Twins : Planet Money Scientists have studied twins for years, hoping to figure out how big a role genes play in human behavior. Our very own pair of twin reporters are on the case.
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When I was about 9 years old, my sister and I were brought to a university campus in Los Angeles to be studied.

ELIZA HELM: I remember going up in the elevator and being very intimidated just about the whole setup.

S HELM: This is my sister, Eliza. So they split us up. Eliza got taken into a separate room.

E HELM: And I think I was the first one with the cap.


OK. Sally, tell me more about this cap.

S HELM: Well, it was a shower-cap-looking thing.

E HELM: It was, like, gel and the hole - and the little holes. And there were wires coming out of your head.

S HELM: Yeah, like, electrodes.

K DUFFIN: The scientists put this thing on her head, connected those electrodes to her scalp. And then they turn on the TV.

E HELM: And then they showed us videos that were disturbing or sad or scary.


RICKY SCHRODER: (As T.J.) Wake up.

K DUFFIN: In the other room, a different researcher is quizzing little Sally, and the questions were weird.

S HELM: Yeah. They asked me, like, have you ever hit anyone with a brick, broken bottle or bat? No. Have you ever pinched anyone? Yes. Have you ever pinched anyone besides your sister? No. We also had to play a computer game. We finished mazes. My sister and I played Bop It together while the researchers watched us. And in our 9-year-old brains, we're just trying to figure out, like, what are they seeing right now?

E HELM: Why are you showing me this? There has to be something.

K DUFFIN: There was something - a big something. Sally and Eliza are very important to science because they are not just sisters; they are twins.

S HELM: Yes, we are fraternal twins. And you, Karen, you also have a twin.

K DUFFIN: This is true. I do. We are identical twins.

S HELM: Marie?


S HELM: Wow, you do really sound a lot like Karen.

M DUFFIN: (Laughter).

K DUFFIN: Sally, last week, we got all of our twins in the studio to get to the bottom of this twin study and the many, many other ways that twins have been studied because - I promise I'm not just saying this because I'm a twin - but we twins, we are this lucky break for science.

S HELM: Twins are a natural experiment in genetics, and they have helped scientists answer what is perhaps the biggest question of all. How much control do we actually have over our own lives? Do we have, like, free will?

K DUFFIN: And this is not just a philosophical question, or even a scientific one. The answer has real-world implications for things like how the government spends money, how we design our laws - for a lot of the things we talk about here at PLANET MONEY.

S HELM: Twin studies have helped us understand things about our mental health, our earning capacity, our ability to learn, our propensity for addiction and whether we hit people with bats, broken bottles or bricks.


M DUFFIN: Hello, and welcome to PLANET MONEY. I'm Karen Duffin.

E HELM: And I'm Sally Helm.

S HELM: Just kidding. That was our twins.


K DUFFIN: Today on the show, how did we get to a place where 9-year-old Sally and Eliza were sitting in a chair with electrodes sticking out of their heads for science?

S HELM: We go deep on the history and design of the twin study. We perform a micro twin study on our own sisters. We learn what those researchers were studying with that electrode cap. And we explore the dark side of twin science.

K DUFFIN: We decided to do this show today in part because of a very strange coincidence.

S HELM: By the way, happy birthday.

K DUFFIN: Right.

E HELM: Happy birthday.

M DUFFIN: Your birthday is the same as ours.

S HELM: Yeah. You and I and our twins - all four of us were born on the same day.

K DUFFIN: This is true.

S HELM: And that day is today, August 2, the day that this show is airing. And that is what got us thinking about twins and the experience of being a twin.

K DUFFIN: Yes. And the experience of being a twin is one of constantly being studied. Pretty much anytime we meet someone new, people first freak out about how much we look alike, gesture alike, as you can hear, talk alike. And then the questions begin, which are always the same.

S HELM: Yes. My sister and I don't look alike, but we get these, too. We ran through these with our twins. So one we always get - can you read each other's minds?

K DUFFIN: Do you know what Eliza is thinking, Sally?

S HELM: No, nope.


S HELM: All right. What else?

M DUFFIN: Do you feel each other's pain?



S HELM: Who's older, Marie?

M DUFFIN: I have two minutes on Karen, so I lord that over her.

E HELM: Two minutes of wisdom...

M DUFFIN: Right?

E HELM: ...Experience.

M DUFFIN: Yeah, she looks up to me.

E HELM: I find the exact same thing, Marie.

K DUFFIN: This kind of fascination that we experience every day - that fascination is what sparked the very first twin study all the way back in the 1800s. The researcher was a man named Francis Galton.

ALISON COOL: Galton was the one who kind of, like, harnessed that existing cultural fascination and turned it into this scientific proof of his theory of heredity.

K DUFFIN: This is Alison Cool. She is like the Russian nesting doll of twin study researchers. She is herself an identical twin who studies researchers who studied twins. She's also studied the history of twin studies. And she is an assistant professor of anthropology at the University of Colorado Boulder.

S HELM: Alison told us that Francis Galton was a gentleman of science in the late 1800s. He was actually Charles Darwin's cousin. And while cousin Charles had obsessed over finches, Galton became obsessed with twins. He was trying to understand why parents and their children - basically why relatives had such similar traits - in other words, heredity. And the most similar-seeming relatives that he could find were twins.

K DUFFIN: So Galton found a bunch of twins, had them answer a bunch of questions. The terms identical and fraternal had not yet been coined, but he knew that some twins were what he called similar, and others were dissimilar. So he asks for information about their appearance, their behavior. And based on what he hears back from these twins, he makes this argument.

COOL: The similar twins stay similar because they have this, like, inborn nature. And the dissimilar twins stay dissimilar because they have different natures.

S HELM: And in the paper that he publishes, he uses this phrasing that we now probably all recognize. He talks about nature and nurture.

K DUFFIN: OK, stepping back for a sec. Studying heredity like this, or what would become known as genetics - it can bring you to this crossroad. Like, for example, say a researcher discovers that genes play a role in some particular disease. Well, you could take that information and say, that's great; that helps us figure out how to spend money to better research or treat this disease. But if you assume that genes are fate, then you could take that same information and go down a very dark path. Instead of trying to treat that disease, someone could say, let's breed that disease. Let's breed that gene out of humanity.

Francis Galton went down that dark path.

COOL: Eugenics literally means, I think, the science of being well-born, but also a social movement about making sure that only the fittest are able to reproduce.

S HELM: These ideas led to forced sterilizations, even outright murder. Eugenics took hold in many different countries, including the U.S.

COOL: All of the kind of, like, biological research in that era had that flavor.

S HELM: The flavor of?

COOL: The flavor of, like, weird racism.

K DUFFIN: These ideas and the twin studies that Galton used to reinforce them showed up again when the eugenics movement was taking its most terrifying form.

COOL: There's a direct trajectory from Galton to the - like, the Nazi twin experiment.

S HELM: Nazi researcher Josef Mengele performed grisly human experiments, including many experiments on twins. He infected them with typhus. He did surgery without anesthesia. He used twins to study heredity in hopes of propping up the Nazi program of so-called racial hygiene.

K DUFFIN: So understandably, for decades after the war, attention shifted away from heredity, and people started focusing more on nurture, on the effects of the environment.

S HELM: But then about 1980, this crazy story crops up.


JOHNNY CARSON: I'm looking forward to meeting these next two gentlemen.

S HELM: It was a big pop culture moment, and it ends up sort of reinvigorating twin studies and pushing the pendulum back towards a focus on genes.

K DUFFIN: There was this highly publicized case...


CARSON: They are identical twin brothers who were separated when they were three weeks old.

K DUFFIN: ...Of some identical twins who had been separated as babies and then reunited as adults. And they had all of these crazy similarities. For one thing, they were both named Jim.


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

S HELM: Now, obviously, there is no gene for being named Jim. That is just a coincidence. But a researcher at the University of Minnesota, Thomas Bouchard, he hears about the Jims and thinks, these are the research subjects I have been looking for, identical twins raised apart.

K DUFFIN: And to understand why his study ended up becoming so important, you have to first understand how normal twin studies work, studies of twins raised together.

S HELM: So the reason why twins are so amazing for science is because you have this natural comparison. You've got identical twins, who inherit identical genes, and fraternal twins, who share, on average, 50% of their genes, like normal siblings. And, actually, we are not just going to explain all this. Back to our sisters in the studio.

K DUFFIN: We are here to conduct a micro twin study.

S HELM: Mini scientific study.

M DUFFIN: I'm in.

E HELM: I believe in us.

K DUFFIN: In the studio - in the collective studios today, we have a set of identical twins and a set of fraternal twins.

S HELM: And that is what you need for a twin study.

K DUFFIN: That's right.

S HELM: Of course, you need many more pairs of twins than this.

K DUFFIN: So what twin researchers will do is they will pick a trait or behavior, something that they want to study.

S HELM: If, on the whole, identical twins are more similar to each other than fraternal twins are, then that suggests that genes or DNA play a role in why some people have that trait. For our mini study, we picked lateness as the trait to look at. Are you a late person or an on-time person? And my fraternal twin sister said...

E HELM: Sally is the late one, and I am the on-time one.

S HELM: So we are different on that trait.

K DUFFIN: And then I asked my identical twin, Marie. How are you on lateness?

M DUFFIN: I - it'd had to have been - be a very intentional act to become a more on-time person, so...

K DUFFIN: I would say that I'm the same. Like, I have been working very hard to not be a late person.

In our micro twin study, the fraternals were different, and the identicals were similar.

S HELM: So I think we found a fairly strong genetic link here today.

M DUFFIN: No more studies needed.

S HELM: So back to the Jim twins and that researcher, Thomas Bouchard in Minnesota, who wanted to study twins raised apart.


JIM SPRINGER: We both named our first sons the same, James Allan (ph).

S HELM: So Bouchard sees twins like the Jims, and he thinks, all right, in many cases, these twins had pretty different environments growing up. So if these identical twins raised separately still end up similar, that is super convincing evidence for the influence of genes.


JIM LEWIS: Both of us like light beer and...


CARSON: How about soft drinks?

LEWIS: Pepsi.


CARSON: Both of you?

K DUFFIN: So Bouchard and his team find more than a hundred twins raised apart to study. And they find DNA does seem to play an important role, even for things like how religious the twins are, how liberal, whether they were attention-seekers, whether they were assertive.

S HELM: Many twin studies - studies of twins raised together, twins raised apart - have found results like this. We looked at one study that basically analyzed thousands of twin studies and summarized what those studies suggest about how important genes are, how much they seem to contribute to the differences that we see between people.

K DUFFIN: DNA seems to be very important on the health of, very specifically, your eyes, ears, nose, throat, skin and bones. For psychiatric health, it's a little bit less. But still, genes seem to play a role in about half the variability. And even in things that seem sort of intangible, like social values, genes do seem to play a role. Like, there was this one big twin study that examined rule-breaking.

LAURA BAKER: We kind of loosely call it the USC twin project or twin study.

K DUFFIN: This is Laura Baker. She is a behavioral genetics expert and a professor of psychology at USC.

BAKER: A longer name might be the risk factors for antisocial behavior twin study.

S HELM: I was wondering if it had sort of, like, an official name.

BAKER: We kind of have a name in our publications, and then we have the name in our correspondence because we didn't want to freak people out.

S HELM: Yeah, yeah, totally. I could see. Would you like your child to be a part of the risk factors for antisocial behavior study?

BAKER: We want to kind of predict whether your kids are going to become incarcerated or not. Would you like to participate?

S HELM: One person who got a letter inviting her kids to participate was my mom.

K DUFFIN: And she said, I have some psychopaths in the house. Would you like to study them?


S HELM: OK, Laura Baker was, indeed, the lead researcher on the study that my sister and I were a part of, the study with the electrode cap from the beginning.


S HELM: After the break, what that study found and what modern twin studies have taught us about nature and nurture.


S HELM: What Laura's study, the study that I was in, was looking for was something that they called antisocial behavior. So on the far end, that's like psychopath behavior. On the sort of lower end, they were talking about things like aggression, rule-breaking, lying.

BAKER: I had a son who was exactly your age at the time we started the study. So I just thought of all the things that he and his friends might've done.


S HELM: The researchers went looking for all manner of rule-breaking behavior, and that's why they asked all those questions about, like, the bricks, broken bottles, bats, about pinching. I remember saying no to so many of those questions and feeling like I had just never done anything rebellious or cool.

But, OK, the electrode cap in that laboratory - that was used to measure our responsiveness to scary or sad situations.

BAKER: We'd have hooked you up to a variety of different devices to be able to measure your - basically, the part of your nervous system that's involved in kind of fight or flight.

S HELM: One theory is that psychopaths or antisocial individuals are less responsive to stimuli and to emotions like fear and sadness. That is why they had us watch those sad, sad movies - to see how much we reacted.


SCHRODER: (As T.J.) Wake up.

BAKER: It was from "The Champ."

S HELM: Oh, yeah. Yeah, yeah. I knew it was a fighty something or other.

BAKER: Yeah. And little Ricky Schroder - his dad was a fighter. And his dad dies in a boxing match. And he's sobbing and sobbing. It's been used in so many studies because that is, like, one of the saddest film clips of all time.

S HELM: Basically, if you don't cry at "The Champ," there is something wrong with you.

K DUFFIN: Sally, did you cry?

S HELM: I don't think I cried.

K DUFFIN: OK, confirmed (laughter).

S HELM: I think I just - but I think I was sad. I don't know. OK, anyway, Laura's study did find that boys who exhibited psychopathic traits - so kids who were more likely to be manipulative or who lied more - their brains did react less to stuff like that movie scene. And more broadly, the study found that genes do seem to play a large role in antisocial behaviors.

K DUFFIN: And this finding is kind of troubling in a way.

BAKER: It's kind of like, what are we going to do with that? You know, is it going to impact our justice system? Is there a possibility that people will get mistreated because they're identified as having a genetic predisposition?

K DUFFIN: And that would be a moral mistake, but also a scientific one because twin studies have helped us move way beyond the idea that genes are fate.

COOL: Pretty much anything that seem to have some kind of genetic influence is also interacting with other genes and with the environment. Like, there's this interplay happening all the time.

K DUFFIN: Alison Cool again, the researcher of twin researchers. And almost everyone we spoke with told us that the question, is it nature or nurture, is totally outdated. It is obviously both.

COOL: You can't really separate out nature from nurture in this really clean way because the whole thing is this, like, dynamic, complex system of interactions.

S HELM: There is a lot of work being done right now to tease out and understand these complex interactions. Like, one question, why is it that sometimes people have a gene but it doesn't get expressed? There's a whole field that studies that called epigenetics. And also, the whole genome has been mapped. So we don't necessarily need twins to help us see genes.

K DUFFIN: And that led us to wonder, very reluctantly, does science even need twins anymore? We asked Laura about this.

S HELM: So are twin studies still useful in a world where we can look at all the genes?

BAKER: Yeah, that's an interesting question. Turns out twin studies are still really useful because you can - you can do things especially to study environment.

K DUFFIN: Because identical twins, for all of our identical genomes, we can still live extremely different lives, like my identical twin sister and I. I am right-handed; she is left. She's queer; I'm straight. Before radio, I spent years in corporate America; she is an artist. By looking at twins like us, researchers can start to zoom in on what aspects of the environment matter most to how differently people turn out.

S HELM: Back in Galton's day, genes were this invisible thing that twins allowed us to see, the mystery twins could help solve. But there's a lot of invisible stuff in the environment, too.

COOL: We think about genes a lot, but what we really know nothing about is the environment. Like, it's the error term. It's - anything that's not genetic is the environment. But, like, what is the environment?

K DUFFIN: Right. We're so busy trying to sequence the genome. But, like, we need to - if this is the real question, nature - what's nature, what's nurture - we have to sequence the environment, too. Like, don't worry, science. Twins - we're on the job.

S HELM: We got this.


S HELM: If you are a twin, send us an email - Singletons are also welcome to write. You can also find us on Facebook, Twitter or Instagram. We are @planetmoney. And while you are there, why not send your favorite PLANET MONEY story to a friend? Give us a review, good or bad. We are here for it all.

K DUFFIN: Our show today was produced by Darian Woods and Nick Fountain. Our supervising producer is Alex Goldmark. Bryant Urstadt edits the show.

S HELM: Special thanks to Nancy Segal and Jay Joseph. Also, our NPR colleagues at Hidden Brain did a great episode about twin studies. Check that out. They do great work.

K DUFFIN: And our friends over at Radiolab just finished an amazing series on intelligence, which goes deep on some of these questions about genetic research that we touched on here. If you liked this episode, you will like that.

We cannot sign off today without saying this is our very last week with the great Sally Helm, and we are so sad. Sally, you have been such a star here at PLANET MONEY, and we are going to miss you so much.

S HELM: I am going to miss you guys. And I'm going to be around. You'll hear me on the airwaves soon.

K DUFFIN: Good, we insist on it. I am Karen Duffin.

S HELM: And I'm Sally Helm. This is NPR. Thanks for listening.


K DUFFIN: Marie, would you like to tell the story of when we switched places?

M DUFFIN: Yeah, it worked out really well for me, but not so much for Karen because - what was it, sixth grade?

K DUFFIN: Yes, we had a substitute.

M DUFFIN: And I was a little bit of a troublemaker, whereas Karen was the angel she is today. And, of course, during the course of the class, I got in trouble, so I got my name on the board with check marks, except my name that day was Karen. So Karen got check marks.

S HELM: And check marks were bad.

M DUFFIN: Oh, yeah. That meant detention.

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