Having older brothers increases chance of being gay, study finds
Journal of Sex Research last week affirmed the longstanding theories of a “fraternal birth order effect,” in which “men’s propensity for homosexuality increases with the number of older biological brothers they have,” the researchers noted in an article for The Conversation.The scientists — Christine Ablaza, Francisco Perales, and Jan Kabatek — noted that past research into fraternal birth order effect had involved small samples.
To counter this, researchers used Dutch population registers and followed the lives of nine million people born between 1940 and 1990.While the registers don’t explicitly state sexual orientation, researchers used same-sex partnerships and same-sex marriages to determine an individual’s sexuality.They argue that, despite excluding all gay people who haven’t registered an official partnership or marriage, their findings demonstrate “clear evidence” of fraternal birth order effect on homosexuality.Dutch men with three older brothers are 41% more likely to be in a same-sex relationship than men with three older sisters.They are also 80% more likely to be in a same-sex relationship than men with three younger brothers, which researchers say show that being the youngest sibling increases the likelihood of homosexuality — and that the differences increase with the number of older male siblings.However, even one older brother can make a difference.
Dutch men with one older brother are 12% more likely to enter a same-sex union than men with one older sister, and 21% more likely than men with one younger brother or sister.“The sex of older siblings wields a considerable influence over same-sex union formation,” researchers noted. “On the other hand, the sex of younger siblings plays little to no.
