It's a medical mystery, one of the biggest debates of our time: Is there a gene that determines whether you're gay or straight? Some scientists think so, saying sexual identity is mostly biological. Others say it's a learned behavior.
Not that kids need science to confirm what they already know. Zach O'Connor, now 18, says he knew he was different from the time he was 5 or 6. Most of his friends were girls, and he once asked his parents for a Barbie Dream House.
"I guess I didn't want to associate myself with guys, 'cause I was like, 'well, I don't want any feelings to happen,'" he told us. "'So I'll associate myself with girls."
Zach says he didn't know why he was different. He just knew he liked boys — not romantically, not then. But at about 11, his feelings began to deepen, and around seventh grade he realized he was gay. Two years later he told his mother, whose reaction astounded him.
"She said, 'Have you done your homework?'" Zach said.
"I think he expected me to start getting hysterical or something," recalled Cindy O'Connor. "I said, 'Zach, you know what? Who you choose to love is your business."
Both Cindy and her husband, Dan, believe homosexuality was probably in their son's DNA. For them, there is no medical mystery. But a proven genetic link might help other parents understand what they saw with their own eyes.
Chris and Deborah Hawkins were convinced their son David was gay when he was just 3.
"I saw feminine traits," Deborah said. "He was, like, prissy." She showed us home videos of David as a little boy, pointing out that "his run was a little flamboyant … his walk, his mannerisms."
David, now in his early 20s and openly gay, said he doesn't remember feeling different from his playmates.
"We would play king and queen and of course, I was the king and he was the queen, you know," said his mom, laughing.
"That was my role," recalled David. "I identified more with that kind of personality rather than the gallant hero."
Deborah Hawkins was raised in a fundamentalist Christian community in Greenville, S.C., and taught that homosexuality was something little boys picked up from TV or movies. She said she had no place to turn, no one to consult, and when David came out to her at 15, she felt pure terror. Then, as she read up on the subject, she came to believe that her son's sexual identity was biological, not behavioral.
But Dr. Stanton Jones, a clinical psychologist and evangelical Christian, says genetics plays, at best, just a small role.
"The scientific evidence doesn't support it," he said. And he's written a book, "Ex-Gays? A Longitudinal Study of Religiously Mediated Change in Sexual Orientation," illustrating his belief that gay men can be converted to heterosexuals.
Can they? Dr. Alan Sanders, a psychiatric geneticist at Evanston Northwestern Healthcare Research Institute, is currently heading the biggest study ever undertaken on sexual orientation. He's looking at the genetic makeup of more than 700 sets of gay brothers.
"I think the evidence is pretty convincing already that a substantial contribution to sexual orientation comes from genetics," he said. "It's probably the single biggest factor that we know about."