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Researcher Delves Into Genetics to Find Answers as to Why Twins Are So Different


December 16, 2004

Nancy Segal
Nancy Segal, professor of psychology, has been interested in twins practically her whole life. A twin herself, she has explored not only twin similarities, but also their differences. The published author, pictured displaying her book Entwined Lives, recently appeared on national television to discuss the relationship between identical twins.

“I like to tell people I’ve been studying twins from the time I was a little girl,” says Nancy Segal, professor of psychology. “As a twin myself, my sister and I always had something that set us apart from most other children.”

However, as fraternal twins, Segal and her sister had no more in common, genetically, than ordinary siblings. “My sister and I have very different interests and don’t particularly resemble each other,” she admits.

However, with identical twins – who share the same DNA – it’s a different story. In fact, in addition to looking alike, they often share personality and intelligence traits as well.

While studying identical twins that were separated at birth, Segal was impressed by their similarities, despite their different home environments. Many of the separated twins held similar jobs, had similar mannerisms, liked the same kinds of food and entertainment, and frequently felt an immediate bond upon first meeting.

More recently, Segal has appeared on television shows such as “Good Morning America” and “20/20” to discuss the relationship between identical twins. What is driving this latest surge in interest is a case in which identical twins – girls – were born, and years later, one has taken the steps to become a man.

“This is where it gets interesting for scientists and researchers,” Segal says. “Here are two identical twins – one is perfectly happy with her gender but, for the other, being female has always created conflict. If their DNA is the same, why is there this dilemma? We hope that by studying these twins, we can better understand what causes issues with transgender.”

After studying twins for decades, Segal always finds something new to investigate.

“I’m probably having more fun than I should,” she laughs. “But it really is an interesting field and my studies aren’t just beneficial to twins. We’re looking at the influences of genes and environments, and trying to determine the influence each has. This is of benefit to everybody.”

Some of the findings Segal and twin researchers have discovered debunk previously accepted medical and psychological assumptions. For instance, Segal believes that only DNA testing can definitively prove whether or not twins are identical or fraternal.

“For instance, with some identical twins, the placenta splits in two, and so, from outward appearances, it seems that they would be fraternal,” she says. “With some fraternal twins, the placenta fuses into one. That’s why with same-sex twins, the only real way to determine whether they are identical or fraternal is a DNA test.”

The personalities of identical twins also are of interest to Segal.

“Some of the commonalities are incredible,” she says. “For instance, we discovered a case where twins, taking a test in the same class, made almost identical mistakes. In fact, in some instances, universities wanted to penalize identical twins because they’re convinced they’re cheating...only to discover that it simply wasn’t true.

“With identical twins, they seem to have similar processing skills.”

One of the benefits of being an identical twin is that there appears to be a mutual understanding between the two. It’s almost as if you have a built-in best friend.

“Twins love being twins,” says Segal. “It’s as if you have something special that most other people don’t.”

While they do share a special bond, most twins also have friends independent of one another, as well as interests that the other may not share.

Yet many identical twins speak of a close bond that exists for them. Segal has discovered that if one twin dies, the other twin feels this loss often more acutely than non-twin siblings do.

“For a twin, this is a companion who has been with them from birth and shares their genetic blueprint,” she says. “What is most helpful to these twins is talking to others who have lost a twin. There seems to be a sort of empathy that is difficult for an ‘outsider’ to understand.”

And, according to her research, parents love having twins...despite all the teasing about “double trouble.”

“Of course, parents see the differences between the twins pretty quickly and usually have no trouble distinguishing one from another,” she adds.

Now she is turning her research to another “twin” group – those she calls “virtual twins.” These are children who were adopted at the same time and are the same age. They are part of a study called TAPS (Twins, Adoptees, Peers and Siblings). Additional participants are welcome.

“Although they are not genetically related, the virtual twins are raised in the same environment and we want to compare them with fraternal and identical twins,” Segal says. She anticipates the three-year study of 350 families to be completed by late spring.

In her decades of research, has Segal discovered any surprises?

“I didn’t anticipate that my work would lead to applications in a legal context,” she reports. Segal is occasionally asked to speak before juries about the special relationship between identical twins when one twin has died due to injury or negligence.

“I also didn’t realize how much twin research – others’ as well as my own –would fit into the context of learning how much of a role genetics play,” she continues. “Some are using this research in the human genome project to look at which genes are triggered and which are not as it relates to identical twins, and some are looking at it when they discuss cloning. Twins are like living laboratories in some instances.”

Finally, she notes with a laugh, “I never realized how much twins like being studied. The twins and parents I’ve been involved with have all been so cooperative and understanding. It certainly makes conducting research much more pleasant.”

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