Poseidon's Mermen

“Older brothers make a man more likely to be gay, and not for the reason most people assumed. Each older biological brother raises the odds by about a third, and the effect only shows up with brothers who shared the same womb - not adopted, not step, not foster.

The home environment doesn't move the needle. Pregnancy does.

The leading explanation comes from Anthony Bogaert at Brock University, whose 2018 paper in PNAS identified the mechanism: mothers of gay men with older brothers had unusually high levels of antibodies to a protein called NLGN4Y, found only on male fetuses.

The immune system appears to learn to respond to "male" with each pregnancy, and the response can shape later brain development.

The math is consistent across cultures, decades, and continents - one of the most replicated findings in the entire field of orientation research.”