MONDAY, June 26 (HealthDay News) -- Adding to the theory that homosexuality may be hardwired and not the product of environmental factors, new research again confirms that the more older brothers a male has, the more likely he is to be gay.

Researchers have known for years that a man's likelihood of being gay rises with the number of older biological brothers. But the new study found that the so-called "fraternal birth order effect" persists even if gay men were raised away from their biological families.

Study author Anthony F. Bogaert said the findings add to a growing body of evidence that links homosexuality to nature, not nurture.

"The research suggests that the development of sexual orientation is influenced before birth," said Bogaert, a professor of community health sciences at Brock University in St. Catharines, Ontario, Canada.

Bogaert and a colleague first reported the older-brother effect a decade ago. According to Bogaert, men with no older brothers have about a 2 percent to 3 percent chance of being gay. If they have three or four older brothers, the rate goes up to about 5 percent.

About 20 studies have reinforced the link between fraternal birth order and male homosexuality, Bogaert said. However, no similar link has been found in lesbians.

Bogaert said he launched his new study to better understand the fraternal birth order effect. "Is it a biological phenomenon? Or is it psychological or have to do with rearing?" he said.

To find the answer, Bogaert examined surveys of 944 Canadian men, both gay and straight, about their sexuality and their families.

The older-brother effect was constant regardless of whether the men were raised with natural, adopted or stepbrothers. It also didn't matter if they weren't raised with their biological mothers.

If gay younger brothers and older brothers don't have the same home environments, what do they have in common? "They shared the same uterus, the same womb, the same mother," Bogaert said.

His findings are published in this week's issue of the Proceedings of the National Academy of Sciences.

The next step is to figure out exactly how biology contributes to homosexuality, said Sven Bocklandt, a postdoctoral researcher at the University of California, Los Angeles, who applauded the new research.

"For me as a molecular geneticist, this study opens a new path of research into the biology of sexual orientation," Bocklandt said.

According to Bocklandt, one avenue of study would be to test the blood of mothers with multiple male sons to see whether their immune systems might be more likely to target genes linked to homosexuality.

Earlier this year, Bocklandt was co-author of a study that found that nearly one-quarter of women with multiple gay sons showed signs of genetic differences from other women.

More information

The Council for Responsible Genetics has more genes and sexual orientation.