Did you know that the more older brothers a man has, the more likely they are to be gay?

quote:

---

If you're a guy with an older brother, there's an increased chance you're gay.

Scientists have noticed this pattern in previous research, but now they think they have a biological explanation as to why, and it starts long before birth.

The researchers say that if their findings can be replicated, we may know at least one of the biological reasons some men are gay.

Many factors may determine someone's sexual orientation, but in this case, researchers noticed a pattern that may be linked to something that happens in the womb. The phenomenon is related to a protein linked to the Y chromosome (which women do not have) that is important to male brain development.

Researchers think it's possible that when a woman gets pregnant with her first boy, this Y-linked protein gets into her bloodstream. The mother's body recognizes the protein as a foreign substance, and her immune system responds, creating antibodies. If enough of these antibodies build up in the woman's body and she gets pregnant with another a boy, they can cross the placental barrier and enter the brain of the second male fetus.

"That may alter the functions in the brain, changing the direction of how the male fetus may later develop their sense of attraction," said study author Anthony Bogaert, a Canadian psychologist and professor in the departments of psychology and community health sciences at Brock University.

Earlier research has shown that the more older brothers a boy has, the more of a chance that boy will be attracted to men. A 2006 study showed that with each brother, the chance that a man will be gay goes up by about a third.

The study builds on research Bogaert and his co-authors have been exploring for more than 20 years. Since their initial research that noted the trend, other research -- although not all studies -- have detected the phenomenon, even across cultures. One found that a man's chances of being gay increased even if he was raised apart from his older brother.

---

LINK

Basically whenever a women gets pregnant with a boy, they develop antibodies that target a protein linked to the Y chromosome that males have but females do not. That protein influences the regions of the brain where sexual attraction is moderated. The more exposure a male has to those antibodies, the more likely they will be attracted to the same sex. And those antibodies usually build up with every male baby they have. It's very interesting how biology works.