[music playing]

Lesley Stahl: These bedrooms belong to twins who are nine years old. One room is all camouflage, airplanes, and military toys. The other? Well, see for yourself. A pastel canopy, stuffed animals, white horses. Not all that surprising for boy/girl twins. Except these twins—

Jared: What's your base coat?

Lesley Stahl: — are Jared and Adam.

Jared: I have the Marines parking only, and then I have—

Lesley Stahl: When I came for a visit, Jared was eager to show me his GI Joe collection.

Jared: I have ones that say like Marine, and SWAT, and then that's where I keep all the guns for them.

Lesley Stahl: Adam, on other hand—

Adam: This is one of my dolls, Bratz baby. This is made by the company that made this and this.

Lesley Stahl: I hate to interrupt you, but I just noticed you have nail polish on with stars.

Adam: And diamonds.

Lesley Stahl: And diamonds. I have to ask you a question. Did you go to school like that today?

Adam: Mm, hm. I just showed them my nails and they were like, why did you to do that?

Lesley Stahl: Adam's behavior is called childhood gender nonconformity, meaning a child whose interests and behaviors are more typical of the opposite sex. Research shows that kids with extreme gender nonconformity usually grow up to be gay. Danielle is Adam and Jared's mom. How early on did you notice this difference?

Mom: About 18 months. He was really into asking for a Barbie. He wanted a Barbie.

Lesley Stahl: At 18 months?

Mom: At 18 months.

Lesley Stahl: And what about Jared? What was he asking at 18—

Mom: Fire trucks, fire trucks.

Lesley Stahl: Michael Bailey, a psychology professor at Northwestern University is a leading researcher in the field of sexual orientation. They've grown up in the same house. They've been treated the same way.

Professor Bailey: To me, cases like that really scream out. Hey, it's not out there, it's in here. There's no indication that this mother is prone to raise very feminine boys, because his twin is not that way.

Lesley Stahl: So you don't think it's nurture?

Professor Bailey: I don't think that nurture is a plausible explanation.

Lesley Stahl: If you can spot a child's future sexual orientation before he even knows he has one, doesn't that prove its genetic? Studies have shown that homosexuality runs in families. So genes must be the answer. But then, the researchers tell you, identical twins can have different sexual orientations.

We found Steve and Greg in New York. As identical twins, they have the same upbringing, have the same DNA. And yet, Greg is a gay and Steve is straight. When you meet people, and they find you're gay and you're not, what do they say?

Greg: Are you sure? How can that be? Yeah, sure. Everyone is curious about that.

Lesley Stahl: There were signs, even when they were little kids. Their mother told us Steve loved sports and the outdoors, while Greg like helping out in the kitchen. But it wasn't until high school that Steve became convinced Greg was gay. Did you say anything to him?

Steve: I did, actually. I think the way I worded it was something like, you know, Greg, if you're gay, it's OK. I'll still love you the same. And he gave a very philosophical answer. He said something like, well, I love the soul of a person and not the physical being. And in my mind I was like, yep, he's gay.

Greg: I wasn't ready just yet.

Lesley Stahl: Does this prove that it's not genetic?

Professor Bailey: What it proves is it's not completely genetic. They have the same genes.

Lesley Stahl: So does that bring us back to the mother and father?

Professor Bailey: No.

Lesley Stahl: But that's environment.

Professor Bailey: That's environment, but that's not the only environment. There's also the environment that happens to us while we're in the womb. And scientists are realizing, that environment is much more important than we ever thought it was.

Lesley Stahl: And this newborn rat pup, oddly enough, may hold important clues to what happens in the womb. Dr. Marc Breedlove at Michigan State University says he can take this a male rat and make it behave like a female for the rest of its life, and vice versa for a female, just by altering the hormones it's exposed to right at birth. Because rats are born underdeveloped, that's roughly the same as altering a third trimester human fetus in the womb. But first, he said, I would need a crash course in rats sex.

DR. BREEDLOVE: He's the Romeo.

Lesley Stahl: He's Romeo. Ooh, look at him. Male rats, like Romeo, mount any rat that comes their way. In the mating process, the female— that's her being dropped on top of him— perform something called lordosis, where she lifts her head and rump. Now what if Romeo, here, goes after a male?

DR. BREEDLOVE: The male will seem profoundly indifferent.

Lesley Stahl: But Breedlove says, he can change all that. He gave this female rat a single shot of the male sex hormone testosterone at birth. Now, grown up, she will never perform lordosis. But this male rat will. Take a look. He was castrated birth, depriving him of testosterone. So you created a gay rat.

DR. BREEDLOVE: I wouldn't say that these are gay rats. But I will say that these are genetic male rats who are showing much more feminine behavior.

Lesley Stahl: So this could be the answer to the genetic question. It may not be genes. It could be hormones.

DR. BREEDLOVE: That's exactly the question that we're all wondering. This business of testosterone having such a profound influence, does that have some relevance to humans?

Lesley Stahl: While biologists look at hormones for answers about human sexuality, other scientists are looking for patterns in statistics. And hard as this is to believe, they have found something they call the older brother effect.

Professor Bailey: The more older brothers a man has, the greater that man's chance of being gay.

Lesley Stahl: What? Say that again.

Professor Bailey: The more older brothers a man has, the greater the likelihood that that man will be gay.

Lesley Stahl: Is that true?

Professor Bailey: That is absolutely true.

Lesley Stahl: Absolutely true?

Professor Bailey: Absolutely.

Lesley Stahl: If this comes as a shock to you, you're not alone. But it turns out it's one of the most solid findings in this field, demonstrated in study after study. And the numbers are significant. For every older brother a man has, his chances of being gay increase by 1/3. Older sisters make no difference. And there's no corresponding affect for lesbians. A first born son has about a 2% chance of being gay, and the numbers rise from there. The theory is, it happens in the womb.

DR. BREEDLOVE: Somehow, the mother's body is remembering how many boys she's carried before. The favorite hypothesis is that the mother may be making antibodies when she sees a boy the first time, and then affect subsequent boys when she carries them in utero.

Lesley Stahl: You mean, like she's carrying a foreign substance?

DR. BREEDLOVE: And if you think about it, a woman who's caring a son for the first time, she is carrying a foreign substance. There are some proteins encoded on his y chromosome that her body has never seen before, and that her immune system would be expected to regard as invaders.

Lesley Stahl: So that's a theory?

DR. BREEDLOVE: That's the theory.

Lesley Stahl: It's still not proved. And it gets even stranger.

DR. BREEDLOVE: One of the things we've only found out, lately, is that older brothers affect a boy only if the boy is right-handed. If the boy is—

Lesley Stahl: Wait a minute.

DR. BREEDLOVE: I know. If the boy is left-handed, if his brain is organized in the left-handed fashion, it doesn't matter how many older brothers he has, his probably of being gay is just like the rest of the population.

Lesley Stahl: You can give yourself a headache trying to apply all the theories to real people. Greg and Steve are both right-handed, and they do have an older brother, so maybe that's why Greg is gay. But they also have several gay relatives, which suggests it could be in the genes, except where does that leave Steve? Adam and Jared, fraternal twins, have older brothers, but they are ambidextrous. And then there's the question of how something in the womb could affect one twin but not the other. There are many more questions, at this point, than answers.