Name _____________________________________ Period _____ Date ______________
Double Take
Directions: Read and answer the questions.
Identical twins form when a fertilized embryo splits into two embryos
early in development. These twins get the same genes from their
parents, but such siblings aren't the same people.
“It’s a unique birth situation,” says Nancy Segal, a psychologist and twin researcher at California State
University, Fullerton. She’s the author of two books about twins and a twin herself. “You feel a little bit
special.”
Twins offer scientists the perfect opportunity to study what makes people who they are, Segal says.
That’s because twins share more in common than ordinary siblings. Yet, twins still end up being
different from each other in important ways — both physically and socially.
By probing these similarities and differences, scientists can begin to figure out which qualities we are
born with and which ones result from our experiences. In science, these questions lie at the center of a
classic debate called “nature vs. nurture.”
“People study twins not because they’re interested in generalizing about twins,” says Matthew McGue, a
psychologist at the University of Minnesota, Twin Cities. (The neighboring Minnesota cities of St. Paul
and Minneapolis are called the Twin Cities.) Rather, he says, scientists study these people pairs to learn
about the human condition.
Fraternal twins, such as the ones shown here, form
when two eggs are fertilized and grow together in the
womb. These siblings get a different mixture of genes
from their mom and dad.
Seeing double
There are two types of twins.
Identical twins begin life in the womb as a single fertilized egg. The egg begins to grow normally into a
single embryo. Then, for unknown reasons, the embryo splits in two. This usually happens during the
first two weeks of growth. About nine months later, two babies are born that often look so similar even
their parents can have trouble telling them apart.
The second type of twins is called fraternal. These twins develop when two eggs are fertilized. Fraternal
twins are much like regular siblings. They just happen to grow together inside their mother. These twins
can be boys, girls, or one of each. Identical twins, on the other hand, always belong to the same gender.
What makes twins interesting to scientists is their DNA. This molecule acts like the instruction manual
for life. Stretches of DNA are called genes. And genes determine the color of your eyes, how tall you
are, which diseases you might be likely to develop and more.
People have billions of cells in their bodies. In every cell, DNA is grouped into 23 pairs of threadlike
structures called chromosomes. One chromosome in each pair comes from your mother. The other
chromosome in each pair comes from your father.
During reproduction, genes from both parents get scrambled into a new combination of chromosomes in
the child. In ordinary siblings and fraternal twins, each person gets a different mixture of genes from
mom and dad. That explains why you might have your mother’s mouth and your father’s eyes, while
your brother has the opposite combination.
In identical twins, on the other hand, each person gets exactly the same genes from each parent.
Genetically, these siblings are like clones of each other.
Ever since the discovery of DNA in the 1950s, scientists have wondered how important genes really are.
Do these microscopic snippets determine whether we like sports, are good at art and everything else
about us? (That’s the “nature” side of the debate.) Or are our personalities a result of the way we’re
raised and the experiences we have? (That’s the “nurture” side.)
Researchers now know that the answer lies somewhere in the middle. Genes (nature) determine our
potential. But the environment (nurture) often determines whether genes are turned on or off.
Identical twins illustrate that concept perfectly. If genes, or nature, alone determined everything about
us, you’d expect identical twins to be identical in every way. Despite their similar looks, however, twins
often prefer different types of music, friends, clothes and more.
Studying these differences can help scientists figure out what makes us all the same, and what makes us
all different.
“Twins,” Segal says, “give us a beautiful natural experiment.”
Tracking twins
At the University of Minnesota, Twin Cities, researchers have been tracking twins for more than 30
years. Starting in the 1970s, scientists there started bringing different types of siblings (and their
families) into the lab.
This study includes identical twins, fraternal twins and a third group called virtual twins. This last
category includes siblings who are genetically unrelated but are the same age and grew up in the same
home. One sibling might be adopted for example, while the other is a biological offspring of the parents.
Researchers chose to compare these groups because twins in all three categories share a very similar
environment growing up. But they differ in how similar their genes are. So differences among groups
reveal how important genes are in different situations.
For their research, the scientists initially collected a variety of information from each family, using
questionnaires, DNA samples, brain wave patterns and more. Every few years since the work began, the
scientists have followed up with the same families and repeated many of the same tests.
The Minnesota researchers now have information about more than 10,000 people. From this large set of
data, the scientists (along with similar researchers elsewhere) have turned up lots of interesting results.
One finding is that, in many ways, identical twins are far more alike than fraternal twins, even when
those identical twins are raised apart. And fraternal twins are more alike than virtual twins.
These similarities are true for a large number of personality traits, such as how outgoing people are, how
aggressive they are and what types of decisions they tend to make. These results suggest that genes play
important roles in determining our personalities.
Identical twins also tend to become more similar to each other with age. That’s probably because as they
get older they have more control over what they do and how they live.
“When you’re a baby, your parent or caretaker completely controls your physical and social
environment,” McGue says. “As [identical] twins get older, they create more similar environments for
themselves when given a choice.”
Name _______________________________________ Period _____ Date ________________
Double Take
Directions: Read and answer the questions.
1. Why are twins such a good model to study to prove “nature vs nurture”?
2. How are identical twins formed?
3. How are fraternal twins formed?
4. What are some of the characteristics the DNA/genes decide?
5. What explains why you might have your mother’s mouth and your father’s eyes, while your
brother has the opposite combination?
6. What is another term for “identical twin”?
7. Explain the nature nurture debate.
8. What do scientists think is the answer to the nature nurture question?
9. Why are identical twins a good model to study?
10. What are the three “groups” tracked by the University of Minnesota?
11. What are virtual twins?
12. What is the biggest difference between the three groups studied?
13. Describe results from the study.
Similar but different
Twin studies have also given scientists insight into mental illnesses.
Not long ago, people believed that schizophrenia, autism, depression and other mental illnesses resulted
from poor parenting or negative experiences. Then twin studies came along to shake up that view.
Scientists found that if one identical twin has schizophrenia, the other twin has a 50 percent chance of
developing the disease. (Schizophrenics often hear voices that don’t exist, among other symptoms.)
When one fraternal twin has the same illness, on the other hand, the other one has only a 10 percent
chance of having it. The rates are even lower for virtual twins. Other mental illnesses show similar
trends in twins.
Those statistics suggest that the genes you’re born with might set you up to develop mental illnesses, if
environmental conditions trigger those genes into action.
“Twin studies have had an extraordinary impact on mental illness,” McGue says. “Showing that genetic
factors are important has been terribly important.”
Now that scientists know genes are important in these cases, researchers can begin to zero in on exactly
which genes are involved in setting people up to develop these diseases. The work might eventually help
doctors develop better treatments for such illnesses.
At the same time, researchers want to know why sometimes only one twin develops a serious disease.
What is it about the environment that pushes certain genes to turn on or off?
One possibility is that what happens in the womb influences how genes end up behaving many years
later. This is something scientists are currently investigating. One day, the work might lead to simple but
life-changing advice, like what moms should eat when pregnant.
These days, scientists are using twin studies to investigate everything from the complexities of genetics
to the reasons why people vote the way they do.
“Twinning is a mystery,” McGue says. “There’s no end to questions.”