Birth order: That's so gay!

The number of biological older brothers correlated with the likelihood of a man being homosexual, regardless of the amount of time spent with those siblings during childhood, Bogaert says. No other sibling characteristic, such as number of older sisters, displayed a link to male sexual orientation.

By accounting for potential psychological effects of having older brothers, Bogaert's data "strengthen the notion that the common denominator between biological brothers, the mother, provides a prenatal environment that fosters homosexuality in her younger sons," say neuroscientist S. Marc Breedlove of Michigan State University in East Lansing and his coworkers in a comment to be published with the new report.

The release of maternal antibodies that boost a boy's probability of becoming gay is a provocative but untested hypothesis, Breedlove and his coworkers note.

It makes sense, though, in light of previous failures to find any older-sibling influences on female homosexuality, they say.

Bogaert reports that the age of the mother also appears related to the sexual orientation of her sons, "with homosexual men's mothers being significantly younger than the mothers of heterosexual men".