Where does homosexuality come from?

From Salon:

What makes a person gay? Is it genetics, upbringing, or some combination of the two? Over the past few decades, a slew of scientific research has bolstered the notion that sexuality is, at least in part, innate. Studies of the sexual behavior of various animal species have shown that homosexuality is not just a human phenomenon. Then there is the curious finding that the number of older brothers a male has may biologically increase his chances of being gay.

Now Simon LeVay, a former Harvard neuroscientist, has written, “Gay, Straight, and the Reason Why: The Science of Sexual Orientation,” a comprehensive, engaging and occasionally quite funny look at the current state of the research on the topic. LeVay is one of the leading authorities in the field: Back in 1991, he discovered that INAH3, a structure in the hypothalamus of the brain that helps to regulate sexual behavior, tended to be smaller in gay men than in straight men. It was a watershed moment in our understanding of sexual orientation (the study was published at the height of the AIDS epidemic, when the disease was widely regarded in religious circles as divine punishment for the sin of being gay) and the first scientific finding to support the idea that gayness might be more than just a lifestyle.

More here.