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Identicial twin studies suggest that homosexuality is not entierly genetic. Basically this means that if you have an identical twin (shares 100% of genes) who is homosexual, then while you are more likely to also be homosexual, you won't necessarily be. I can't remember details though, ie how much more likely, and can't really be bothered going and finding finding them. This suggests though that environmental factors play a role. This doesn't mean environmental as in you 'chose' to be gay, ie could be diet, birth complications or plenty of other things that may effect ones biology and stuff.
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You're right, it is nurture. In the womb.
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While biologists look at hormones for answers about human sexuality, other scientists are looking for patterns in statistics. And hard as this is to believe, they have found something they call "the older brother effect."
"The more older brothers a man has, the greater that man's chance of being gay," says Bailey.
If this comes as a shock to you, you’re not alone. But it turns out, it’s one of the most solid findings in this field, demonstrated in study after study.
And the numbers are significant: for every older brother a man has, his chances of being gay increase by one third. Older sisters make no difference, and there's no corresponding effect for lesbians. A first-born son has about a 2 percent chance of being gay, and the numbers rise from there. The theory is it happens in the womb.
"Somehow, the mother's body is remembering how many boys she's carried before," says Breedlove. "The favorite hypothesis is that the mother may be making antibodies when she sees a boy the first time, and then affect subsequent boys when she carries them in utero."
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