My Brother Made Me Gay!

My attention was drawn yesterday to an article in New Scientist with the following introduction: You would think I would have known about the fact that men with older brothers are more likely to be gay, but this was news to me. The Fraternal Birth Order Effect does seems to have been well established since its discovery in the 1990s. The explanation proposed at the time attributed the effect to a prenatal biological mechanism, as the association is only present in men with older biological brothers, and not present among men with older step-brothers. Hypothesized to be the result of a maternal immune response to male foetuses, whereby antibodies neutralize male Y-proteins thought to play a role in sexual differentiation during development, it would leave some regions of the male brain in the ‘female typical’ arrangement – resulting in attraction to males. The origin of the effect seems plausible to me, though I’m not an expert on such matters. I have no idea why it has taken 30 years…