InsideOut: Interview with Nancy Segal

by Owen Lipstein, Amanda Schmidt November/December 2008

The nation’s preeminent twin researcher, Dr. Nancy Segal, is a professor of developmental psychology; director of the Twin Studies Center at California State University/ Fullerton; and author of Entwined Lives: Twins and What They Tell Us About Human Behavior (Plume, 1999), and Indivisible by Two: Lives of Extraordinary Twins (Harvard University Press, 2005). In this interview, she tells us why studying identical twins separated at birth offers us clues into ourselves—from our choice of occupation to the way we hold our beer.

InsideOut: Let’s start with you. You have a twin.

Nancy Segal: I have a fraternal twin sister. She and I were so different from a very early age [that] as a child, I used to think: Here we are, same parents, same experiences. Why are we so different? It was this question that has guided my work ever since.

IO: The implications of twin research are very provocative, yet a little scary in the sense that they make us think our identities might be hardwired.

NS: I don’t like to use the word “hardwired” because human beings are very flexible, and we like to think of genes acting in a probabilistic fashion, not a deterministic one—even though genes do explain a lot more of the variation among people than what would have been thought, particularly in some unusual behaviors. We still have a whole range of choices that we make freely.

IO: Can you tell us about the case study, from your book Indivisible by Two, of the twins who both became firemen?

NS: Well, reared-apart twins, particularly reared-apart identical twins, are a marvelous test of the extent to which genes and environment influence our behavior. The firemen twins exemplify that. These are two guys, raised apart in different towns in New Jersey, never knew they were twins, met by accident, and showed a whole range of unusual similarities, from the ordinary ones like intelligence, but also to things like wearing big belt buckles.

IO: Or the way they hold their Budweiser.

NS: Yeah, the Bud. I think that what these twins teach us is that even some of our unusual, quirky habits that we think we just picked up randomly, or even [from] copying somebody—really do have a genetic basis.

IO: You talk about what you call the variations on common themes, and refer to the twins of whom one is transgendered.

NS: I think of them as variations on a common theme because most of their personality traits, interests, [and] mannerisms were so similar. I expected much bigger differences between them, given this very fundamental difference of gender identity—to the point that one actually changed gender. When I met the second twin, in many ways it was like visiting the first one, except for this one fundamental difference.
I get a lot of calls from parents who have very young children—sometimes twins, sometimes not—who show these tendencies to want to be another gender. Some of these parents worry that something they did to the child caused this, but it’s comforting when I tell them that something happened to the child, most likely prior to the birth, that you are not responsible for, and if you treat this child differently than your other kids, it’s because this child is eliciting different treatment from you.

IO: Do we tend to be attracted to a certain type of person?

NS: Yes and no. I don’t think we’d be attracted to everybody, but I think there is a range, which also may be partly determined by our genes. If you think about the people you’ve been romantically attached to, they could all be pretty different, but still, they can reflect different facets of you.

IO: You’ve been researching twins for quite a long time. What’s happened in the last couple of years that you’ve found particularly notable?

NS: Actually, two things. We always say that identical twins are genetically identical, but that is really not quite the case. In fact, a paper that came out this year talked about what are called “copy number variations” or CNVs, where you can have actual genetic differences in identical twins. Th[is] small study actually linked some [CNVs] to one twin having Parkinson’s and the other one not.
Then I think a year or two ago, another study came out that talks about epigenetic differences. This has to do with certain genes turning on or turning off. I mentioned this before when I talked about the transgender twins, where one twin might have had certain genes turned on, one off. They still have the same genes, but one [set is] expressed and one is not.

IO: Twins are so intriguing. The rest of us rarely get to see ourselves that clearly.

NS: This is the fascinating thing about identical twins, because you can see yourself as another life, perhaps. One identical twin told me that she always felt like she was too fat. When she met her sister for the first time, the sister was actually heavier than she was. The sister looked terrific, so it changed her whole sense of her body image.
[There’s] no question that twins have held fascination with people for years, beginning in literature, ancient Roman times with plays, and Shakespeare. Shakespeare was a father of twins. When I did my dissertation at the University of Chicago, I would come down to breakfast in the dorm and people would ask me about my research. Now, no one asks you about your research at breakfast, but I would get asked because people love twins. People would love somebody who thinks like them, and who they can really feel safe and secure with, and this is what twins are naturally born into.
People always question their life choices, and it is very easy to assign events in life to a life choice, but I think that now people are starting to consider more the biological side of the question, as we’re understanding more what the links are between biology and behavior. This gives people a whole other source of explaining who they are and who their children are.

IO: So how do we navigate our own, or our children’s, choices?

NS: I would say you were born into this world with a lot of potentials, a lot of genetic potentials that have been passed down through the generations in your family, and you have many, many choices based on those predispositions. Now you are free to choose those.