One of the most consistent environmental explanations for homosexuality is called the “fraternal birth order effect.” Essentially, the more older brothers a man has, the more likely he is to be gay. The effect doesn’t hold for older or younger sisters, younger brothers, or even for adoptive brothers or stepbrothers.

According to Ray Blanchard, a psychiatry professor at the University of Toronto, the reason could be that the mother’s body mounts an immune attack on the fetus of her unborn son. As the report authors explain:

Male fetuses carry male-specific proteins on their Y chromosome, called H-Y antigens. Blanchard hypothesized that some of these antigens promote the development of heterosexual orientation in males … Because these H-Y antigens are not present in the mother’s body, they trigger the production of maternal antibodies. These antibodies bind to the H-Y antigens and prevent them from functioning.

With the H-Y antigens not functioning, it could be that the “be straight” signal in the fetus’s brain never flicks on.

Blanchard believes that this phenomenon grows stronger with each boy a woman bears. Studies have found that a man without older brothers has about a 2 percent chance of being gay, but one with four older brothers has a 6 percent chance. (Meanwhile, other studies have found the relationship to be weak or nonexistent.) As psychologist Ritch Savin-Williams writes in an accompanying commentary, the outcome for any given baby boy might depend on the timing of the immune response and the fetus’s susceptibility to the antibodies.

According to the report, Blanchard now plans to test mothers of gay and straight men for the presence of these antibodies. If proved out, fetal birth order could do a lot to fill in the missing explanations for homosexuality. But gaps will remain, such as why some firstborn sons are gay, why some identical twins of gay sons are straight, and why women are gay, to name just a few.

The review-paper authors do rule out one explanation for homosexuality, however: That tolerance for gay people encourages more people to become gay.

“Homosexual orientation does not increase in frequency with social tolerance, although its expression (in behavior and in open identification) may do so,” they write.

That reasoning—that a tolerant society somehow encourages homosexuality to flourish—has been used to support anti-gay legislation in Uganda, Russia, and elsewhere. These laws do marginalize and shame gay people, the authors write. But they won’t do away with a sexual orientation that’s ubiquitous, enduring, and—whether through genes, or hormones, or antibodies—perfectly natural.