Nobody really knows what "causes" homosexuality. Then again, nobody really knows what "causes" heterosexuality, which goes to show how little we really know' about sex at all.

There has been, of course, a long-running debate about the origins of homosexuality. Are gay people "just born that way," as many gays claim? Does their homosexuality spring from something in their upbringing? Or are nature and nurture both involved? There's no firm answer yet, but over the past decade, researchers from a variety of fields' have begun turning up compelling new evidence that there may be some innate, biological difference between gays and straights.

Some of these findings are really intriguing. In late 1991, for example, a researcher at the Salk Institute for Biological Studies in San Diego reported what appears to be the first real evidence of an innate difference between the brains of homosexuals and heterosexuals. Neuroscientist Simon LeVay, Ph.D., reported that in a group of heterosexual men that he autopsied, one tiny cluster of neurons in the hypothalamus (a deep structure of the brain known to be involved in regulating sexual behavior) was more than twice as large as the same structure in the brains of a group of women and homosexual men. Since the homosexual men had died of AIDS, some critics charged that these brain differences might have been caused by the disease. But when Dr. autopsied a gay man who'd died oflung cancer, he found the same thing.

A variety of studies have also shown that being gay tends to run in families suggesting, perhaps, that there may be some genetic component to sexual preference. In one recent study of homosexuality in twins, researchers from Boston University School of Medicine and Northwestern University reported that if one identical twin is gay, the other is almost three times more likely to be gay than if the twins are fraternal. Because identical twins share the same set of chromosomes, the study raises the intriguing possibility that there may be a "gay gene" that is as yet undiscovered.