Is homosexuality biological?

Mark Joseph Stern looks at the evidence:

Some of the strongest current evidence that some people are born gay is based on a phenomenon called the fraternal birth order effect. Several peer-reviewed studies have shown that men with older biological brothers are likelier to be gay than men with older sisters or no older siblings. The likelihood of being gay increases by about 33 percent with each additional older brother. From these statistics, researchers calculate that about 15 to 30 percent of gay men have the fraternal birth order effect to thank for their homosexuality.

Why does this happen?:

One of the leading explanations is called the maternal immunization hypothesis. According to Ray Blanchard of the University of Toronto, when a woman is pregnant with a male fetus, her body is exposed to a male-specific antigen, some molecule that normally turns the fetus heterosexual. The woman’s immune system produces antibodies to fight this foreign antigen. With enough antibodies, the antigen will be neutralized and no longer capable of making the fetus straight. These antibodies linger in the mother’s body long after pregnancy, and so when a woman has a second son, or a third or fourth, an army of antibodies is lying in wait to zap the chemicals that would normally make him heterosexual.

Or so Blanchard speculates. Although the hypothesis sounds reasonable enough, it’s premised on a number of assumptions that haven’t been proven. For instance, no one has shown that there is a particular antigen that controls sexual orientation, let alone one designed to make men straight. And if that antigen does exist, does it control orientation only? Blanchard refers to its antibody attackers as “anti-male,” implying that the antigen controls for various aspects of masculinity. But when I asked him about this, he was noncommittal. Moreover, the hypothesis proposes a loose, two-way flow of antigens and antibodies between the fetus (whose antigens spread to the mother) and the mother (whose antibodies spread to the fetus). But this exchange has never been observed—and the antibodies and antigens in question are hypothetical, anyway. If they do exist, there’s no assurance that they perform this placental pirouette.

Does this mean that homosexuality is pathogenic?:

If Blanchard is right, then (at least some) gay people are indeed born gay, but there’s still something wrong with them. The hypothesis turns homosexuality into a birth defect, an aberration: Gay people are deviants from the normative mode of heterosexuality. We may have been born this way, the hypothesis implies, but that’s not how it was supposed to happen.

What about a cure?:

If homosexuality is truly biological, discrimination against gay people is bigotry, plain and simple. But if it’s a birth defect, as Blanchard’s work tacitly suggests, then being gay is something that can — and presumably should — be fixed.

Let us hope that if homosexuality does turn out to be biological then scientists will be able to come up with a way to solve it.