Directions: As you read annotate using the marks included in the box. Underline or highlight the line(s) that correspond
to the annotation mark you make.
Purpose of Study P
Background Information B
Independent Variable (Cause or thing that they are changing) IV
Dependent Variable (Effect or thing that they are measuring DV
Experimental Design/Data Collection EX
Conclusion(s); What did they learn from the study? !
By Emily Sohn
Dec. 10, 2008
I have a friend who looks just like me. We both have light brown hair that we wear pulled back, often in pigtails. We
dress in the same types of sporty clothes. Our glasses have thick rims and a blue tint. We are both journalists, athletes
and moms to little kids. Even our husbands look alike, and even they get my friend and me mixed up sometimes.
Everywhere we go, strangers ask us if we’re twins. My friend and I are not even related. But it’s fun to feel like I’m
looking in a mirror when I look at her. And the attention we get helps me imagine what life must be like for actual twins.
Being a “pretend twin” is also fun for me because I’ve always wondered what it would be like to have a sibling who
seemed just like me, but was actually a different person altogether. And I’m not the only one who is fascinated by these
rare pairs. Lots of scientists are, too. “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
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.
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.). Genes (nature) determine our potential. But the
environment (nurture) often determines whether genes are turned on or off.
If genes 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.”
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.”
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.
Post Reading Activity:
On a separate sheet of paper, answer the following questions using COMPLETE SENTENCES. Sheets of loose-leaf paper
must be stapled together and should not have edges from being torn out of a notebook.
1. Explain what the difference between fraternal and identical twins? And what happens differently in the womb
to give rise to one type of twins or the other?
2. The University of Minnesota study also tracks “virtual twins.” What are these?
3. What happens to the differences among twins as they get older?
4. What is the difference between “nature” and “nurture”?
5. What kinds of information can the study of twins tell us or help non twins?
6. Even identical twins can have different traits — from height and health to personality. What factors can lead to
7. Some scientists study twins who were separated at birth and raised by different families. What can these
investigations tell us about the role of genetics in people generally?
8. What are 3 major findings or conclusions found in this study?
Collins Type 2
- Write a Collins Type 2 explaining what the difference between “nature” and “nurture” is.
o Give examples of what “nurture” is and use your knowledge of DNA/genetics to explain what “nature”
o What do you think plays a larger role in the person you are “nature” or “nurture”? You must provide at
least 2 pieces evidence/explanations that support your answer. Underline these evidences/explanations
in your paper.
- At least 15 sentences long. Number your sentences like the example below
o Ex. 1) This is the first sentence of my paper. 2) This is the next sentence in my paper. 3) After this
sentence I only have to write twelve more sentences.