Researcher Delves Into Genetics
to Find Answers as to Why Twins Are So Different
December 16, 2004
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
“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
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
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
“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
“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
“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.”
« back to Research