Fraternal birth order correlates with sexual orientation

Viren tipped me off to this fascinating result that reinforces the idea that we really have no clue about what is happening in the earliest stages of neural development. This recent PNAS study found that the strongest predictor of a man being homosexual is the number of biological, older brothers. The effect is independent of non-biological brothers and still holds when the brothers are reared apart. As presented in the paper, the evidence suggests that the early development in the uterus might be different for later children.

Abstract after the jump.

Biological versus nonbiological older brothers and men’s sexual orientation
Anthony F. Bogaert

The most consistent biodemographic correlate of sexual orientation in men is the number of older brothers (fraternal birth order). The mechanism underlying this effect remains unknown. In this article, I provide a direct test pitting prenatal against postnatal (e.g., social/rearing) mechanisms. Four samples of homosexual and heterosexual men (total n = 944), including one sample of men raised in nonbiological and blended families (e.g., raised with half- or step-siblings or as adoptees) were studied. Only biological older brothers, and not any other sibling characteristic, including nonbiological older brothers, predicted men’s sexual orientation, regardless of the amount of time reared with these siblings. These results strongly suggest a prenatal origin to the fraternal birth-order effect.

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