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The renowned Canadian sex researcher Ray Blanchard heaves an exasperated sigh. A much-published, recently retired University of Toronto academic, he is currently looking into why younger brothers are more likely to be gay than their older brothers. And he thinks he and a frequent collaborator might have identified a biological cause, something that happens in the mother's womb as opposed to being rooted in upbringing – nature, not nurture.

This has some gay activists wringing their hands: What if we discover the biological origins of homosexuality and, based on that science, a cure to gayness can be found? "You know, research is not the continuation of politics by other means," he says, after the sigh. "Ours is a legitimate research question."

Blanchard's idea is that some mothers develop "anti-male" antibodies from having already carried XY fetuses, and the antibody can attack the next male fetus that occupies the same womb in a way that affects sexual orientation. If many of the first wave of gay activists wanted scientists to show that the condition was innate, nearly immutable, the latest generation worries that if we find the causes, the cures can't be too far behind.

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And so, if, as Blanchard posits, the alleged anti-male antibodies can be identified, they might also, in some distant future, be neutralized, reducing the number of births of gay-tending boy babies.

The scientific evidence that maybe these babies were, indeed, born this way, has been growing. Assorted studies have found that lesbians on average have longer fingers, that the left-handed are more likely to be gay and lesbian, that gay men are, on average, shorter than their straight peers – it all tends to support the idea that homosexuality is bred in the bone.

As far as the greater incidence of homosexuality among younger brothers, Blanchard and fellow psychologist Tony Bogaert, a professor at Brock University, believe they've eliminated social causes. Having an older but not biologically related stepbrother in the house seems to have no effect on the odds that the boy will become a Friend of Dorothy; also, those who have an older biological brother who grows up in another household, well, they tend to turn out gay more often.

I wanted to learn more about the scientist's ongoing work on why younger brothers are more likely to be gay for personal reasons. I'm the second of three brothers, and the last two of us are gay – yes, a dream come true for our lovely parents. And my partner is the youngest of four boys. What Blanchard and others call the fraternal birth order effect is something that seems to apply in our case. I put some questions to the renowned sexologist.

Is it well-established scientifically that younger brothers are more likely to be gay than their older brothers?

The birth order finding is an empirical finding and at this point it's not a discussable matter. There have been recent studies showing the effect in Italy, the U.K., the U.S., Spain, Samoa and the Netherlands. There's no parallel effect in females.

What is your theory to explain this?

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The finding that having older brothers increases the odds of homosexuality in later-born males, whereas older sisters have no effect on those odds, points toward the mother's immune system. That is the only maternal system, apart from the brain, that could "remember" the number of male fetuses that a woman has carried while ignoring the number of female fetuses.

According to this hypothesis, cells or cell fragments from male fetuses enter the maternal circulation during childbirth or perhaps earlier in pregnancy. The mother's immune system recognizes these male-specific molecules as foreign and produces antibodies to them. When the mother later becomes pregnant with another male fetus, her antibodies cross the placental barrier and prevent the fetal brain's neurons from "wiring-up" in the male-typical pattern, so that the individual will later be attracted to men rather than women.

How are you investigating this thesis?

The way to test it – and Bogaert is doing this – is to get mothers who have had at least one gay son and mothers who have had sons, and none of them is gay. Then you draw blood from both groups, and compare the blood with regard to evidence of past immune reactions for male-specific antigens.

What do you make of activists' concerns that, if scientists find the so-called anti-male antibodies, then the next step will be to combat or neutralize them, and then there'll be fewer gay younger brothers?

People who want to make political arguments can cite this research in any way they want. When people start issuing dire predictions about how the information is going to be used and what impact that information is going to have in the larger society, then …

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But what about the argument that finding a biological cause of gayness is one relatively quick step away from finding the "cure"?

If you're going to argue that the fraternal birth order is dangerous because it might allow intervention to prevent homosexuality in the fetus, then you have to take the same stance with regard to genetic research. Once the relevant genes are all established, or you know which combination of them puts your fetus over some unacceptable threshold of a homosexual outcome, you abort.

Many scientific questions are asked out of curiosity, and people are actually very interested in questions like why are some people gay, and why are most other people not. That interest does not necessarily betoken a malign intent or even necessarily a positive activist intent. There's a lot of research on why some people are left-handed …

Surely figuring out why some people are gay is more charged than why people are left-handed?

Research on the causes of homosexuality is more fraught because opposing sides in the culture wars want to do something with the information. I don't think that is a reason for scientists to walk away from a research area. Not everyone is a culture warrior.

Is what you've proposed just a scientific version of the old canard, of blaming the mother?

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No, no, it's not. She would have no control over this effect. It wouldn't be her fault at all.

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