Twins: Why we are who we are

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Twins have provided some of the most interesting research into human individuality. Both  identical twins and fraternal twins, raised apart or together, can help scientists evaluate what individual traits are heritable. 

Nancy Segal, director of the Twin Studies Center at California State University, Fullerton has spent her career researching twins. She recounts her first encounter in 1979 with Jim Lewis and Jim Springer, identical twins separated at birth and reunited at the age of 39.  

KCRW’s Jonathan Bastian talks with professor Nancy Segal and discusses why twins provide the perfect insight on why we are who we are. 

The following interview excerpts have been abbreviated and edited for clarity. 

KCRW: What got you interested in studying twins?

Nancy Segal: “It actually started in the womb for me because I'm a fraternal twin. I have a sister who looks and acts very differently than me. And as a child, I was so fascinated by that because here we were, two kids exactly the same age, raised by parents in the same home with so many experiences, and yet we went in very different directions. So I began to get very interested in psychology. 

I was always very interested in the differences between identical and fraternal twins in terms of their closeness, their intimacy and their social connection. I remember in school, I would see identical twins and they seem so close. And yet my sister and I, while we had a lot of family loyalty, we didn't have that sort of twin mystique that many people think about. And I was fascinated by that, and so that has informed most of the research I've done in subsequent years. 

So my doctoral dissertation was called “Cooperation, competition and altruism within twin sets: A reappraisal.” And what I did was I gave puzzles to young identical fraternal twins and watched them solve them. It was just wonderful because just by acting naturally, identical twins told us so much about how they interact. They were happy, they were sitting close to one another, they were successful. This was a joyful experience. But the fraternal twins, they were uninvolved, they were working individually, it just didn't have the same feeling to it. So that has informed my research in many, many different ways. 

When I got to the University of Minnesota, I worked on the study of twins raised apart with Dr. Bouchard. I was wondering if you would see that same thing mirrored in twins raised apart who newly reunited. That is, would identical twins meeting for the first time show a closer social connection than fraternal twins? That's exactly what I found. 

So how do we explain all this? I think it's that there are common genes [that] underlie their similar traits in intelligence, interest, personality, temperament. But it's not so much the similar traits. It's the perception of those traits, that if I see bits of myself mirrored in you, I'm going to feel a social attraction towards you.”

Tell us a bit more about this famous Minnesota study you were part of .

Segal: “That study began in 1979. When two twins in the state of Ohio, Jim Lewis and Jim Springer, met one another at the age of 39, they had a very long list of similarities. For example, both twins had married women named Linda and Betty. Both bit their fingernails. They both had part-time jobs as sheriffs. They both drove the same cars. They vacationed in the same area in Florida. They worked at McDonald's part-time. And they had severe headaches at the same time in adolescence. So they had a long list of similarities, which really captured the public and scientific imagination. 

Professor Bouchard, a professor at the University of Minnesota, decided to bring the Jim twins into the laboratory and put them through a very extensive assessment, and perhaps they might find other twins through that. I did not arrive in Minnesota until 1982, but I followed the work very closely. And the Jim twins then led to about 15 other pairs of twins. And by that time, the study was ongoing. I arrived as a post doctoral fellow and stayed there for nine years. 

It was just amazing to see these twins up close and to see their similarities. For example, we had a pair of twins we called the ‘giggle twins.’  These ‘giggle twins’ were raised in England and didn't meet until they were in their 30s and they both would laugh. I mean, they didn't laugh with other people, but with each other incessantly. They both had the same crooked piggy fingers. They both were very uninterested in politics and in reading. They both have the same number of children. And the list goes on and on. 

There was another pair that came to us from England. They arrived independently at the airport wearing seven rings, three bracelets and a watch. How do you explain this? The women liked glitter, they liked jewelry. They had long, slender hands and fingers that showed jewelry off just beautifully. And they could afford to buy these things. So you can see that when you start to decompose a trait into its components, you can start to understand where these genetic influences might come from. 

On the fraternal twins, we saw these kinds of unusual similarities to a much lower degree. I recall one pair where they each had 18 tattoos. But that's the only example I recall from a set of fraternal twins.”

Is there a benefit to keeping twins together and not separating them for their general health? 

Segal: “I am a great advocate for keeping twins together. The twins who reunited as adults are really angry, upset, frustrated, sad that an adoption agency somehow separated them. Maybe families wanted only one twin and for whatever reason, they were separated. Or sometimes the mother died in childbirth, so twins are given to different families. But whatever the reason, these twins are so happy to have met and so regretful of the years they lost together. 

There’s a big controversy in the twin world about whether you should separate twins in school. And my opinion is that you should not have any kind of policy, but this should be done on a case by case basis. If the twins do better together, keep them together. There's this sudden fear that they will never develop a separate identity, which is completely unfounded. And research shows that these twins do better when they're together. I tell teachers — put them at different tables so they can mix and mingle, but they have each other as security. So I really believe that schools have to rethink those policies very seriously.”

What are your thoughts on raising kids, and the empathy we show towards kids and the environments in which they grow up? 

Segal: “I think parenting is one of the most important responsibilities that mothers and fathers have. Parents should worry less, and pay more attention to the children. I tell parents that you don't bring your children up, they bring you up. The reason I say that is because parents respond in certain ways to certain children, and it's the parents’ responsibility to identify and then nurture whatever interests, talents and abilities that child shows. 

I also believe that parents who have two children should do the same thing and to treat them equally, meaning giving each child the exact same opportunities.”



  • Nancy Segal - Professor of psychology and director of the Twin Studies Center, California State University, Fullerton - @nlsegal


Andrea Brody