Double take

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.

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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.

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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.”

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