Birth Order and Same-Sex Sexual Orientation
New research debunks the fraternal birth order effect - sort of
The fraternal birth order effect refers to the observation that the more older brothers a man has, the more likely he is to be gay. The original claim was that the effect is found only in men, and that older sisters have no impact on sexual orientation. The leading explanation for the phenomenon is the maternal immune hypothesis - the idea that male pregnancies trigger an immune response in the mother, which can influence the sexual development of subsequent male offspring.
I’ve always been agnostic about the explanation. Until recently, however, I was convinced that the the effect itself - the association between older brothers and male homosexuality - was firmly established. My confidence stemmed in large part from a 2018 meta-analysis synthesizing a quarter century of research on the topic.
Apparently, though, I was wrong. Several recent papers have significantly complicated the picture.
The most recent of these papers challenges both pillars of the original hypothesis. First, it finds that older brothers increase the likelihood of homosexuality in both sexes, not just men. Second, it shows that older sisters increase the chances of homosexuality as well, albeit in men only. These findings are shown in the graph below.
To be clear, there is a birth order effect for same-sex sexual orientation. But there’s no fraternal birth order effect affecting only males. This not only complicates the empirical picture, but it undermines the maternal immune hypothesis, which applies only to male fetuses.
The paper was authored by Jan Kabátek and Ray Blanchard, and was published in the journal Archives of Sexual Behavior. Here’s the abstract.
This study used a recently developed statistical technique to investigate the relations between various elements of a subject’s family background and the odds of that subject reporting a sexual history indicative of a minority sexual orientation. The subjects were 78,983 men and 92,150 women who completed relevant questionnaire items in the UK Biobank, a large-scale biomedical database of volunteers aged 40–69 years. The men were divided into three sexual minority groups—homosexual, bisexual, and asexual—and a comparison group of heterosexual men. The same was done for the women. The analytic procedure consisted of logistic regressions specifically designed to disentangle the effects of birth order and family size. The results showed that older brothers increased the odds of homosexuality in both men and women, and that older sisters increased the odds in men. In contrast, neither older brothers or older sisters affected the odds of bisexuality or asexuality in men or in women. These results suggest that birth order effects may be specific to homosexuality and not common to all minority orientations. The only family size finding was the negative association between family size and the odds of asexuality in both men and women. The outcomes of this study indicate that the maternal immune hypothesis, which was advanced to explain the relation between older brothers and homosexuality in later-born males, might have to be abandoned or else expanded to explain the findings concerning females. A few such modifications are considered.
The paper is open access, so you can read it here for free.
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