Psychologist Nancy L. Segal (www.drnancysegaltwins.org), director of the Twin Studies Center at California State University, Fullerton, has been researching twins for more than two decades. Her books include Entwined Lives; Indivisible by Two; Someone Else's Twin; and the forthcoming Born Together—Reared Apart: The Landmark Minnesota Twin Study. She spoke with National Geographic magazine Assistant Editor Amanda Fiegl.
Why are twins so intriguing?
For me, it's partly because I'm a fraternal twin. My sister and I were very different from an early age, and I was always fascinated with those differences. And I think in our Western culture, we value individual strengths and talents. So when we discover twins, especially identical ones, it runs counter to the way we think things should work. It can also create envy, because there's this sense that twins have someone who completely understands them, a soul mate—and who doesn't want that?
But are twins always soul mates, or is that a myth?
For the most part, identicals are much closer than fraternals. The majority of identical twins have an extremely close relationship and tend to fight less often than do other siblings, although there are obviously exceptions to all rules. In my study on the loss of a twin, using a grief-intensity scale, I found that the level of grief associated with losing one's twin is comparable to losing a spouse. It is high for all twins, but slightly higher with identicals. Fortunately, too few twins had lost children, so I could not make that comparison.
How rare are twins?
Identicals are still relatively rare. The natural twinning rate is about one in 80 births in Western nations, and identical twins are only a third of those. But in the U.S. we're seeing a dramatic increase in the fraternal twinning rate correlated with the increased use of assisted reproductive technologies, which can encourage zygotic division. The rate of identical twinning tends to be about the same around the world, suggesting that it occurs randomly, although recent studies have suggested that it may have a genetic basis in some families.
Is it hard for identical twins to develop a sense of individuality?
Growing up with someone who looks exactly like you is a very unusual situation. I always think of twins as having two identities: the one they share with their twin, and the one they keep for themselves. Many educators have a misguided notion that if twins aren't placed in separate classrooms, they won't develop a sense of self. But you have to handle these situations on a case-by-case basis. Separation may work well for some twins, but not so well for others.
What unique challenges do parents of twins face?
It's more work in that the parents have to be exceedingly fair, because children are exquisitely sensitive to any trace of favoritism. But it's also easier, in that twins have a built-in playmate. Of course, it's also financially harder to raise two, and then there's the sudden empty nest when twins leave home.
Have you ever heard a story that made you wonder if twins had some sort of extrasensory connection, like telepathy?
I've never directly witnessed anything like that. In 2002 there were identical male twins in Finland who were riding their bicycles and both died of a heart attack at the same time. Some people suppose that reflects some sort of unusual communication between the twins; I believe it shows that their physical and health characteristics were very similar. Twins do sometimes say they can sense when the other is in trouble. Of course, when you worry about somebody 20 times a day, chances are that one of those times they are going to really need your help. That doesn't make it telepathy.
Can twins have different sexual orientations?
Of course. Identical twins are not any more or less likely to be gay than anyone else in the general population. But if one twin is gay, that increases the chances for the co-twin. What's really fascinating is when one identical twin is transgendered. I studied a case in which one twin underwent a sex-reassignment operation to become male, while her identical sister remained female. It's hard to explain that, but it probably reflects differences in hormonal exposure before birth.
Is it possible to go around not knowing you have a twin out there?
Absolutely. The landmark University of Minnesota Study of Twins Reared Apart, which began in 1979, found over 130 sets of twins who were adopted and raised apart, and in many cases had no idea. Most were intentionally separated because their mothers died in childbirth, or their parents couldn't financially manage two children. But there have also been a few cases—I've found nine—in which nurses mistakenly switched one newborn twin with a singleton in a crowded nursery, creating a pair of "fraternals" who later discover they are unrelated. For twins in those situations, reuniting is not entirely a happy celebration. Suddenly, their identities and their family member's identities are completely shattered.
Who are the most surprising twins you've studied?
Oh, definitely, Oskar and Jack. They were born in British Trinidad to a Romanian father and a German mother who soon went separate ways. In 1933, when the twins were six months old, their mother took Oskar and his sister back to what had become Nazi Germany, leaving Jack with his father. Oskar was raised Catholic and became a member of the Hitler Youth, while Jack was raised Jewish and joined the Israeli military.
Wow. Did they ever reunite?
Yes, but they didn't hit it off in their first meeting after the war, as young men. After participating in the 1979 Minnesota study, they started spending more time together. When Oskar died of lung cancer in 1997, Jack was grief stricken. What struck me about their story was that they came from such different political and historical backgrounds; most people in their shoes wouldn't bother to try to get along. They also noticed that they shared many habits and quirks. They were so much alike, even though one was raised by a father and one by a grandmother, in totally different cultures. That's the beauty of reared-apart identical twins. They give you new explanations for understanding the blend of genetic and environmental influences on behavior. They really are the perfect research subjects.