A study from our friends up North says that having older biological brothers raises the chances of a male homosexual orientation by a statistically significant rate.
The argument feels significantly more compelling when reading the entire article. There is something disturbing that homosexuality would be attributed to a faulty immune response in utero, and while the immune response theory is plausible, it is anything but the only explanation for an effect on one of the most complicated (and most impossible to study) systems in all science, that of human development in the womb.
"Only biological older brothers, and not any other sibling characteristic, including nonbiological older brothers, predicted men's sexual orientation, regardless of the amount of time reared with these siblings," Bogaert said."
These results provide evidence that a prenatal mechanism, and not social and/or rearing factors, affects men's sexual orientation development."
One possible explanation, he said, was that the mother's immune system builds up antibodies in response to carrying male babies, thus affecting the developing brain of a later male baby and having a bearing on his sexuality.
As always, these studies only identify biological factors for male homosexuals. To my knowledge, no good research has supplied a good explanation for female homosexuality. However, that's probably because, even in research of homosexuality, misogyny reigns supreme.