Researchers are further exploring the notion that having an older brother increases a man’s chances of being gay.
“This pattern has been documented around Canada and the United States, but it goes well beyond that,” Stetson University psychology professor Scott Semenyna told NPR this week.
“There’s been now many confirmations that this pattern exists in countries like Samoa,” Semenyna continued. “It exists in southern Mexico. It exists in places like Turkey and Brazil.”
Known since the 1990s as the fraternal birth order effect, it has been theorized that men have roughly a 33% increased probability of being attracted to men for every older brother they have, according to Semenyna.
Those with one older brother have an approximate 2.6% probability of being gay “and then that probability would increase another 33% if there was a second older brother, to about 3.5%,” Semenyna explained.
People with five older brothers have about an 8% chance.
Potential connections and possible biological associations through parental chromosomes have been studied for years.
A 2022 study, published in The Journal of Sex Research, highlighted an even stronger correlation by examining data from 9 million people in the Netherlands born between 1940 and 1990.
Researchers from the University of Melbourne found that the probability of a man entering a same-sex relationship is 41% greater if he has three older brothers instead of three older sisters — and 80% greater if he has three older brothers instead of three younger ones.
“Interestingly enough — and this is quite different from what has been done before — we also showed that the same association manifests for women,” study author Jan Kabátek told NPR.
“But just the fact that we are observing effects that are so strong, relatively speaking, implies that there’s a good chance that there is, at least partially, some biological mechanism that is driving these associations,” Kabátek added.
Still, experts are quite aware that the purported pattern only affects a few in the masses of the human population.
“The vast majority of people who have a lot of older brothers are still going to come out opposite-sex attracted,” Semenyna said.
