The Los Angeles Times reported in a June 27, 2006 article on a study published by the Proceedings of the National Academy of Sciences regarding the relationship between male homosexuality and number of older siblings:

"In an analysis of 905 men and their siblings, Canadian psychologist Anthony Bogaert found no evidence that social interactions among family members played a role in determining whether a man was gay or straight. The only significant factor was the number of times a mother had previously given birth to boys. ...

Each older brother increased the chances by 33%."

June 27, 2006 Los Angeles Times

The Proceedings of the National Academy of Sciences article by Anthony F. Bogaert, PhD et al., "Biological Versus Nonbiological Older Brothers and Men's Sexual Orientation," (Vol 103, No. 28, pp. 10771-10774) stated:

"Only biological older brothers (reared with or not) and no other sibling characteristic, including nonbiological older brothers and the time reared with older biological or older nonbiological brothers, predicted men's sexual orientation. ...

These results support a prenatal origin to sexual orientation development in men...

A mother's body may have a memory for male (but not female) fetuses because she herself is female, and thus, her immune system may interpret and remember male ... fetuses as foreign. ...

...[T]he link between the mother's immune reaction and the child's future sexual orientation would probably be some effect of maternal anti-male antibodies on the sexual differentiation of the brain."
June 28, 2006 Proceedings of the National Academy of Sciences
(Read a PDF of the article abstract)

S. Marc Breedlove, PhD, Professor of Neuroscience at Michigan State University, told the Associated Press in a June 28, 2006 interview:

"Anybody's first guess would have been that the older brothers were having an effect socially, but this data doesn't support that."
June 28, 2006 S. Marc Breedlove

The Family Research Council issued a statement by its senior researcher, Timothy Dailey, PhD, as reported in a June 28, 2006 article by the Associated Press:

"We don't believe that there's any biological basis for homosexuality. We feel the causes are complex but are deeply rooted in early childhood development."
June 28, 2006 Family Research Council