BROOKE GLADSTONE: This is On The Media, I'm Brooke Gladstone. The double helix structure of deoxyribonucleic acid, or DNA, was discovered just 65 years ago. But mankind has been playing with the notion of what makes us who we are for millennia. Twins especially played a prominent role in ancient mythologies. The founders of Rome, Remus and Romulus twin brothers raised by a giant wolf. Castor and Pollux have twin brothers whose Gemini forms sparkle in the night sky. The birth of twins has been interpreted as good luck and as bad and they themselves as two halves of the lightness and darkness of our nature. That's never stopped. You see it in sci-fi sagas like Star Trek.

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STAR TREK VOICE OVER: Now Data's evil twin plots to destroy the crew.

MALE VOICE: Back off. Go!

STAR TREK VOICE OVER: With the help of a lethal life force on Star Trek: The Next Generation. [END CLIP]

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FEMALE CORRESPONDENT: The Southern California woman who was dubbed the evil twin and convicted of plotting to kill her twin sister could be granted parole. [END CLIP]

BROOKE GLADSTONE: As vehicles for conflict, serendipity and comedies of error.

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PATTY DUKE SHOW INTRO: There're cousins. Identical cousins! [music]

LINDSAY LOHAN AS HALLIE PARKER: So if, if your mom is my mom, and my dad is your dad. And we're both born on October 11th, then you and I are like, like sisters!

LINDSAY LOHAN AS ANNIE JAMES: Sisters? Hallie, we're like twins!

FEMALE CORRESPONDENT: Double trouble! What twin partners in crime are accused of doing. [END CLIP]

BROOKE GLADSTONE: But beyond mere novelty, multiple births can provide scientists deep insights into the age old argument between nature and nurture. A documentary released this summer three identical strangers tells the story of triplets David, Eddy and Bobby. Adopted by three different families and studied under highly unusual circumstances. Spoilers ahead.

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VOICE: They separated these boys at birth.

FEMALE VOICE: The parents had never been told that there were two other children.

MALE VOICE: What was the purpose?

MALE VOICE: Why? How could you not tell us?

MALE VOICE: They're trying to conceal what they did from the people that did it to.

FEMALE VOICE: When you play with humans you do something very wrong.

MALE VOICE: Who would be evil enough to come up with something like this? [END CLIP]

BROOKE GLADSTONE: The secret study orchestrated by psychologist Peter Neubauer sought to examine how three distinct parenting styles might shape identical triplets reared apart. Not until the boys met by chance at age 19, did they learn how their lives had been manipulated for a trove of data that remains largely locked away in an archive at Yale. According to Dr. Nancy Segal, the director of the twin studies center at California State University at Fullerton, the unreleased study was likely inconclusive and definitely cruel.

NANCY SEGAL: Well I think this was a very unethical study. Keep in mind that even though in those times, the practice was that you don't encourage contact. But nevertheless, these children were never told they were twins. And most of these adoptive parents who are hungry for children would have gladly taken two. So to not tell people this very important aspect of their background I think is just horrendous. It's really beyond my comprehension. And I--

BROOKE GLADSTONE: Why?

NANCY SEGAL: Because being a twin is a very important part of one's identity, one's knowledge of one's medical history. And I can tell you that having worked with hundreds of pairs of twins raised apart just how happy they are to have met and how regretful they are that they lost all those years together.

BROOKE GLADSTONE: Twin studies often capture the public imagination but none quite like NASA's twin study which tracked the genetic profiles of two astronaut brothers. One who spent a year in space while his twin lived on earth. The media had a field day with this.

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FEMALE CORRESPONDENT: Scott Kelly and Mark Kelly are the only identical twin astronauts in history. But following a DNA altering space mission, they may not be identical anymore.

FEMALE CORRESPONDENT: A year in space has already made Scott two inches taller than his brother.

MALE CORRESPONDENT: Scientists found 7 percent of Scott's genes did not return to normal after he returned to Earth two years ago. [END CLIP]

BROOKE GLADSTONE: So they're no longer identical twins?

NANCY SEGAL: Space travel altered genetic expression. It did not alter the DNA physically itself. When he returned to Earth the vast majority of his genetic material returned to the way it was initially. See this is a fascinating finding because why identical twins differ in the expression of some traits. That's what we need to find out. I think it will fascinate your listeners to know that if you take diseases like schizophrenia or multiple sclerosis or diabetes you'd expect that the correlation should be about one they have the same genes but it's closer to 40, 50 percent. Something has to trigger the expression in one and keep it quiet in the other. And identical twins reared apart or identical twins who differ in some way can help us find these environmental effects that might help us control and manage disease.

BROOKE GLADSTONE: You've profiled other twins that have been shaped by extreme events, could you tell us about the subjects of your book Accidental Brothers.

NANCY SEGAL: Accidental Brothers follows the life histories of two sets of identical male twins who were born a day apart in Columbia, South America in 1988. One pair of identical twins was born in Bogota, the capital city and the other born in a small remote town 150 miles to the north. But one of the little babies up in the north needed immediate care at a good hospital. So his grandmother brought him down to Bogota. At the same hospital as the other twins were born and somehow in an amazing twist of fate got exchange with one of the other twins. So the wrong baby was brought back to the small town. A week later so they were essentially raised as two sets of fraternal twins when in fact they were unrelated and they didn't know this until they were 25, when the brothers up north moved to Bogota and somebody confused a twin and one pair for twin and the other. I went to Bogota two times to study them and they are wonderful guys. And there were similarities in many personality traits between the real identical twins despite their separate rearing. But you could see that the effects of the different environments had a role. The twins in the city had gone to high schools and good colleges. The twins up north didn't go past fifth grade. But I have to say that living with them you could see the similarities really popping out. And the medical life histories were amazing. The one twin born in the city and raised in the city was basically symptom-free as was his identical twin brother raised in the north. The other twin who was raised in the city who was not born there had a long list of medical complaints as did his twin brother up in the north. So even the different environments and the different access to medical care did not make a difference in their medical histories and physical health which I find quite extraordinary.

BROOKE GLADSTONE: So what did this tell you about nature versus nurture. It was too small a sample, I know but--

NANCY SEGAL: Yes but it's a very dramatic case and so it told me a couple of things. In terms of the abilities, one could certainly see that not having access to education left an indelible stamp on those twins. But I will say that one of the twins who was raised in the small town who should have been raised in Bogota went and got himself a GED, a high school diploma and is now finishing law school. So he always had the drive. He didn't have the opportunity. But I have also become very appreciative of extreme environments and how they influence behavior. I think that this is a very telling case in so many ways.

BROOKE GLADSTONE: So what are some of the myths that you are confronted with on a regular basis that you would like to dispel right now.

NANCY SEGAL: One of them is that twins communicate telepathically. I have carefully reviewed the literature on that and published it in my book, Twin Mythconceptions. I believe that twins often come to the same decisions, perform the same way and make the same choices. But it's not as though they are communicating, their genes simply predispose them to make these similar choices.

BROOKE GLADSTONE: But the special language issue.

NANCY SEGAL: That's, that's different. When we talk about special languages. This is more common in twins but can also occur between same age unrelated infants who were raised together. And because twins tend to spend a lot of time together and perhaps less language experience with an adult. They often will develop their own verbal expressions or physical gestures. And so this may actually hold back their language development. But this in no way implies any kind of telepathy. You know, if you think about how you may have a best friend, you and the best friend kind of exchange a glance at times and the whole situation is summed up in that glance, that's identical twins are so good at in so many domains of behavior.

BROOKE GLADSTONE: Have you seen that YouTube video of the twins, I think they're 18 months old, maybe they're younger than that, and they're just going blah, blah, blah at each other and laughing at each other's jokes.

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NANCY SEGAL: Yeah, Brooke I showed that to my students every semester and I love it. And to me that video captures the evolution of the twin bond and the so-called twin language. Those kids are hysterical and they know what's funny and yet we don't. But I will tell you that I spoke to a mother once who had an older child and twins and she said that the older child can often act as a translator telling her what the twins are up to. And I thought isn't that interesting, maybe a linguist could use those older siblings as research assistants to find out what's going on.

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BROOKE GLADSTONE: So what about the depiction of twins culturally? Identically dressed twin toddlers may delight us. Some twins like the ones in "The Shining" terrify us. There are lots of twins in mythology. Twins as gods, evil twins and so on. Why does the idea of twins so consistently captivate us?

NANCY SEGAL: In the arts in mythology in movies and literature. Twins are very convenient devices for exploring duality. Good and evil. Happy and sad. Now, why is the public so enchanted with twins? I think it's because we all grow up believing, and rightfully so, that we're all individuals. That we look different we behave differently. I think that when we encounter two people who look and act so much alike, it challenges the way we think that the world works. And many of us admire and even envy the intimacy that identical twins convey. They're so accepting of one another, so non-judgemental. And who among us would want to enjoy your relationship with that kind. So for some people seeing identical twins might almost seem claustrophobic, too much togetherness. But on the other hand, they're still intrigued by it. So I think that's why we are so captivated.

BROOKE GLADSTONE: Which brings us back to that documentary Three Identical Strangers for a minute. There is one doctor who had some involvement with the study that the triplets were unwittingly engaged in and she said this.

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NATASHA JOSEFOWITZ: We would prefer that we have some influence over our lives. Wouldn't you rather know that, that you have some control over this. And so finding out, never mind doesn't matter what you do. So I think it's upsetting to people. To see how little influence, they have, how little control they have. We don't like that, we fight that.

NANCY SEGAL: Genes don't tell us what to do, they may predispose us in certain ways but ultimately we make the choice to do something. We know that divorce has a genetic component to it but we can choose to stay with a partner or divorce. We have a lot of control over our lives. It's just that genes may guide us in certain sorts of directions. I think a great example of that is a pair of twins we studied in Minnesota named Jack and Oscar. They were born in 1933. Jack was raised Jewish by his father in Trinidad and Oscar was raised Catholic in Nazi Germany with his mother. Oscar was in the Hitler Youth but he was only 12 when the war ended. These two, of course, had very different political and historical understandings but they both coped with their living situations in similar ways. Jack, by becoming very pro-British in Trinidad because he didn't want people to know of his German background and Oscar becoming very pro-German because he didn't want people to know if his Jewish background. And when they met they spoke to one another and realized that had their positions been reversed they both would have embraced a point of view that they currently disliked. So the context in which we find ourselves certainly can make a difference but we can choose or not choose a certain perspective or a certain way of looking at things.

BROOKE GLADSTONE: You heard me refer a couple of times to nature versus nurture.

NANCY SEGAL: Oh yeah.

BROOKE GLADSTONE: Two things. Two things separate and immutable. Yeah that is really reductive, right?

NANCY SEGAL: Yes. In fact, I should probably call you on that. We never really say nature versus nurture. That's a false dichotomy that was put to rest years ago. What we talk about is nature and nurture or nature via nurture. Nature via nurture is a nice conceptualization of that because if you think about how you put your house together or the occupations you have or how your environment is constructed you see genetic influence on your environment. You know, you choose to put certain paintings on the wall, you choose certain outfits, you choose a certain design of your home. Why do you do that? Because they are pleasing to you in some way. In an individual, nature and nurture are completely intertwined it cannot be separated. We separate them only statistically in a population and that can give us a good handle on what somebody is chance of having Alzheimer's or diabetes or something like that might be. But it's not totally deterministic. We know for example that autism has a high genetic component, we know that from twin studies on the order of about 70 percent. But a given twin pair, if you have one affect the child the other one maybe never ever affected by autism. So you can't generalize from the population to the individual. You can only use it as a very rough guide.

BROOKE GLADSTONE: So you have a fraternal twin, do you ever wish you had an identical one?

NANCY SEGAL: You know, I thought about that sometimes. I'd never want to give up my fraternal twins. But I think, more that I wish we were triplets and there was a fraternal and an identical I think would be kind of fun. I am fascinated by identical twins and the intimacy that they share and the similarities that they display. So I've often wondered what it would be like but I'm very happy with my situation as it is.

BROOKE GLADSTONE: Sometimes I wish I had an evil twin.

NANCY SEGAL: There's no such thing.

BROOKE GLADSTONE: Thank you so much.

NANCY SEGAL: Thank you so much.

BROOKE GLADSTONE: Dr. Nancy Siegel is a professor of psychology at California State University at Fullerton and director of its Twin Studies program and the author of Accidental Brothers: The Story of Twins Exchange at Birth and the Power of Nature and Nurture.

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BROOKE GLADSTONE: That's it for this week's show. On The Media is produced by Alana Casanova-Burgess, Micah Loewinger, Leah Feder and John Hanrahan. We had more help from Asthaa Chaturvedi and Samantha Maldonado. Our technical director is Jennifer Munson. Our engineers this week are Sam Bair and Josh Hahn. Katya Rodgers is our executive producer. Jim Schachter is WNYC vice president for news. Bassist composer Ben Allison wrote our theme. On The Media is production at WNYC Studios. Bob Garfield will be back next week. I'm Brooke Gladstone.

FEMALE CORRESPONDENT: On The Media is supported by the Ford Foundation, the John S. and James L. Knight Foundation and the listeners of WNYC Radio.