Sexual orientation is still, for the most part, a mystery — as a big recent review article on the subject in Psychological Science in the Public Interest showed, there isn’t yet anything close to a coherent theory explaining the full, fascinating diversity of human sexual desire and expression.

Researchers have dredged up some intriguing clues and partial explanations, though. One of the wackiest-seeming such clues is called the fraternal-birth-order effect. Weird as it sounds, if you are a guy, then every older brother you have increases the chances that you are gay. There’s no corresponding effect if you’re female, and for guys the effect only holds for biological brothers, whether or not you even live with them or know they exist.

And the data suggest it’s a pretty big effect, in the grand scheme of things. As Northwestern sex researcher Michael Bailey’s team wrote in that review article, “Assuming that a man without any older brothers has a 2% chance of being homosexual, a man with one older brother has a 2.6% chance; with two, three, and four older brothers, the chances are 3.5%, 4.6%, and 6.0%, respectively.”

A few days ago, a new YouTube personality who focuses on LGBT issues and goes by the online handle “Proud to be fruit” posted an interesting and informative video on this strange effect:

As PTBF points out, the prevailing theory for the fraternal-birth-order effect has to do with antibodies. His rundown is concise and correct, but the Bailey article goes into more detail, summing up a theory proposed by the sex researcher Ray Blanchard as follows: “Male fetuses carry male-specific proteins on their Y chromosome, called H-Y antigens,” and “these antigens promote the development of heterosexual orientation in males.” Since the antigens are not normally present in the female body, Blanchard has theorized, when a mother-to-be is exposed to them as she gestates a boy, they “trigger the production of maternal antibodies.” These antibodies, in turn, “bind to the H-Y antigens and prevent them from functioning, which, in turn, impedes sexual differentiation of brain centers mediating male sexual orientation.”

The more boy-pregnancies, then, the more antibodies, and the less of an impact the H-Y antigens have, increasing the odds that the baby-to-be will end up being gay. It’s too early to say this theory has been proven, but it’s got a certain elegance, in that it helps explain various surprising aspects of the fraternal-birth-order effect: the fact that fraternal birth order affects boys’ sexual orientation but not girls,’ for example, and the fact that it seems to occur independently of whether the boy was raised in the same household as his brothers, and so on.

It’s telling, of course, that even this fairly complicated, interesting effect can only account for a little bit of the overall variation in sexual orientation — plenty of gay guys have zero older brothers, and the theory accounts for zero percent of the world’s lesbians. When it comes to sexuality, there’s just a ton of stuff going on.