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11:00 am
Mon January 2, 2012

Twins Data Reshaping Nature Versus Nurture Debate

Originally published on Mon January 2, 2012 1:42 pm

Almost 150 years ago, English scientist Francis Galton coined the phrase "nature versus nurture" — and proposed that research on twins could resolve the debate.

Genetics have long seemed to weigh heavily in favor of the role of nature in shaping the people we become. But even identical twins are different to varying degrees, and some researchers believe those differences suggest a third influence at work, called epigenetics.

Peter Miller of National Geographic Magazine wrote the magazine's January cover story, "A Thing or Two About Twins." Miller explains how scientists are expanding the field of epigenetics with research on twins.

Copyright 2013 NPR. To see more, visit http://www.npr.org/.

Transcript

NEAL CONAN, HOST:

This is TALK OF THE NATION. I'm Neal Conan in Washington. Almost 150 years ago, an English scientist named Francis Galton coined the phrase nature versus nurture and in the next breath proposed that research on twins could help resolve that debate.

Genetics seem to weigh heavily in favor of nature. Almost everybody knows identical twins who have the same hobbies, career and preferences - right down to snack foods. But even identical twins are different to a greater or lesser degree, and some researchers now believe that those differences suggest there's something besides nature and nurture at work.

If you're an identical twin, how are you different from your twin? Give us a call, 800-989-8255. Email us, talk@npr.org. You can also join the conversation on our website. That's at npr.org. Click on TALK OF THE NATION.

Later in the program, Ann Selzer with the Des Moines Register's Iowa poll, but first Peter Miller joins us here in Washington, in Studio 3A. He's a senior editor at National Geographic, and his article "A Thing or Two About Twins" is the cover story for this month's issue of the magazine. Nice to have you with us today.

PETER MILLER: Thank you very much, nice to be here.

CONAN: And all the evidence about twins suggests the power of genetics. We hear all the time about these twins who grew up apart, for various reasons, and it turns out, well, they are so strikingly similar, it seems that their choices are so heavily weighted by nature, if you will, as opposed to nurture.

MILLER: Well, I think it's true that everybody who has twins in their family maybe knows that there's a mix, that because they share the same DNA, they look alike, they might have some of the same characteristics, some of the same personality traits, but they're different.

They're different people, and scientists are particularly interested in the differences that can tell us, the rest of us, why we are the way we are, for example why one twin will develop cancer or why one twin will have a reading dis - or why both twins will have reading disabilities.

Those kinds of differences are special because twins enable them to eliminate the DNA factor when they're trying to figure out well, what it is that causes these things because they share same DNA. It must be something else.

CONAN: It must be something else, and the obvious answer is, well, how they're raised, it's the environment.

MILLER: Well, that's probably the answer that it is the environment. But what is the environment? I mean, the environment includes which one of them got more nutrition in the womb, for example. I mean, the environment starts very early.

The other reason why scientists really like to include twins is that in general, identical twins, which is the kind of twins that we're talking about here rather than fraternal twins, who are more like brother and sister or more like ordinary siblings, identical twins, who share the same DNA, can help them to teach them about the kinds of traits that we're most interested in.

CONAN: Well, for example, the article you write talks about a pair of twin boys, one of whom - both of them have various degrees of autism, but one is extremely autistic, and the other is marginal.

MILLER: Yeah, remarkable case, two lovely boys, both of them extremely bright, very sweet, but one is - has developed a severe, severe case of autism. And the other is the kind of genius that can't devour enough books and has a laser-like intensity. And so you wonder how could they be so different, how could...

CONAN: I loved the story about him, he was interested in Greek mythology, developed the wings of Icarus to try them out for himself, broke his ankle and then has since studied everything there is to know about anatomy.

MILLER: Exactly, and it's even more fascinating, because autism is a - autism developmental syndrome is thought to be genetically - or thought to be inherited. In other words, there's a very strong likelihood that if one twin has autism that the other will.

CONAN: And so is there an explanation as to why one of the twin brothers is much more affected by the autism than the other?

MILLER: No, not yet. There are - there have been studies that have indicated that conditions in the womb or in the first year of life may have an influence on the development of autism, but what's really interesting is, well, what's the mechanism by which these factors can cause changes in two identical twins.

And that's where scientists are starting to talk about another set, another system of inheritance apart from the DNA sequences. You know, we all share the same code. I mean, we all have DNA that can't be changed, but there's another system called epigenetics which can be changed, and the epigenetic system works to turn genes on and off in a way that can change the traits.

So it's quite possible, and scientists are looking at, well, perhaps autism is one of those traits, one of those diseases, that is caused by epigenetic changes.

CONAN: And these are not likely to be simple answers or one switch here, one switch there. It's likely to be more complicated than that. The analogy you used in the article, which I thought was interesting, was twins are born with the same keyboard, like a piano keyboard. Epigenetics is what you're describing there, enables slightly different tunes to be played.

MILLER: Exactly. We're born with a keyboard. We can't change the keyboard. But there are chemical reactions that can change the volume of the keys or even turn keys off and on. And the keys that are turned off might be ones that we need in order to develop in a proper way. So the hope is that as scientists understand more clearly which genes are responsible for which traits, then we will be able to develop therapies, even chemical therapies, that might affect whether they're turned on or off or not, in other words, epigenetic drugs that can affect things like autism, things like cancer.

CONAN: We want to hear from the twins, identical twins in our audience about how you are different from your twin. Give us a call, 800-989-8255. Email us, talk@npr.org. We'll start with Leslie(ph), Leslie with us from Macomb in Illinois.

LESLIE: Right, I'm 66 years old, I have an identical twin sister named Linda, and we are mirror twins, which means I'm left-handed, and she's right-handed. So that's a little unusual. In terms of being different, we have similar personality types, but we married men who are very different and sort of diverged at that point in terms of religion, politics and so forth, one of us being conservative the other being liberal.

But we still remain very, very close and very close friends.

CONAN: That's interesting, so the divergence you trace to marriage.

LESLIE: Yes.

CONAN: Clearly there might have been some other divergence beforehand that might have led you to be attracted to different kinds of men.

LESLIE: That's true, but I really couldn't put my finger on what it would be.

CONAN: It's interesting, Peter Miller, the divergence chart that's in the National Geographic article shows that identical twins are very close early on in their years, and then as differences mount up over time, they get further and further apart.

MILLER: Yes, and in fact there was a recent study that did link these differences to the epigenetic system I was talking about a minute ago, that twins, when they're say three years old, have similar patterns of gene regulation, and as they experience different things in life, as they have different levels of nutrition or, say, are exposed to smoking or not smoking or the sun, they change.

And their epigenetic profiles change as they become older. So by the time you're 66, although your DNA is the same, the system that manages your DNA is quite different.

CONAN: Leslie, thanks very much for the call.

LESLIE: Thank you for talking to me. I really like your show.

CONAN: Thank you, we wish you and Linda a happy new year.

LESLIE: Thank you very much.

CONAN: Email from Jeff(ph), and excuse me for cutting her off, Jeff in Brighton, Oregon: I am gay while my identical twin brother is straight. I was born first. We were both large, both seven pounds. So it's believed I was in the lower birth position for most of the third trimester.

There is a womb development theory that I think that was either exposed to or denied certain chemicals that I was either exposed to or denied, certain chemicals that led to my sexual development. Thank you for another great topic, very interesting.

I wonder, there's an interesting picture in the article that shows identical twins, one 10 pounds heavier than the other, they're infants.

MILLER: Yes, well, sexual orientation is a very interesting difference in twins. Twins are - all twins are no more likely to be gay, for example, than the average population. But when one twin is gay, it - the likelihood that the other will be gay is about 25 to percent, not that great. I mean, you...

CONAN: Higher than in the normal...

MILLER: Yeah, higher than random.

CONAN: Higher than random but less than certain.

MILLER: But less than you would say would be a very strong influence.

CONAN: And has anybody figured out why?

MILLER: I don't think so, no.

CONAN: It is one of those anomalies. You can see different personality traits, different mannerisms, different speech, you know, mannerisms. But something as fundamental as that you'd think would be shared.

MILLER: You would think so.

CONAN: You would think so, yet it's not. Here's an email, this we have from Leeza(ph), Liza(ph), excuse me, in Flagstaff: I know two men who were identical twins. They went into the exact same profession. Both married, had families, but one of the twins started drinking, became a total alcoholic, lost his family and profession and is now homeless. The other twin is a successful professional and family man.

Alcoholism is strongly genetic, yet we know that predisposition does not mean determinism.

MILLER: No, it's not, exactly, that's a very good example that even though the like - say there's a 60 percent likelihood that you will have alcoholism if - genetically, there's still 40 percent that says you won't. And so it's up to you...

CONAN: And it could be epigenetic, too, and up to you, as well.

MILLER: Yeah.

CONAN: We're talking about twins. If you're an identical twin, how are you and your twin different? Give us a call, 800-989-8255. Email us, talk@npr.org. More in just a minute. Our guest is Peter Miller, a senior editor at National Geographic magazine. "A Thing or Two About Twins" is the January 2012 cover story on National Geographic. Stay with us. I'm Neal Conan. It's the TALK OF THE NATION from NPR News.

(SOUNDBITE OF MUSIC)

CONAN: This is TALK OF THE NATION from NPR News. I'm Neal Conan. When twins descent on the small town of Twinsburg, Ohio, every year for the Twins Day Festival, they catch up with old friends over picnics, participate in look-alike contests, even crown a royal court.

But as the cover story in this week's National Geographic magazine explains, the festival is more than just fun and games these days: It's a valuable research opportunity for scientists who hope to quantify the influence of nurturing versus that of nature.

Identical twins can help explain the influence of environmental factors; the experiences of fraternal twins can help illuminate heredity's role. Identical twins, we'd like to hear from you today. How are you different from your twin? 800-989-8255. Email talk@npr.org. You can also join the conversation at our website. Go to npr.org. Click on TALK OF THE NATION.

Peter Miller is our guest, he's the author of that National Geographic cover story, and let's see if we can go next to - this is Adam(ph), Adam with us from Toledo.

ADAM: Hi, how's it going?

CONAN: Good, thanks.

ADAM: Happy new year's.

CONAN: Happy new year to you, too.

ADAM: Great, so when my brother and I - I'm an identical twin, I was born outside of Charlotte, North Carolina - when my brother and I were in elementary school, we had different teachers, and we had an experience basically where I was pulled out of the IQ test that we took in second grade as students.

And I just violated the rules, apparently. I guess they weren't properly explained to me. But anyway, so my brother was placed in the academically gifted group at our school, and I wasn't. And so we kind of diverged from that point. I assumed that I was, you know, kind of the dumb twin, and he was the smart one.

And so I found my niche as being in the gregarious, social person, and that kind of led through our entire lives. I didn't find out until that - that that was an IQ test until I was 18, and, you know, my academics were pretty good, but my brother developed a much more analytical way of viewing the world.

He went off to study meteorology, and I am a political activist now. So it's pretty interesting.

CONAN: That is interesting because, Peter Miller, reading your article, IQ is one of the most heritable traits.

MILLER: Yeah, that's a fascinating story. It's - I don't know why you might have been pulled out of the IQ test, but I expected you to say that you took the test for your brother, or your brother took it for you, and you ended up with the same score because the IQ test does have a very strong correlation between twins.

They tend to have similar potential in terms of IQ, but what your story also demonstrates is that things that happen to twins separately can have a very strong impact on how they then differ.

I mean, you say your brother went to a different educational program and then was sort of led in a different direction, and it's a very good example of how nature and nurture kind of do a dance where he was encouraged to pursue these certain abilities, and then these abilities probably got better, and then he sought out more experiences that were like that, almost like, you know, like a tall person might play basketball and then be rewarded for being good at basketball and et cetera.

So it's a very interesting story that you tell about, you know, how nurture sort of sent you two in different directions, even though nature had given you the same sort of potentials.

ADAM: And I wanted to further comment, just really quickly, I think it's interesting, and I'm sure that a lot of your identical twins that are listening now can definitely relate to this, but there seems to be a societal pressure for twins to, you know, be different from each other.

And my brother and I are, you know, thick as thieves. We're still incredibly close, and interestingly enough, even though my brother went to study hard sciences, he's actually now a journalist. So he's working in a more gregarious, you know, social field than I think he originally intended to. And...

CONAN: So that divergence chart might find you guys coming closer together in some respects.

ADAM: That's right, and I went to grad school to study political science. So I kind of went into this - you know, even though it's not a hard science, it's very - it's deeply analytical. So it's kind of interesting. But I think maybe that's something to discuss further about social pressures on twins to, you know, kind of branch out and be their own people as opposed to just I guess allowing themselves to be as similar as a lot of times they are.

CONAN: Well, that's interesting, and Adam, thanks very much for the phone call, there are so many pictures in - that accompany this article of twins who prefer to dress alike, indeed the two truck-driving twins who have the exact same beard and look precisely the same, of course they're identical twins, but nevertheless, people who prefer to be the mirror of their sibling.

MILLER: Yeah, when you go to the festival in Ohio, it's really overwhelming to see - to be surrounded by so many people who look exactly like each other. And after a while, you really begin to feel like you were the odd one, you know, that you're the singleton in a sea of twin-world.

And as you said, the twins that like being twins, really this is a great opportunity for them to celebrate.

CONAN: Let's see if we can get another caller in. This is Harvey(ph), Harvey calling us from New Haven.

HARVEY: Yes, hi, I'm a doctor at Yale University. I happen to have a set of identical twins, and I also happen to be a person who studies this. So I think one other part of the story needs to be added, and that is identical twins are different depending on when they split.

There are twins that are formed 20 percent of the time at the two-cell stage, and those two cells end up forming two separate placentas, and those twins are the most identical, identical twins, although ironically because they're two separate placentas, the OBs will often sometimes mistakenly say no, your twins are not identical.

Eighty percent of the time, the twins are actually mirror image, and Linda brought this up when she called. And those twins are really quite different. Our daughters, for example, are mirror-image identical twins, and although they look similar, they have very different personalities, and part of the reason we now understand is that in the first five days, what's happening is that different genes are turning off on different parts of the embryo.

And when they split on Day Five, which is probably considered a mistake, actually, one of the babies becomes - comes from the right side of the embryo, and the other comes from the left side, and those two sides actually have different genes that are turned off, and so they can never be the same once that happens.

CONAN: So that embryonic history - of course OBs, obstetricians you're talking about - and I wonder, Peter Miller, is this something you encountered in your research?

MILLER: Yeah, yeah, identical twins can never be completely identical just because of that, because they start being different from, you know, the moment that the egg splits.

CONAN: Harvey, let me take advantage of your expertise. There's an email we have from Tim(ph) in Minneapolis, who writes: The development of epigenetics has changed discussion of inherited traits forever. Epigenetics is the expression of proteins unique to each individual. Any discussion of genetics without incorporation epigenetics is flawed.

HARVEY: I think that's true, and we're finding that both in studies of twins and also just in assistive reproductive technology, in the sense of what we do in the laboratory before we put an embryo back may be slightly different than what happens in nature.

So I think we've taken this for granted, but the identical twin story is a nice example of how splitting an embryo, Day Five, you end up with really two distinct people that have quite a few differences. Even though they have the same DNA, the DNA is being expressed differently in each of those individuals.

CONAN: Interesting, Harvey, thanks very much.

HARVEY: My pleasure.

CONAN: Appreciate it. Important to point out, as you do in the article, though, Peter Miller, this is all - well, I hate to use the word embryonic. It's early days yet.

MILLER: It really is. There's so much more to learn about the way that the epigenetic system works. It's not totally new because cancer researchers have been studying this intensively for perhaps 20 years, but what's really advanced the science is that computers and other technology have enabled us now to understand and to study the genome at a microscopic - at such a detailed level that we can now actually sort of hope to put our finger on what's - how the epigenetic processes are actually working on the DNA.

CONAN: Let's go next to Kathleen(ph), Kathleen with us from Oakland.

KATHLEEN: Hi, I have an identical twin. We are both 57. And we are extremely different in both personality and in medical history, or I should say, medical background. But we are - I mean, physically very identical and, you know, have the similar gestures, exact same voice, that kind of thing that made it very difficult for people to tell us apart for most of our lives - not anymore. But the personality differences were from day one, and there seems to be almost all the evidence points to the fact that we were the very early splitting twins, the 20 percent.

My sister has very severe autoimmune disorders and I have none. And, of course, anyone who's - who knows the research on this will know that a lot of it has to do with, you know, something that only affects identical twin girls, which is the deactivation of X chromosomes, which, of course, starts in utero - starts very early on. And of the diseases that my sister has, for instance, is Sjogren's disease, and I don't have Sjogren's disease, or any other type of autoimmune disorder.

CONAN: And the kinds of differences Kathleen is talking about, particularly those medical differences, Peter Miller, those seems to be the ones that scientists are most interested in? The others are, well, interesting, but if we could figure some of these out, we could have a direct impact on, not just on those people - all of us.

MILLER: Yeah, I think - I was just thinking that I'm sure that the researchers in these fields would love to be able to study your case, and if there were other people like you, to draw some conclusions from it. Because as you mentioned, in many ways, it sounded like you two share a lot of traits - your physical appearance and so on.

KATHLEEN: Yes. And not only that, but things like - I'm sure we - I think someone told us once that we were within one point of each other on IQ...

CONAN: Mm-hmm.

KATHLEEN: ...best, which as I know is quite common and doesn't surprise me a bit. But our entire approach to life, led by a personality trait, is completely different and has been, I would say, since we were born.

CONAN: And have you been in any twin studies?

KATHLEEN: We are in the northern - yes, we're in the Northern California twin registry and have been for about 30, 35 years. They don't seem to do too much. I'm not sure why not. I mean, you know, like three to four years, they ask us about being in something.

CONAN: Well, maybe a vacation in Twinsburg.

KATHLEEN: What?

(SOUNDBITE OF LAUGHTER)

KATHLEEN: I really haven't concentrated on being a twin for most of my life. It's just not something that takes up a lot of my consciousness. And I would say that that is exactly the opposite with my twin.

CONAN: Interesting. Interesting. Kathleen, thanks very much for the call.

KATHLEEN: OK. Bye.

CONAN: We hope your sister does well.

KATHLEEN: OK. Bye.

CONAN: This tweet, we have from Lindsay's Moon(ph): Polar opposites - she still loves 'N Sync at the age of 22. I've got Led Zeppelin lyrics tattooed to my feet. And there are some profound differences indeed. We were talking about the article, "A Thing or Two About Twins." That's the cover story in the January 2012 issue of National Geographic. You're listening to TALK OF THE NATION from NPR News. Bryce is on the line. Bryce with us from San Jose.

BRYCE: Hi there. How are you doing?

CONAN: Good, thanks.

BRYCE: I was just, you know, clicking through the radio stations here and I stumbled upon your discussion about twins, and I happen to be an identical twin.

CONAN: How are you different from your twin?

BRYCE: Well, up until senior year of high school, we were pretty much physiologically and psychologically identical. I'd say within the psychological part of it, we complement each other. So to each other, we're different, almost like a yin yang. But to everyone else, we basically seem to be perfectly identical in mannerisms in gestures and, you know, speaking style - even, like, down the voice and body language. However, we went to different colleges, and, you know, I stayed on the West Coast and he went out to the East Coast.

And over the course for about six years, we really diverged. These days, when people meet us, I'd say only half of them guessed that we're twins right off the bat. And that's because, you know, I'm a little bit thicker, more broad shouldered, have filled out - I'm probably a good 25 to 30 pounds heavier than he is. My face has changed a lot. We're very, very different at this point. And on top of that, through our experiences, I say that we've changed psychologically as well. And it's been kind of interesting to see our pasts diverge. And I'm actually aware of how epigenetics play into that, and I guess that our story attests to how environmental conditions, habits, diets et cetera, et cetera can change a person.

CONAN: Yeah, it's interesting. The size, Peter Miller, so many twins seem to be so identical. Identical twins seem to be the same size. Diet doesn't seem to play into it, or they seem to have similar diets in any case.

MILLER: Well, that seems to be that weight is one of the traits that seems to be strongly heritable.

CONAN: Strongly heritable...

MILLER: Yeah.

CONAN: So Bryce sets an important - an interesting distinction there.

BRYCE: Yeah, definitely. I mean, we - like I said, in high school, we were, you know, pretty much spot on. We're still the same height, of course, but the weight has really changed a lot, you know? I went from being maybe two or three pounds heavier than him and just a little bit more muscular to being 25, 30 pounds heavier than him than - and filled out. I mean, I would say, I'm not overly - I have like a athletic build, maybe got a few pounds here and there, but he's almost gaunt, in my opinion.

CONAN: Hmm.

BRYCE: Yeah.

CONAN: That's interesting. Thanks very much for the call.

BRYCE: Yes. Have a good one. Thanks.

CONAN: We're talking about how scientists are studying identical and fraternal twins to study what makes us tick, a combination of nature, nurture and a relatively new field called epigenetics. We'll be back with more from our guest, Peter Miller, who's the author of the cover story in this week's issue of the National Geographic Magazine. It's titled "A Thing or Two About Twins." Stay with us. I'm Neal Conan. It's the TALK OF THE NATION from NPR News.

(SOUNDBITE OF MUSIC)

CONAN: In a few minutes, the latest from Iowa. We'll take a look at the Iowa poll published by the Des Moines Register Saturday night, but let's continue our conversation with Peter Miller, who wrote National Geographic's cover story for this month, "A Thing or Two About Twins." You can find a link to it at our website. Go to npr.org, click on TALK OF THE NATION.

And we've got some people still on the line and some emails coming in. This is from David in Fredericksburg, Virginia: Have there ever been any cases where parents had difficulty or inability to tell their identical twins apart?

MILLER: Oh, that's funny. I think - yeah, I think it happens. I spoke with one mother at Twinsburg. When their baby girls were three weeks old, she had to paint one toenail pink and the other toenail purple in order to make sure that she didn't feed one twice.

CONAN: Interesting. And a related question, this is from Mary in Minneapolis: Are twins better than average at telling twins apart?

MILLER: That's a very good question. No, I don't think that they have any better ability than that - than we do.

CONAN: ...more finely attuned than the rest of us, yeah.

MILLER: I don't think so.

CONAN: Let's go to Mandy, Mandy with us from Herkimer in New York.

MANDY: Hello.

CONAN: Hi, Mandy.

MANDY: How are you?

CONAN: I'm well. Thank you.

MANDY: Thanks for taking my call. I have a twin sister. I'm from upstate New York, and she lives on the West Coast. Growing up, we were extremely identical, physically, but we always had different talents. She - I love to brag about her - she was the quiet one, but she has an amazing public speaking skill, and my frustration right now, because it scares me to death.

(SOUNDBITE OF LAUGHTER)

MANDY: And, yeah, I was the outgoing one. So I could charm anybody in the room as long as we were, you know, having a conversation together and you didn't put me behind a podium. She has rhythm. I can't dance a beat - to this day. And she could play snare drums. She was the best in our state. She was the one who didn't want to go away to college, and I did. I went out of state. She stayed close to home, and yet somehow I'm five minutes from my parents' house, and she's on the West Coast. So it's really interesting the way our paths ended up diverging.

And we were compared, so much, as kids that we really tried to be individuals. You know, we love being twins. She's my best friend. Thank God for email and texting and the like. But I'll visit her out in Oregon, and we feel like a freak show, you know? We feel like we can't eat our meal at a restaurant without being stared at. We can't walk on the street without people slowing down because our pace and our steps are the same, you know? And we don't try to look different, because we're not around each other to compare. So I think sometimes, now, we look more alike than we did before because we don't - we're not, you know, saying, hey, what are you wearing? I don't want to look like you today. So...

CONAN: So you're not consciously trying to assert your individuality, yeah.

MANDY: Exactly, exactly. And then I ended up being a scientist, and she is a social worker. So I'm the more analytical-minded, and she has more of a, you know, a language - linguistic side. So - but, you know, we're extremely close, just not geographically.

CONAN: Were you one of those sets of twins that developed your own secret language?

MANDY: No, but we do play a game with each other. She'll call me or I'll call her, and we literally say, guess what I'm doing. And within, like, one or two guesses, she can guess exactly what I'm doing. And it could be something brand-new that I've never done before. So we have an intuition.

CONAN: That's interesting. There's a - the picture, and I'm blanking on the name of the tennis players in the article, the twins who play...

MILLER: The Bryans.

CONAN: ...yes, who play doubles tennis. And, of course, they're accused by their opponents of using telepathy to defeat them. Mandy, thanks very much. And please tell your sister happy new year for us.

MANDY: Thank you very much.

CONAN: Appreciate the phone call. As these studies go ahead, where do - the researchers you talk to, where do they think this goes next?

MILLER: Well, there's a lot of basic work that needs to be done at the molecular level to understand the mechanism in which epigenetic processes are affecting which genes. And we still really don't know what causes so many of the diseases that we have. Only a handful of diseases are caused by problems with single genes, and those have pretty much been identified. But when you get into the more complex problems that people - the kind of diseases that we suffer, the solutions are going to be complex.

CONAN: Interesting, our last caller referred to expressions of individuality. An email from Janine in San Mateo: I wanted to say, on the subject of nature versus nurture, one thing non-twins may not take into account is the identity issue. Some of the differences may not be a matter of nurture, but instead of a twin, whether consciously or not trying to assert an individual, unique identity instead of constantly being lumped together with his/her twin. I believe this is partly the case with my twin and I.

We were more alike when younger and completely different today. We hated being called the twins all the time and being mistaken for each other by pretty much everyone outside our family. We wanted to be our own people. The gay, non-gay twins are an interesting case and may well very well be an issue of identity assertion that caused that. Well, that's on the theory that being gay or not is an identity assertion. But that's questionable. But other traits...

MILLER: I've heard that a lot from twins that only twins can understand the kind of pressure that society exerts upon twins to kind of be - to play that role of being twins, you know. And I don't think any twin likes being called, you know, you're the twin or one of the twins.

CONAN: Well, let's get the twins in on this.

MILLER: Yeah.

CONAN: And finally, an email - this is from Beth. Any studies on identical triplets?

MILLER: Yeah. I don't know. I don't know about identical triplets.

CONAN: There can't be so many of them as there are twins, obviously, an order of magnitude, at least...

MILLER: Studying them would be very difficult, exactly.

CONAN: Peter Miller, thanks very much for your time today. Fascinating article.

MILLER: Thank you.

CONAN: Peter Miller's article in the cover story of this week's National Geographic is called "A Thing or Two About Twins." You can go to npr.org, click on TALK OF THE NATION to find a link to it. He joined us here in Studio 3A. Transcript provided by NPR, Copyright NPR.

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