ATLANTA, Georgia (CNN) -- Can you tell whether someone's gay just by the way he or she walks?

Psychologists and geneticists are trying to answer the question of whether sexual orientation is a choice.

David Sylva wants to know. He straps bright red lights to people's bodies and videotapes them walking in the dark. He then shows the videotape to observers (who won't be biased by clothing or hairstyles since the walker is in the dark) and asks them to guess the walker's sexual orientation.

Sylva's observations focus on the physical characteristics of the individual's stride, such as the closeness of the knees.

Why does Sylva, a graduate student at Northwestern University, care so much about how gay people walk? Because he's one of a growing number of researchers who think sexual orientation may be as basic as how you walk, something inborn that you don't choose.

His premise reflects a growing belief among Americans, according to a new CNN/Opinion Research Corporation poll. For the first time a majority of respondents -- about 56 percent -- said they don't believe a person can change his or her sexual orientation. In a similar poll in 2001, 45 percent said orientation couldn't change. In 1998, 36 percent held that belief. The sampling error for Wednesday's results is plus or minus 4.5 percentage points. Watch more on research into the nature of sexual orientation »

A growing number of psychologists and geneticists are working on the "nature versus nurture" question -- a question that's set off a highly charged political debate about whether people choose their sexuality, or whether gayness is determined by their DNA.

Take Richard Lippa, a professor of psychology at California State University at Fullerton. His studies show that gay people are twice as likely to be left-handed. He also collects photos of hair whorls -- those circular swirls you see atop a man's head. He says about 10 percent of the general population have whorls that rotate counter-clockwise, but about 20 percent of gay men have counter-clockwise whorls.

Lippa acknowledges that studying hair patterns sounds strange. "It sounds a little like the 'Twilight Zone' or voodoo science," he says. But to Lippa, a link between sexual orientation and something that's clearly inborn (like handedness or the way hair grows) speaks volumes. His theory: You can't choose your whorl, and you can't choose your sexuality, either.

"You're born with either a clockwise or a counter-clockwise hair whorl. It's fixed, it's biologically determined. No one's going to argue that your hair whorl is influenced by learning or culture," he says.

Lippa says his next step is see whether there are specific genes that control sexual orientation.

Douglas Abbott thinks Lippa won't find a thing.

"There is no evidence of a 'gay gene,' " says Abbott, professor of child and family studies at the University of Nebraska.

Abbott points to studies that look at the sexual orientation of the offspring of gay people. "If homosexuality was caused by genetic mechanisms, their children would be more likely to choose same-sex interaction," he says. "But they aren't more likely, so therefore it can't be genetic."

For Abbott, the answer to the nature-vs.-nurture question is very clear. "I think the primary causes of same-sex behavior are environmental and personal choice and free agency," he says. "Can someone change their orientation? The definitive answer to that is, "yes.' "

That makes Gerulf Rieger laugh. "Ask a bunch of straight guys [if they could switch to being gay] and they would tell you, 'Are you kidding me?' " says Rieger, a lecturer in psychology at Northwestern University. "So the other way around doesn't work either."

In his research, Rieger shows videotapes of men and women talking about the weather. Observers have been able to predict with great accuracy whether the person talking is gay or straight. "Even within seconds, people are pretty good at figuring out who's gay and who's not," he says.

Like Sylva with his illuminated walkers, Rieger thinks his research points to genetics, and not choice, as the source of sexual orientation.

"It doesn't seem to be the social environment, it doesn't seem to be the parents or peers that make you gay," he says. "It seems to be something that comes from within." E-mail to a friend

Elizabeth Cohen is a correspondent with CNN Medical News. Senior producer Jennifer Pifer, associate producer Sabriya Rice and intern Rachel Zelkowitz. contributed to this report.

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