As a child, Nancy Segal, PhD, knew that she was a twin, but she didn't always feel like one. She and her sister, fraternal twins, didn't look anything alike and had few interests in common. Meanwhile, a set of identical twins whom she knew from school seemed incredibly similar and in tune with one another.
Segal was fascinated by the contrast. "I would think: My sister and I have the same parents, the same school, some of the same friends — why are we so different?"
Years later, in graduate school, Segal turned her attention to studying the topic that had fascinated her since childhood. In the 1980s and '90s, she worked on the landmark Minnesota Study of Twins Reared Apart. That research, which looked at identical twins who were reared apart and only met as adults, found remarkable similarities between those virtual strangers, suggesting that genes strongly influence aspects of our personality, intelligence and other traits that had long been thought to be mainly shaped by environment.
Today, Segal continues to study twins reared apart, as well as other "twin-like" pairs, such as look-alike but unrelated "doppelgangers," to see what they can tell us about how genes and environment affect both twins and everyone else.
Over the years, has your research backed up your childhood intuition about the differences between fraternal and identical twins?
Yes, it has. A lot of these studies of twins, particularly the Minnesota Study of Twins Reared Apart, have shown that genes are pervasive. They affect virtually every measured trait. And because fraternal twins share only half their genes on average, it makes sense that they can be very, very different, just like two ordinary children in a family can be quite different.
Just about every behavior we studied had a genetic component, and that includes some very surprising ones, like religiosity and social attitudes. Even in those, identical twins reared apart were more similar than fraternal twins reared together.
Media coverage of the Minnesota study has often focused on some wild coincidences, like the "Jim twins," who were identical twins separated at birth who didn't meet until age 39. Both were named Jim, had first wives named Linda and second wives named Betty. Both chain-smoked, drove Chevys, served as sheriff's deputies and vacationed on the same beach in Florida. These are fascinating coincidences, but are they scientifically interesting? What is the line between fun anecdote and scientific data?
I look at many of these similarities as science because I think they help us form hypotheses that we can go on to study. I'm not so quick to dismiss the anecdotes. You do see many more of these resemblances in identical twins than in fraternal twins, and, like I said, they let you formulate hypotheses that you might not have thought of otherwise.
For example, we studied these identical twins who were 32 when they met. They were both volunteer firemen, and they had similar full-time jobs: One installed chemical fire suppression systems, and the other installed burglar alarms. They both wore big belts, with big buckles and dangling keys, and always carried a knife.
That sounds like a lot of coincidence. But maybe something about these identical twins made them gravitate toward similar experiences and types of people in their environment. That's why I think that identical twins, even when raised separately, have more similar environments than other people — because environments are partly of our own making.
For example, who goes into firefighting? Maybe vigilant people, with an altruistic bent — you want to help people. You're also inclined toward risk-taking, and fires are dangerous. And then maybe being a firefighter makes you a little more vigilant, so you want to always carry a knife. I'm not saying this is right in all respects, but the story makes you entertain ideas you wouldn't have thought of otherwise.
So the twins' similar personalities could lead to these seeming coincidences? So for example, if they're both risk-takers and vigilant, there's a limited set of careers that would appeal to you. Fighter pilot, maybe …
Right, or hazmat team or EMT. All those jobs have similar requirements. But these twins happened to be firefighters.
What's the significance of studies like this for the vast majority of us who don't have an identical twin?
Twin studies have two levels to them. One is all about the twins themselves: all the unique aspects of twinship, being a twin, or parenting a twin.
But twin studies also have so much to reveal about the genetic and environmental aspects of behavior and disease. That's where these studies are for everybody. For example, if you find identical twin pairs in which one twin has a disease and the other doesn't — they both have the predisposition for it, but only one has it. So what triggered it? If you can identify those environmental triggers, then you can use that to understand susceptibility in a non-twin population.
Right now you're working on a study of Chinese twins who have been adopted by Western couples and are being raised apart. How does this work expand on the Minnesota study, and what do you hope to learn from it?
The Chinese twins study is tracking them as they age, whereas the previous reared-apart twins were found as adults.
We're only at the beginning of the study, but I'm hoping to be able to see whether there are certain environmental events that might trigger differences between the two children. With respect to their rearing, are they following similar trajectories?
I've only published one study so far. It looked at the mothers' reports of the first time the children met each other. The mothers were impressed by the closeness the children developed, so quickly. It was quite amazing.
What else are you working on?
I'm studying "virtual twins," which are same-age children raised together who are not biologically related. It started during my last years in Minnesota, when a mother called me and said, ‘You would love my children.' And I said, "Tell me why." What happened was that she had trouble conceiving, so she put in for adoption. But then she got pregnant, and the babies ended up being delivered just a couple of days apart.
So these children are not biologically related, but are being raised together in a very twin-like situation.
I also have an online study of identical twin adults and their relationships with their nieces and nephews. They are as genetically related to them as to their own children because they're genetically the same as their mother or father. And I compare their relationships to fraternal twins, who have the ordinary aunt-uncle/niece-nephew relationship. I have found that the identical twins do have a closer relationship with the children of their twin.
I'm also studying unrelated look-alikes — people who are "doppelgangers." They are not related but, in some cases, look as similar as identical twins. I find them just fascinating. This research helps refute the contention by critics of twin studies that identical twins are more alike simply because people treat them the same because they look alike. My reasoning is that if that's the case, then these unrelated look-alikes should be as similar as identical twins raised apart. And they're not. And when they get together they don't form a close social relationship because their interests are so different. They have nothing in common other than looks.
There are now more twins than there used to be. Have you seen a corresponding increase in interest in your work?
Yes, there are more fraternal twins. Two-thirds of the increase is due to fertility treatments, and the other third is because mothers are having children later, which is linked to more fraternal twins. We're also seeing a slight increase in identical twinning. That's partly because we can better manage twin pregnancies now, [so more twins are likely to survive pregnancy and childbirth]. And also because when you do in vitro fertilization, you're micromanipulating the embryo outside the womb and that increases the chance of it splitting, so you're seeing more identical twins.
I don't know if I have seen more interest in the research as a result, though. All I can say is that people always find twins interesting. Whenever I travel, say I'm on a bus or a train or a plane, and I discuss what I do, everybody is fascinated. People are fascinated by twins. I've thought a lot about why that is. I think partly it's their uniqueness — most people are singletons. But I also think, with identical twins in particular, they challenge our belief in the way the world works. We grow up thinking that everyone is unique and looks different. When we see people who act and look so much alike, it challenges our assumptions about the way the world works.