Alice Dreger is a professor of clinical medical humanities and bioethics at Northwestern University’s Feinberg School of Medicine. She has written for The New York Times, The Wall Street Journal, and The Washington Post.
(blurb from The Pacific Standard)
Her article, here quoted, was published in The Pacific Standard, on May 12, 2014. It can be found here, if you would like to read it in full.
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.
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.
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.”)