Here is an excellent article on the biology of sexual orientation. We all know this is a contentious issue — are we born with an orientation, or is it a ‘choice’ that people make? — and the article just lays the facts out for us and points out some of the lacunae in our knowledge.
First, I’ll confess to my own position on that nature-nurture debate: it’s both and it’s neither, and the argument is misplaced. There is no template on the Y chromosome that triggers a sexual response when Pamela Anderson enters the visual field, but there almost certainly are general predispositions that are a product of genetics, development, and learning. Even if there were no genetic component at all, it shouldn’t matter in social policy or in our interactions with other people: my own heterosexuality is fairly strongly fixed and was acquired before I was really aware of it, and I’m willing to see other’s homosexuality, no matter what its source, as equally fixed, and changing it as both undesirable and unjust.
The article discusses animal studies, in particular with mice and rats, and there it is clear that sexual orientation is established by a biological factor, hormone exposure. Hormone exposure in developing brains causes remodeling that fixes a behavioral response, and in lab animals, you can tinker with that directly, removing a hormone or adding an exogenous one, and controlling for other variables in the environment. You can’t do that with people — can you imagine the reaction to an NIH grant proposal that suggested injecting pregnant women with hormones to induce homosexuality? Can you imagine anyone agreeing to being that kind of experimental subject? — which means of course that there is a dearth of direct evidence for such phenomena in us. We rely on natural experiments that accidentally manipulate embryonic hormone exposure, and then we try to pick apart the effect in a clutter of other uncontrolled variables. The authors point out that there is precisely one study that shows a statistical effect of modified prenatal exposure to a gonadal hormone that is correlated with later sexual orientation.
There is one very interesting explanation for the apparent difference between rodents and humans. Rodent sexuality is tied to hormone levels throughout their life, with cycles of sexual interest and activity in both males and females. Humans have uncoupled sex from hormonal regulation to a much greater degree, so we can have sex any time of the month or any season of the year (and isn’t that a good thing?). This suggests an interesting evolutionary hypothesis: perhaps the lability of sexual orientation in humans is a side-effect of unshackling sexual behavior from tight cyclic hormonal regulation. Freeing us from the bonds of the spring rut or the monthly estrus means human brains aren’t quite as locked in to a single sexual stimulus. (This can’t be the whole explanation, of course, since even species with restricted mating seasons can exhibit same-sex mating behaviors.)
Anyway, you know what they say: read the whole thing. It’s a good, measured article that promotes a sensible view of sexual orientation and also recommends more research on the issue.