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.

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 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.

eyecrave/iStockphoto

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.”

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.”

TEACHER’S QUESTIONS

Here are questions related to this article.

More than 1,400 high school students, selected from a pool of millions, came to Portland, Ore., in May to present their science projects and compete for $3 million in scholarship money and other prizes.

Music was the focus of several research projects at ISEF this year. That’s probably not surprising. Music is a popular form of entertainment, and young people pay a lot of attention to it. Rap, rock, folk, jazz, R&B, country, klezmer: Almost everyone has a favorite type.

Classical feel

Classical music is the genre of choice for Richie and Ryan Huynh, 15-year-old identical twins from Champlin, Minn. The freshmen perform with a couple of orchestras. Richie plays the viola. Ryan plays the violin.

It doesn’t matter whether they’re performing or listening to classical music, the twins say. Either way, it makes them feel good.

“It’s a great way to relax and escape,” Ryan says. “Sometimes, we play all night long.”

Richie and Ryan Huynh of Champlin, Minn., did a science project on the effect of listening to classical music on a person’s white blood cell count.

E. Sohn

Classical music makes them feel so good, in fact, that Richie and Ryan wondered if music calms more than just the mind. For their project, they studied how classical music affects the body.

For 6 weeks, 11 volunteers (all 14- and 15-year-olds) listened to classical music for 20 minutes every day. They were told not to do anything else at the same time. Another 10 volunteers carried on with their normal lives—with no classical music. Each week (except for a 1-week vacation break), the school nurse drew blood from all 21 students.

Richie and Ryan then took a drop from each blood sample and smeared it on a slide. On each slide, they counted white blood cells, which are responsible for fighting diseases and germs. The more white blood cells you have, the healthier you are. The counting involved hours of work every day after school.

The results showed that, over the course of the 6-week experiment, the white blood cell count went up and down in both groups. Consistently, however, the students who listened to classical music had higher counts than those who didn’t.

“We did some research and found that people with less stress have higher white blood cell counts,” Ryan says. “We think it has to do with relaxation.”

Just sit back and let Mozart stream in for a little while each day, the brothers say, for a healthier, less stressful life.

Tough rock

Hardcore punk rock or heavy metal probably would not produce the same kind of benefits, another ISEF project suggested. Aggressive music might even hurt you.

That’s what Rebecca Wilder, an 18-year-old senior from Mentor, Ohio, discovered. Her study showed that the kind of music you listen to while you drive could actually affect your chances of getting hurt in a car crash.

Rebecca Wilder of Mentor, Ohio, studied the effects of different types of music on driving ability.

E. Sohn

For her project, Rebecca brought 50 teenage volunteers to an arcade, one at a time. Each student played three rounds on a driving simulation game called “California Speed.” Each round lasted 3 minutes.

During one round, participants used headphones to listen to an aggressive rock song called “Y’all Wanna Single” by Korn. During another round, they listened to a weepy country song called “Don’t Take the Girl” by Tim McGraw. In a third round, they drove in silence. The order of rounds was random. Throughout the experiment, Rebecca counted crashes.

Her results showed that aggressive music might be the most dangerous. Players crashed an average of 10.7 times in 3 minutes while listening to the Korn song. They averaged 6.1 crashes during the sad country song and 8.4 crashes when there was no music.

In reports of other research, Rebecca found evidence that music affects emotion centers in the brain. She speculates that aggressive music makes people aggressive, whereas slow music relaxes them. Driving in silence might encourage people to space out and crash more, she says.

Car accidents killed 6,300 teens between the ages of 15 and 19 in 1998, Rebecca notes. She hopes that her research might inspire Intel, the sponsor of ISEF, to invent a chip that can block certain radio stations on car radios. Keeping young people from listening to the hard stuff when they’re on the road could save lives, she says.

Violin sounds

Saving lives was not on Linden Webster’s mind last summer. She was too busy playing the violin.

Linden Webster of Midlothian, Scotland, plays her violin as part of her physics project studying what factors affect a violin’s quality.

Intel

For 6 weeks, the 17-year-old from Midlothian, Scotland, recorded herself playing notes on four violins in a special, echo-less room called an anechoic chamber. She plugged the recordings into a computer program that turned the sound into graphs. Her goal was to figure out what makes one violin sound better than another violin.

“When I was given the opportunity to complete this project, I suddenly realized I knew nothing about how a violin makes sound,” says Linden, who has been playing since she was 8 years old. “I was embarrassed by how little I knew,” she says.

Linden’s work demonstrated how essential certain vibrations are. When a violin is played, it vibrates. Depending on the violin, vibrations at certain frequencies are much stronger than those at other frequencies. These special frequencies are known as resonances.

High-quality violins have resonances that occur at particular frequencies, Linden says, so they sound better.

Her insights might some day come in handy for violinmakers, Linden says. “Using physics can help us find the best way to make violins and to make them cheaper,” she says.

In the meantime, Linden has a better understanding of how her own violin works. “Now, when something goes wrong,” she says, “I’ll have a better idea why.”

Math beat

Studying music can do more than boost your physics and performance skills, another young violin player found. It can also make you better at math, says Ana Peterlin, a 13-year-old freshman from Fairbanks, Alaska.

Ana Peterlin of Fairbanks, Alaska, holds a CD containing a musical piece that she composed and analyzed mathematically.

E. Sohn

Ana composed a retrograde inversion canon—a piece of music performed simultaneously by two people. One person plays from the first note to the last, while the other plays from the last note to the first, reading the music upside down. She named her composition “Upside Down and Right Side Up!”

Ana then compared her piece to one of Mozart’s retrograde inversion canons, called “Table Music for Two.” Both compositions have the same beat, the same key, and the same range of musical notes. But they differ considerably in the types of notes that are used. Ana’s piece, for example, has many more sixteenth notes.

In the process of analyzing the compositions, Ana discovered some unexpected mathematical patterns in Mozart’s piece. For one thing, he divided his piece into two sections, where one part is longer than the other. It turns out that the ratio of the length of one part to that of the other is a special number called the golden ratio.

Mozart himself was fascinated by numbers. As a kid when he was learning arithmetic, he once covered the walls of all the rooms in his house with figures. When Mozart was 14 and busy with music, he still found time to ask his sister to send him extra math exercises. The margins of one of Mozart’s musical compositions contain calculations of his chances of winning a lottery. He used math in a variety of ways to guide how he composed some of his musical pieces.

Ana was delighted to see her passion for music intersect with her passion for math. “Both help each other,” she says. “Music stimulates your brain, and you use math so much in music. You can’t have one without the other.”

Watch out, math teachers. Singing multiplication tables may be next!

Going Deeper:

Word Find: Project Music

Additional Information

Questions about the Article