A Canadian study reported that boys with more older biological brothers tended to be gay.
Previous research had revealed the more older brothers a boy has, the more likely he is to be gay, but the reason for this phenomenon was unknown...
Professor Anthony Bogaert from Brock University in Ontario, Canada, studied 944 heterosexual and homosexual men with either "biological" brothers, in this case those who share the same mother, or "non-biological" brothers, that is, adopted, step or half siblings.
... the link between the number of older brothers and homosexuality only existed when the siblings shared the same mother.
The amount of time the individual spent being raised with older brothers did not affect their sexual orientation.
One report contained a conservative objection:
Tim Dailey, a senior fellow at the conservative Center for Marriage and Family Studies disagreed...
“If it is indeed genetically based it is difficult to see how it could have survived in the gene pool over a period of time,” Mr. Dailey added.
It takes a bit of imagination. For instance, here's a simple hypothesis to consider: a reduction in sibling rivalry for large broods. A gay brother is less likely to fight a heterosexual brother for access to fertile females. Since males are capable of impregnating many females, there is little penalty for reducing net male fertility.