Nature Plus Nurture

Of all the motley forms of humanity, surely the one type that is made-laboriously, painfully-and not born would seem to be transsexuals. These men (and, rarely, women) submit to hormone therapy and complicated surgery to mold the bodies they were born with into the bodies they want. But new research suggests that biology sculpts transsexuals long before surgeons get near them. According to neuroscientists led by Dick Swaab of the Netherlands Institute for Brain Research, one tiny region in the brains of six male-to-female transsexuals is only 52 percent as large as in other men, they report in the journal Nature-and almost exactly the size of the region in female brains. "Our study," they conclude, "is the first to show a female brain structure in genetically male transsexuals."

Other research has also seemed to strengthen the biology-is-destiny camp, at least for sexual orientation. A 1991 study found differences in the brains of gay and straight men. Two years later researchers led by Dean Hamer of the National Cancer Institute reported evidence of what has come to be called a "gay gene." Last week Hamer and colleagues unveiled, in the journal Nature Genetics, stronger evidence that a region of the X chromosome, long enough to contain 200 genes, is linked to homosexuality. The evidence: gay brothers are more likely to both inherit this region than are their straight brothers. But instead of clinching the case for predestination, the studies are prodding scientists to search for how experiences influence sexuality.

Consider the male transsexuals. Perhaps they are not born with a brain that tells them they are really female, suggests neurobiologist S. Marc Breedlove of the University of California, Berkeley. Instead, "social and other experiences"--how friends and parents treat them--might make these men feel they are women. These experiences might then feed back on the brain. Three decades of research "have made it clear that experience can dramatically alter the structure and function of the brain," Breedlove points out.

Even a "gay gene" might not mean that homosexuality is predestined. As Hamer admits, it is only "in some families but not in all" that gay brothers inherit the suspect stretch of the X chromosome; in many gays this DNA is absent. That suggests that the DNA is not necessary for homosexuality. In fact, it may not be sufficient, either. Of identical twins in which one brother is gay, the other twin is gay only about half the time-even though he has the identical DNA. Clearly, experiences, relationships and happenstance-this hodgepodge called environment-play a role in shaping sexuality. Psychiatrist William Byne of New York's Mount Sinai Medical Center spins a theory in which a gene that makes a boy avoid rough sports might hurt his relationship with a father who adheres to sex-role stereotypes, but not with a more enlightened father. The impaired relationship might induce brain changes that make the first boy gay but not the second.

None of the studies on gay brains, gay genes or transsexual brains has been replicated by other labs. One of Hamer's ex-collaborators even accused him of selecting only data that support his hypothesis; Hamer calls that a "wild accusation." Even if the findings hold up, they barely address the deeper question: how genes, brains and life itself interact to shape sexuality.

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