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 to find a similar effect with older sisters. In the interest of full disclosure, I should state that I have an older brother and no other siblings.
My worry with studies like this is not their scientific validity but the use that others may make of the findings. In the wrong hands, this feature of pre-natal development could easily be presented as some sort of defect, leading to all kinds of nasty ideas about how to “correct” it.
In the Dark 
September 18, 2026 at 8:38 pm
I thought this had become general knowledge by now. The reason it has never been big news is that the statistical correlation, while clear, is weak. So even if the mechanism has been identified correctly it is far from causative.
September 18, 2026 at 8:43 pm
No doubt there are many factors at play so one mechanism is unlikely to explain everything.
It may indeed be general knowledge but this is the first I had heard of it.