SCIENTIFIC ANALYSIS OF TWINS YIELDS CLUES IN THE QUEST TO LEARN WHAT DETERMINES A PERSON'S SEXUALITY -- "60 MINUTES" SUNDAY

What determines whether a person is gay or straight has been the focus of social, religious and political discussions for years that almost always come down to the question: is it nature or nurture? While scientists say a definitive answer is years away, their studies -- including some of twins -- are providing some early and interesting clues to inform the debate. Lesley Stahl reports on this fascinating subject in a 60 MINUTES story to be broadcast Sunday, March 12 (7:00-8:00 PM, ET/PT) on the CBS Television Network.

Studies of a pair of 9-year-old twins raised by the same mother in the same house who exhibit opposite tendencies seem to rule out the nurture argument. Adam paints his nails and plays with dolls; he asked his mother for a Barbie doll when he was just 18 months old. His twin brother Jared has as many GI Joes as Adam has girl dolls and prefers his camouflage bedspread to his twin brother's pastel-colored one. He wanted fire trucks when he was 18 months old.

Adam's behavior is an example of childhood gender nonconformity, say scientists. Research shows that most children with extreme tendencies toward gender nonconformity grow up to be gay.

But is it nature then? Another set of twins, Greg and Steve, grew up in the same house, Steve liking sports and the outdoors while Greg liked helping out in the kitchen, says their mother. Today, Greg is gay and Steve is straight -- but they are identical twins with the same DNA. So how can it be nature?

Some scientists say the key to what determines sexuality could be in the womb. In a study of rats, researchers discovered they could alter the sexual behavior of the rodents. The rats are born premature and approximate humans still in the womb. Scientists manipulating levels of the male hormone testosterone in the premature rats could alter their sexual behavior at maturity, causing males to act like females and vice versa.

Another interesting observation scientists have made is a relationship between the amount of older brothers a boy has and the likelihood he will be gay. Studies suggest the more older brothers a boy has, the greater chance he will be gay.

But at this early stage in a still under-funded field, there are many more questions than answers. The emerging consensus among scientists, though, is that people are born gay. The exact mechanism is still a mystery.