The Older Brother Effect

             I’m the youngest in my family, so I always tried to pin a lot of stuff on my older brother growing up, but never did I think I could blame him for me being gay. That is, until I came across this finding (LINK) a while back; it examined several variables, including finger length, vocal pitch, and childhood gender conformity, among other things, to see which were most strongly correlated to being gay. Interestingly enough, the one with the highest correlation was fraternal birth order, however this only applies to men.

            This phenomenon, also referred to as the “older brother effect,” is the strongest known predictor of sexual orientation in males. According to several studies that have been conducted around the globe (LINK), each older brother increases a man’s odd of being homosexual by 28-48%. So if the average rate of homosexuality is 5%, then on average, one in twenty guys are gay. But for those who have one older brother, their odds are closer to 7%. There is a limit, however, because after three or four older brothers the rate levels off around 10-12%. Still, I find it fascinating that with even one older brother, the differences are observable and statistically significant.

            So where does this increase in the probability of being gay come from? When the study was repeated with step-brothers, there was no correlation, so the cause seems to be biological and occurring in the shared womb. Researchers have pointed to the prenatal environment in which the fetuses form. With each boy that the mother has, her body produces a certain amount of antibodies to combat the unknown male proteins or perhaps something found in the Y chromosome, which females lack. So with each older brother, these antibodies increase exponentially. These antibodies may interfere with the normal hormonal washes that occur in the womb and are responsible for the sexual orientation, behavior, mannerism, and attractions of that person later on in life.

            Now, I don’t think the mother’s body is trying to kill the child, but maybe this is just nature’s way of ensuring that not too many heterosexual males exist in the same family. Too many straight brothers could lead to excessive competition or aggression, especially when it comes to dating and mating. Maybe, after the first male, the need to reproduce is not as demanding, so the probability becomes slightly tweaked, going from 5% to 10% over the course of several male offspring.

            Overall, there is always a rate of homosexuality. The fact that it increases with the number of older brothers just makes whole subject more interesting and seemingly intentional in my mind. So thanks big bro for increasing my odds, I couldn’t be happier with the results, and you can rest assured that I won’t be stealing your girlfriend anytime soon.