Male Sexuality May Be Decided In The Womb

If you are male, having more older brothers makes it more likely you will be gay - and a new study suggests the basis of this is biological rather than environmental. The crucial factor influencing the likelihood of male homosexuality may be how many brothers were born before you to the same mother, not how many brothers you were brought up with.
The “fraternal birth order effect” - the finding that each additional older brother increases your chances of being homosexual by about 30% - has long been dogged by the suggestion that social factors rather than biological ones underpin it.
Some proposed that perhaps rough-and-tumble play between brothers, or even sexual abuse, may have led the impressionable younger boys to become gay.
Now Anthony Bogaert at Brock University in St Catharines, Canada, has largely ruled that out. He examined four population samples of homosexual and heterosexual men - 944 men in total.
The fourth sample included gay men who had grown up with non-biological male siblings. Bogaert reasoned that if simply being raised around a lot of older brothers had produced the effect, it should not matter whether they were born to the same mother or not.
In fact, it did matter: only the number of biological older brothers predicted sexual orientation in men, Bogaert found. This was true even when the biological older brothers lived separately. “It’s pretty strong in suggesting a prenatal origin,” he says.
from New Scientist