Men with Older Brothers More Likely to be Gay

June 26, 2006 — -- The number of older brothers a man has may influence his sexual orientation, researchers say.

A first-born son has a 3 percent chance of being homosexual, which is standard for the population. However, the fourth son's chance of being homosexual doubles to about 6 percent.

Sexual orientation researcher and study author Anthony Bogaert, at Brock University in Canada, studied 944 heterosexual and homosexual men in Canada, with either biological or non-biological (adopted or step) brothers.

Previous theories suggested that the older brothers' psychosocial interactions with their younger brothers influenced their sexual orientation. If this were true, then the leading factor would be that the younger brother was raised together with older brothers --biological or non-biological.

In Bogaert's study, only the number of biological older brothers, regardless if they were raised together, increased the chances that the younger brother would be homosexual.

"In fact, [the gay men] had more biological older brothers who they were never reared with, which means there's probably some biological prenatal factor to account for this older brother effect," Bogaert said.

His research suggests that, in at least some cases, homosexuality is biological and may account for about one of every 7 gay men in North America.

"This is an important contribution," said Dean Hamer, a researcher at the National Cancer Institute, who discovered genetic links to sexual orientation. "It's possible to really show this is a biological rather than social or psychosocial effect."

This older brother effect "does not explain everyone, but this is definitely a part of it," said Sven Bocklandt, a researcher at the David Geffen School of Medicine at UCLA.

The order in which the sons are born to the same mother, called the fraternal birth order, seems to be the source of the biological factor.