Gay men often claim that even as children they knew they were somehow "different" from other boys. Many say that sense even preceded puberty. And yet, though researchers have tried for decades to identify a biological basis for homosexuality -- which seems to be present in all human societies -- they have mostly come up dry. Tantalizing clues have surfaced: gays are more likely to be left-handed, for instance. But in the end, there has been little proof that biology is sexual destiny.
Now new research offers evidence that there may indeed be a physiological basis for sexual orientation. In a...