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A researcher has found strong evidence that the more biological older brothers a man has, the more likely he is to grow up to be homosexual. Investigators say the finding strongly suggests a biological basis for sexual orientation.

Society has long debated whether homosexuality is learned behavior or is genetically determined.

A study by Anthony Bogaert at Brock University in St. Catherine's, Canada, provides strong evidence that sexual orientation may be the result of biological processes that happen in the womb. He studied nearly 1,000 men and found that the more older brothers a man has, the more likely he is to grow up to be homosexual.

There was no affect if the older siblings were stepbrothers. Even more striking ...

"If you have a biological brother, and you never even lived with that biological brother, this leads to a higher likelihood of male homosexuality," said Anthony Bogaert. "And so it suggests that there is probably some non-learning or non-environmental factor that is affecting sexual orientation."

But having a lot of older brothers does not automatically mean a man will be homosexual. The study finds for each biological brother that precedes him, a man's chances of being homosexual increases by three percent.

Bogaert says the older sibling effect has not been seen in women.

Other researchers are looking at how a pregnant mother's immune reaction to her youngest son might in some way influence his sexual orientation later in life.

Again, Anthony Bogaert of Canada's Brock University:

"Given that there is perhaps a strong predisposition to be gay or not to be gay that is determined early on life, prenatally or even genetically, then this suggests that the idea that [homosexuality] is some kind of choice that people make and they are making bad choices and that kind of thing, I do not think that is really supported by the kind of research that we have been doing," he said.

The research on the influence of biological older brothers on male homosexuality is published in Proceedings of the National Academy of Sciences.