the researchers who have looked at this phenomenon think it may involve some kind of immunological response a woman’s body exhibits to carrying male fetuses, a response whose effect on male fetuses grows stronger with each successive male-fetus pregnancy. This etiology remains theoretical. But the effect does not. In spite of the long-running “gene-gay versus turned-gay” discussions of homosexuality, we have far better data evidencing womb-gayness than we do gene-gayness or turned-gayness.

So what does this have to do with Mormons? Well, given the relatively large size of Mormon families, on average, it is highly likely that gay men are relatively more common among Mormons than among the general population, where family size is, on average, smaller. It’s not just that each Mormon family would have, on average, more sons than the average American family; it’s that the population of Mormons would include more gay men per capita than the general American population.

Put that fact together with a study that purported to show that men who are homophobic are more likely to be sexually aroused to homosexual stimuli and another purporting to show that homophobic men are more likely to be aggressive towards gay men and imagine, in turn, that gay (Mormon) men who are forced to be closeted are more likely to become homophobic.

It’s just hard not to wonder if the Mormon declaration of war over Prop. 8 doesn’t have a little something to do with womb-gayness.

The fraternal birth order effect, incidentally, is a great starting point for telling just-so stories of evolution. For instance, isn’t it interesting that traditional religious societies often feature large families and polygamy? So wouldn’t a neat way to explain this be that the fraternal birth order effect co-evolves successfully with polygamy, because in a situation where you have one brother snatching up a bunch of wives, it would be good if a lot of the other brothers didn’t care? (When I asked Blanchard what he thought about this ironic possibility, of traditionally homophobic religious societies producing more gay sons on average, he responded wryly, “This proves that God is supporting my research.”)

Blanchard has calculated that it would be impractical to try to use knowledge of the fraternal birth order effect to engineer having or avoiding a gay son, but it is nonetheless interesting to ponder what would happen if the general population actually knew about womb-gayness. After reading an old New York Times Magazine cover story, "Her Body, My Baby," I found myself wondering whether potential parents screening women who might be their gestational surrogates might not start asking, “Just how many males have been in your womb, ma’am?”

When I put this possibility to Blanchard, he responded that it had already happened: “A man phoned me from the States and wanted advice about hiring a potential surrogate mother who had already conceived several male offspring. Despite my statistical arguments, he concluded that he didn’t want to hire her if there was any increased chance that she might produce a gay son. He said, ‘That’s not really what I want ... especially if I’m paying for it.’”

A version of this post originally appeared on the author's personal site.