The more biological older brothers that a man has, the more likely he may be gay, according to a recent study performed at Brock University in Ontario, Canada.

The study was conducted by Anthony Bogaert, a professor of community health sciences and psychology at Brock University, and was published in the Proceedings of the National Academy of Sciences on June 28. It aimed to find whether or not older brothers needed to be biological in order to see the increase in homosexuality.

The study found that the link between homosexuality and the number of older brothers only occurred when men were biologically related to the brothers.

David Puts, a post-doctoral research fellow in the MSU neuroscience program, along with neuroscience professors Cynthia Jordan and Marc Breedlove, wrote a response to the research that was published in the same issue of the journal.

"It really does go a long way, saying older brothers have the same effect whether or not you grow up with them — that you just need the biological connection through the mother," he said.

Bogaert studied 944 heterosexual and homosexual men who were living with either non-biological or biological families.

The subjects showed the increase in homosexuality even when they were raised in homes away from their biological brothers, he said.

Bogaert said the findings show that growing up in a household with many older brothers does not increase a boy's likeliness of being gay, and the cause for the link is due to something in the prenatal stages of life.

Bogaert was one of the two researchers involved in a 1996 study that found that gay men tend to have more older brothers, and said a number of studies have added to the evidence.

He said it is still unclear what happens in prenatal stages that increases chances of homosexuality, but it appears to involve the environment of the mother's womb.

When carrying a male baby, a woman's body may identify proteins found on male cells as foreign and build a stronger immunity to them with every additional son she carries, Bogaert said.

The woman may produce antibodies for the proteins, which may in turn affect the male fetus. This could cause him to be gay, he said.

Bogaert said further studies will have to be done to see if this is really the cause for the link.

"The bottom line for the study is there is some kind of prenatal factor," he said.

Puts is conducting research with Jordan and Breedlove on the fraternal birth order effect in mice, which aims to find what brain regions are involved in the development of sexual orientation.

"If older brothers are having some effect on sexual orientation, there ultimately has to be some kind of effect on the brain," he said.

Puts said there are many other factors that contribute to homosexuality in males besides the number of biological brothers.

"This fraternal birth order effect probably only accounts for about 20 percent (of homosexuality in men), so that means there are other causes," he said.

The study concludes that at least one factor of homosexuality doesn't appear to have anything to do with social rearing, he said.

Felicia Anderson, president of BRIGHT — the lesbian, bisexual, gay and transgender caucus for Brody Complex — said homosexuality should be respected regardless of what causes it.

"Research is great, but ultimately there shouldn't be some underlying medical excuse for someone to be accepted," she said.