Twins as a window into nature and nurture | Hidden Brain In December 1988, two pairs of twin boys were born in Colombia. One twin from each pair was accidentally given to the wrong mother — a mistake that wasn't discovered for decades. The twins' story is a tragedy, a soap opera, and a science experiment, all rolled into one. It also gives us clues about the role that genes and the environment play in shaping our identities. We talk with psychologist Nancy Segal about her work with twins, and her encounters with these now-famous brothers. For research related to this episode, please visit

What Twins Can Tell Us About Who We Are

  • Download
  • <iframe src="" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
  • Transcript


This is HIDDEN BRAIN. I'm Shankar Vedantam. In October 2014, psychology professor Nancy Segal heard a story. It was about her specialty - the study of twins. Nancy had made a career of studying unusual cases of twins, but she had never heard a story like this one.


VEDANTAM: It's December 1988. A woman from a remote farming village in Colombia makes a daylong trek to a hospital. She gives birth there to a pair of identical twin boys. One of the twins needs more advanced medical care that isn't available.

NANCY SEGAL: One of them was extremely sick.

VEDANTAM: Since the mom is still recuperating, a relative takes him to another hospital six hours away in Bogota, the capital. At the Bogota hospital, another set of identical twin boys has just been born. Their mother is a seamstress who lives in the city.

SEGAL: The scene was quite chaotic.

VEDANTAM: The hospital staff is tired and overworked.

SEGAL: It was approaching Christmastime. People were thinking about the holidays.

VEDANTAM: Now, nobody knows exactly what happens next, but something goes wrong.


VEDANTAM: The baby from the countryside is switched with one of the babies born to the mother from the city.


VEDANTAM: By the time all the babies are sent home, one boy from each pair of identical twins is in the wrong family. Each family thinks their twins are fraternal. The homes the boys grow up in are in totally different places - 150 miles away from each other and a world apart.

SEGAL: One - the lively capital city of Bogota. And the other in a very remote farming village. And they'd had no modern amenities, no schooling - things of that sort.

VEDANTAM: Researchers who study twins knew of many examples of identical twins raised apart. But two sets of identical twins where one from each pair was switched into the other home - it had never been documented before. It brought up fascinating questions. Twenty-five years later, when the truth was discovered, Nancy says it was also deeply painful.

SEGAL: It was also a tragedy in the sense that two of the twins grew up in a home where they were not biologically connected, and they discovered in a matter of minutes that their identities were totally shattered. They weren't who they thought they were. They belonged in other families. Their parents were not their parents. In short, everything about their life was basically a lie.


VEDANTAM: This week, we'll delve into the story of the Bogota twins - a tragedy, a soap opera and a science experiment all rolled into one. And we'll also find out why twin studies can act as a natural experiment to test some of the biggest questions about human nature. How much of our lives is shaped by upbringing? How much by genes? Are we the masters of our own fate, or are we all destined to become our mothers? Insights and provocations from twin studies - this week on HIDDEN BRAIN.


VEDANTAM: Nancy Segal is a professor of psychology at California State University, Fullerton. She's the director of their Twin Studies Center. She wrote about the Bogota twins in her book, "Accidental Brothers: The Story Of Twins Exchanged At Birth And The Power Of Nature And Nurture." In both the remote farming village and the city, each set of mismatched brothers had very different outlooks and personalities. In each family, one brother seemed outgoing, the other reserved.

SEGAL: And in fact, people would kid the twins and the parents and say, are you sure your twins, and how could you have two boys that look so different? But people, again, are sensitive to fraternal twin differences. People know that ordinary siblings in a family who had the same genetic relatedness as fraternal twins often look and act very differently. So it was not such a tremendous surprise.

The other interesting thing, though, is that in Bogota, the mom had been told she was having identical twins, and this might have been information gotten from an early scan. It's possible. But it could be that, you know, sometimes these things can be unreliable. And so when she had fraternal twin boys, she accepted it and was quite pleased.


VEDANTAM: William and Wilber, the two brothers who grew up in the farming village, eventually moved to Bogota and get jobs behind the counter at a butcher shop. It's the summer of 2013. They're now 25. And it's here where their lives begin to intersect with the brothers in the city - Jorge and Carlos - where it starts to become clear that someone, somehow, has made a terrible mistake.


VEDANTAM: A co-worker of Jorge - again, one of the twins raised in the city - runs into William at the butcher shop.

SEGAL: One day, two women went to the butcher shop to buy some meat. Now, one of those women was a co-worker of Jorge's, and her first impulse was, what is Jorge doing behind the butcher counter? Is he moonlighting as a butcher on weekends? Is he short of money? And she was really baffled when he didn't greet her. Meanwhile, the friend that she was with said, don't be silly. That is my boyfriend's cousin. So they're going back and forth, back and forth on this. And finally, he said, my name is William. I'm her boyfriend's cousin. I don't know Jorge. And that was it.

But when Laura, this friend, went back to see Jorge the following week at work, she told him about it, they had a good laugh about it, and nothing really happened.

VEDANTAM: But something about the incident sticks with Laura. She tracks down a photo of William and brings it to Jorge's desk. She tells him to sit down; he's about to be shocked. Jorge stares at the image on Laura's phone. He swears. That's me, he says. One thing leads to another, and the two biological brothers meet for the first time at a public square in Bogota. The scene is captured on someone's phone. A friend, offscreen, sounds shocked.



VEDANTAM: Jorge and William look exactly the same. Another friend breaks the ice.


UNIDENTIFIED PERSON #1: (Speaking Spanish).

VEDANTAM: "William is whiter," he says. But other than their complexion, they are like mirror images - both handsome with delicate features. Both have an easy laugh.



VEDANTAM: They steal glances at each other and shift from side to side.


UNIDENTIFIED PERSON #2: (Speaking Spanish).

VEDANTAM: "Oh, my God," says their friend, "this can't be true." Jorge says, "I can't believe it."



VEDANTAM: To an onlooker, this moment would look like nothing - just young men joking around. Its meaning would sink in only later.


UNIDENTIFIED PERSON #3: (Speaking Spanish, laughter).

VEDANTAM: But right now, they have so many questions. William and Jorge go to a nearby coffee shop, where they find they have instant chemistry.

SEGAL: They got along well. They understood the world the same way. And that was something rare for them because neither one of them shared it with their fraternal twin - their so-called fraternal twin brother.

VEDANTAM: Up next will be the reunion of the other identical twin pair, Carlos and Wilber. And here's something I've left out of the story - Wilber was also at the square that night, watching from the sidelines. It was decided, then and there, that he should meet his identical twin Carlos as soon as possible. The three young men get in a cab to go find Carlos.


VEDANTAM: The reunion is hard for Carlos. It meant he was supposed to have grown up on a farm, not a city. It meant his brother was not his real brother, his sister was not his real sister, his mother was not his real mother.

SEGAL: Because you've got to remember that Carlos was a city boy. He loved his mother who had passed away a few years earlier and to discover that she might not be his mother was absolutely devastating to him.

VEDANTAM: The first time he saw his identical brother Wilber, Carlos went out of his way to try and find differences between them.

SEGAL: But he looked in vain because they looked exactly the same. Both of them were very, very interested in appearance. Both of them got their eyebrows waxed - very into fashion. They both liked similar types of girlfriends. They both called themselves the crybabies because they were very sensitive as children. They were always complaining and always kind of undergoing various medical complaints, whereas Jorge and William had no medical complaints whatsoever. We gave them long medical histories, and Jorge and William just had - handed them in blank. But Carlos and Wilber had long lists of various types of pains and aches and other ailments.

VEDANTAM: Did the brothers stay in touch after the mix-up was corrected and revealed?

SEGAL: Oh, yes. In fact, they call themselves - well, I call them the band of brothers because they do that. And they have stayed in touch. They make decisions based on four. They have a common email address. They really are very close. And, you know, what's important is that there are two ways they could have handled this - one would have been complete despair, and the other is simply accepting and moving on. And so they and their family members say, look, we did not lose a nephew, we did not lose a brother, we did not lose a son; we're gaining. And I think that they should be applauded for that kind of an attitude and for carrying it out so successfully.

VEDANTAM: As a researcher, obviously you've been fascinated by, you know, how the bonds of genetic connection might prompt, you know, similarities. But when you went down to Bogota, did you also find that the bonds of familial connection, growing up in the same house or growing up in the same environment, did that form or forge connections that are also still apparent today?

SEGAL: Yes. Each pair of unrelated brothers feels a tremendous loyalty toward the other one. So Jorge wondered initially if perhaps Carlos would feel left out, and so he went and got a tattoo of Carlos on his chest, next to the one he has of his mother. And in Spanish, it says, my sacred family, and that was very touching for Carlos. So they maintain a lot of family loyalty, as do William and Wilber. Now, William and Wilber were in the army together. They were in the army together. They went to Bolivia together, and they watched out for one another a great deal when they're there. And of course, they worked together for many years in that butcher shop.

So the loyalties are there, but at the same time, the twins will tell you that there is disagreement, there is animosity, there was friction. And they don't have that immediate understanding with the unrelated brother that they so quickly forged with the related brother when they met for just a short time.

VEDANTAM: You've studied a number of other twins - twins raised together, twins raised apart, identical twins, fraternal twins. We'll talk about the science of twin research in the second half of our conversation and what this means in terms of parenting and other issues. But I want to look at a couple of other examples first. Tell me about the Jim twins.

SEGAL: Well, the Jim twins - Jim Lewis and Jim Springer - were identical twins born in Ohio and grew up about 30, 40 miles apart, and they met when they were 39 years old. One of the families had been told the other twin had died, but she later learned that was not the case. And the twins eventually decided it was time to meet, and they did.


JOHNNY CARSON: I'm looking forward to meeting these next two gentlemen. You may have heard or read about my next guests...

VEDANTAM: In 1979, just a few months after the twins first met, they shared their story on "The Tonight Show Starring Johnny Carson."


CARSON: Would you welcome, please, Jim Lewis and Jim Springer?


VEDANTAM: The similarities were incredible.


JIM SPRINGER: We both - well, Jim's been married three times. I've been married twice - his first wife named the same as mine.

CARSON: His first wife is the same name as yours.

SEGAL: They both married different women named Linda. They both divorced them and married women named Betty. And then one of the Jim twins divorced Betty and married Sandy. And so I imagine that Betty is a little nervous waiting in the wings to see what's going to happen.

VEDANTAM: (Laughter).


SPRINGER: We both named our first sons the same, James Alan. Oh, and we both named our dogs Toy.

SEGAL: And both of them had light blue Chevrolets. They both liked to vacation on the same three-block strip of beach in Florida. And it's amazing they didn't meet there. They both used to bite their fingernails. They both used to scatter love letters around the house to their wives. And they both enjoyed woodworking. So their similarities went on for a long time. And what's interesting is that they were the ones who first launched the Minnesota study of twins raised apart directed by professor Thomas Bouchard at the University of Minnesota. And this pair got so much attention that other pairs suddenly came out of the woodwork. And before Bouchard knew it, he had 15 pairs in the list.

VEDANTAM: Now, of course, when you're a researcher and your studying twins like this, it's difficult to distinguish between things that are actually points of connection and things that are coincidences. In other words, the fact they both married a woman named Linda might be a coincidence. It might have nothing to do with the fact they're identical twins. How do you go about distinguishing between those two things, telling the things that are coincidences from the things that you think actually are driven by some form of connection?

SEGAL: Yeah. Well, first of all, let's say that maybe being attracted to women with a certain name is not random. We don't know that for sure. But the way we can distinguish coincidence from real scientific hard data that's got a genetic basis is that you compare identical twins with fraternal twins with reference to a particular attribute. Now, we had different kinds of quirky habits that we would see in some identical pairs. So, for example, one of the identical pairs always read books from back to front. In another identical pair, both twins used to scratch their ears with paperclips. So you can't start looking for paperclip scratching across pairs or for reading books back to front. What you can look for are matched unusual habits. And if you find matched unusual habits more often in identical twins than you do in fraternal twins, this gives you some clue that the genes may be driving that behavior at least in part.

VEDANTAM: You looked at another pair of twins who had some remarkable things in common, including some of the behaviors you just described. Tell me about the giggle twins.

SEGAL: Oh, the giggle twins. They were from Great Britain. And when they met for the first time, they discovered that with each other but with nobody else they would giggle uncontrollably. They even did this to me once on a TV set, which was really irking the producers. But there was something about the other one that just set them off. But they also had other similarities in that besides their identical appearance, they both had the same crooked pinky fingers. They both disliked politics. They both had a habit of sort of wrinkling their nose, something they both called squidging. So they had a number of similarities as well. But we called them the giggle twins because that was their hallmark feature.


VEDANTAM: After the break - the science of twin studies and what they can tell us about the mysterious dance between nature and nurture.


VEDANTAM: Nancy, researchers long ago had an insight. Some twins are fraternal and some twins are identical. What is the genetic difference between such twins, and how do you and other scientists use these differences in what are called twin studies?

SEGAL: Identical twins result when a single fertilized egg divides sometime between the first and 14th day after conception. So these twins share all their genes in common and are always of the same sex. Fraternal twins result when a woman releases two eggs at the same time, and they are separately fertilized by two sperm from the father. So these twins share half the genes on average just like ordinary brothers and sisters.

So what we do is, as scientists, we gather large numbers of both types of pairs, and we compare the similarities with respect to intelligence, personality, running speed, interests, food preferences. And in virtually every study that I've seen, identical twins tend to be more alike than fraternal twins, which suggests that the genes do play a role in the development of that trait. On the other hand, identical twins are never perfectly matched. There are always some differences. And so that becomes exciting because we can look to see what are those differences and what - how do we explain them? What environmental effects can we use?

Now, twin studies are not just for twins. That's a very important point. Twin studies are a model. They're a research paradigm for how we understand genetic environmental influences on development in the population at large.

VEDANTAM: So there have been hundreds of studies looking at this idea. What does this body of research tell us, Nancy, about things like personality and intelligence, the things you just mentioned?

SEGAL: Yes. Well, there had been a lot of studies in both of those areas, and we find that for intelligence, general ability, that has about a 70 to 75 percent genetic influence. We find for personality, closer to 50 percent. But let me be clear about what those numbers mean. It does not mean, say, that for personality I could take a person and slice their extroversion or sociability into a genetic and environmental component - not at all. Those are completely intertwined within individuals. What it does mean is that if I study personality in a population or a group, there's going to be variation from person to person. Not everybody is the same. About half those differences are explained by the genetic differences among the people, and about half the differences were explained by environmental differences.

VEDANTAM: In some ways, what you're pointing out is that the superficial take on what twin studies mean is actually perhaps not accurate in terms of depicting reality. I mean, when it comes to individuals, your environment can shape who you become in profound ways just like your genes can shape who you've become in profound ways. You've, in fact, studied identical twins who have turned out very different, sometimes because of environmental differences that begin right in the womb.

SEGAL: That's right. There was a pair of twins I studied once who differed in height by 4 inches when they were 7 years old, the time that I studied them. And what happened in that case was that one of the girls, the one who was shorter, had a marginally attached placenta, which means that she did not get enough nutrition prenatally. As they grew up - I was shocked when I met them the first time because they looked like the same version of each other but just one very small and one very big. And then when I went back to see them again in their young adult years, it turned out that the taller twin was 7 inches taller than the other one. So that was something that had a lasting imprint on these still identical genes.


VEDANTAM: Differences in the womb led these twins to look very different. Similarly, other environmental differences can act on two people with the same genes and reshape them dramatically. One of the best examples of this might be the Bogota twins that Nancy studied. Consider, for example, one of the brothers who grew up in the wrong home, William. He was born to the city mother but raised on a farm. Even as a small child, he was studious and drawn to learning. But his circumstances put brakes on his natural inclinations. Opportunities for education in his poor farming village, Nancy says, were greatly limited.

SEGAL: They were not available, and so he decided to join the army. And in the army, once you complete your service, you are entitled to take some sort of officers training course. And that seemed perfect for William. It would have meant going to school and being paid for by the government. But it turned out, unfortunately, that while he was in the service, he had contracted a disease called leishmaniasis. And while he recovered from it, that put him completely out of the running.

VEDANTAM: William's dream of getting an education through the army was dashed. And by the time he discovered the truth about his origin, he had resigned himself to not getting a proper education. He had begun to work at the butcher shop in Bogota. This is when the co-worker of his identical twin happened to stop by the shop. What happened in the years that followed is even more revealing about the power of the environment to reshape our genetic inheritance.


SEGAL: With William, there were just fewer opportunities he could even pick from - that was his trouble - until he got down to Bogota. Now, what's happened to William since then and since he's met his brother, Jorge, he is now enrolled in law school in Bogota. He's in his second year of it now. And with the help of Carlos, actually, who was not related to him genetically or environmentally, Carlos helped him research law schools, and now that's where he's looking to continue his education. He has political aspirations. He wants to go back to the town that he grew up in and become the mayor of the town.

VEDANTAM: Carlos was the other brother who grew up in the wrong home. He was born to the poor family from a rural area but was raised in Bogota. His life, too, was dramatically reshaped by living in the city.

SEGAL: Carlos was working as a financial analyst, having completed college and several advanced certificates. And he's very much a man about town. He loves nice clothes. He loves cultural activities. And for a long time, he had a lot of trouble with William, saying, well, why didn't William do this and why didn't William do that? But he really didn't understand that there were no opportunities; there just weren't any.

VEDANTAM: For an academic discipline, twin studies have drawn an extraordinary amount of controversy. There are debates about its central paradigm, which is that differences between identical twins and fraternal twins are explained by genes. But if families and teachers treat identical twins differently than they treat fraternal twins, could some of the differences between the groups be driven by upbringing - nurture rather than nature? Underlying many of these debates is an unease; if genes determine our intelligence and personality, what does this say about differences we see in society between people?

Although we've seen how it's impossible to separate the effect of genes from the environment in any person, it's tempting to explain differences between people by pointing to their genes. Why is one person rich and another poor? Genes. Why do we see different outcomes in the workplace between a man and a woman? Genes. If biology explains intelligence, why bother with public education? After all, if we are smart, it's because of our genes. If we are dumb, it's because of our genes. I asked Nancy what she made of such arguments.

SEGAL: Well, that's a real misunderstanding of the way that the genes work because, after all, behavior is a function of genes being expressed in an environment, and by moderating or fixing the environment in different ways, you can alter gene expression. Now, some behaviors are harder to alter than others, but everybody can improve. Everybody can develop new skills and get better at that. People can't all be the same. We're not all the same. We're not all one big identical twin. But everybody probably has talents, interests, all kinds of abilities that are undiscovered because they've never had a chance to express them.

VEDANTAM: Just as people are wrong in thinking that genes determine everything about our lives, those who say the environment explains everything are also missing something crucial. The environments we find ourselves in are themselves shaped by the proclivities and propensities that are encoded in our genes. Especially when it comes to human behavior, nature and nurture really are inextricably intertwined.

SEGAL: Yes, exactly. We all pick certain items in our environment that are compatible with who we are. And so while one person might be drawn to dance lessons, one person to music, one person to art, these are the things that mean that we fashioned our environments. So environments in a sense are genetically influenced, and then within those environments certain genetic propensities might be expressed.

VEDANTAM: So in some ways, what I'm hearing you say, Nancy, is that our genes, indeed, are very powerful in shaping us, but they are not necessarily deterministic. It's one thing to say that intelligence is heritable, but when you say, you know, 62.35 percent of your intelligence came from your genes, we might be guilty of false precision.

SEGAL: I agree with that. And I think that some of the theories people have had from this research is that once you've got your genes, you're stuck with them, and that's not really true. We are the ones who make decisions about what we will do, how hard we will work, how we will go about getting our goals. After all, genes are just propensities; they're not deterministic. I think that we can all take tremendous pride and happiness in those things that we accomplish.


VEDANTAM: I understand that you yourself have a twin?

SEGAL: That's correct. I have a fraternal twin sister, Anne. I'm the older twin by seven minutes, but I'm also the shorter twin by 4 inches. And we are very different in our outlooks, in our appearance, in our talents. But we have a lot of family loyalty, and I think there's probably no one I love as much in my family as my sister.


VEDANTAM: Nancy Segal is professor of psychology at California State University, Fullerton, and the director of their Twin Studies Center. She is the author of "Accidental Brothers: The Story Of Twins Exchanged At Birth And The Power Of Nature And Nurture." Nancy, thank you for joining me today on HIDDEN BRAIN.

SEGAL: It's been an absolute pleasure.


VEDANTAM: This episode was produced by Laura Kwerel and edited by Tara Boyle and Parth Shah. Our team includes Rhaina Cohen, Jenny Schmidt and Thomas Lu. Our unsung hero today is Luis Clemens. Luis is an editor at NPR, and he cares deeply about opening doors for young journalists. I often see him in the cafeteria in the mornings helping interns craft cover letters and get ready for job interviews. He's a walking reminder of how we can all help change the trajectory of other people's lives by giving them opportunities. Luis played an important role in getting our very own Parth Shah to HIDDEN BRAIN, where we now can't imagine what life was like before him. Thanks, Luis.


VEDANTAM: For more HIDDEN BRAIN, you can follow us on Facebook and Twitter. If you like this episode, please tell one friend about it and help them subscribe to our show.


VEDANTAM: I'm Shankar Vedantam, and this is NPR.


Copyright © 2019 NPR. All rights reserved. Visit our website terms of use and permissions pages at for further information.

NPR transcripts are created on a rush deadline by Verb8tm, Inc., an NPR contractor, and produced using a proprietary transcription process developed with NPR. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.