Yes, she's strange and different...but not THAT different.

21 August 2006

Do brothers make you gay?

From the September, 2006 online issue of Discover magazine:

R&D: Boys with older brothers are more likely to be gay.
But is it nurture or nature?
By Stephen Ornes

"Biological Versus Nonbiological Older Brothers and Men's Sexual Orientation," published in the July 11 Proceedings of the National Academy of Sciences.

Can brothers really make you gay? Ten years ago, a study by Anthony Bogaert and Ray Blanchard outed a surprising relationship between siblings and sexuality: Boys with older brothers are more likely to grow up gay. In the wake of the "gay gene" media frenzy, Bogaert and Blanchard's results had plenty of doubters. But over the course of the past decade, a number of independent studies have verified this "fraternal birth-order effect." So now Bogaert is asking the next logical question: Do older brothers exert their influence psychologically or biologically?

To explore his hunches about homosexuality, Bogaert turned to Canadian classifieds. He placed ads in both gay-oriented and general-interest publications in Toronto, Montreal, and Niagara, inviting gay, bisexual, and heterosexual men with both biological and adopted older brothers to get paid to participate in his study. The ads attracted 521 usable subjects, whose data Bogaert combined with information from previous research studies, giving him a total of 944 subjects.

The subjects provided information about siblings and ranked themselves on a 7-point sexual spectrum that ranged from "exclusively homosexual/gay" (1) to "exclusively heterosexual/straight" (7) in terms of both attraction and behavior. Bogaert then compared the "degree" of homosexuality to data about older siblings. He found that having biological older brothers, not adopted brothers, was significantly linked to sexual orientation, regardless of whether a gay man was raised with them or not. The effect was so strong that Bogaert estimates that about 1 in 7 gay men can chalk up their sexual preference to having an older biological brother.

Moreover, the more brothers a man has, the more likely he is to be gay. According to Bogaert, every older brother increases the probability that a man is homosexual by 33 percent. The average estimate of the base rate of male homosexuality is around 4 percent of the general population. So a man with 1 older brother has around a 5.3 percent probability of being homosexual. For the youngest of 3 brothers, that figure rises to 7 percent. Hypothetically, a man with 9 older biological brothers, according to Bogaert's estimates, has about a 50 percent chance of being gay. Notably, Bogaert's estimate breaks down at 12 brothers, when the probability exceeds 100 percent.

A psychology professor at Brock University in Ontario, Bogaert has been studying human sexuality for the last 15 years. Although he acknowledges there is still room for the nature/nurture debate, he notes that his results suggest homosexuality has a strong biological basis. But how that works remains unknown. "A mother's immune system may 'remember' certain male factors," suggests Bogaert. He speculates that certain immune proteins or hormones may then cross the placental barrier in later pregnancies and somehow predispose a developing fetus to homosexuality. Regardless of how it works, says Bogaert, "I think it does push the pendulum toward the nature side."

  • On 8/21/2006 7:38 PM, Blogger Maya's Granny said…

    I'm relieved that the effect is biological and not because older brothers are doing something to younger ones!

    The fact that it takes so long to raise children and so much energy, I believe, accounts not only for the fact that only humans live beyond their reproductive years, giving grandparents to help, but also that there are some people who love children, want to raise them, but don't marry and produce their own.

    The advantage to a small hunterer/gatherer tribe of having men who can stay with with the women while the men are away and who the men don't have to regard as sexual rivals would be great. Also, the advantage of having women who had no children of their own to go gathering with the mothers and children. I mean, if I was designing it, I'd factor them into the equation.

  • On 8/22/2006 12:45 PM, Anonymous estrogenius said…

    I wrote about my impressions on this study back in June. You can read the dialogue here if you like :)


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