Double Take: How Twins Influence Research

Being a twin is sometimes fun and sometimes frustrating, but did you know it’s an advantage in medical research?

“Researchers love twins!” says Susan Griffith, M.D., a research librarian from the University of Kentucky in Lexington, who works for the National Organization of Mothers of Twins Club/Multiples of America. She fields questions about twins and triplets from all over the country.

Her initial interest began when she had twin sons 28 years ago.

The National Institutes of Health has a National Twin Registry and says, “Twin studies are considered to be the gold standard for investigating the relative importance of genetic susceptibility and environmental influences.”

Scientists are able to do so because of the unique development of twins, including their shared experience in utero and their common childhood environment. “Equipped with a better understanding of how certain traits are influenced by our genes and environment, scientists can gain insight that may improve disease treatment and prevention, as well as our understanding of what drives certain human behaviors,” says Emily Lilley, administrator of the Mid-Atlantic Twin Registry at Virginia Commonwealth University in Richmond, Virginia.

When you are looking to prove whether something is “nature versus nurture” it’s helpful to have subjects that share every single gene. For years, twin studies have been used to prove whether something is nature (both twins do it or have it) or nurture (differences between them appear over time). But, scientists now wonder, does our body’s own “eco-system” change over time?

Neither Nature nor Nurture?

Griffith says current research focuses on epigenetics, which studies changes in our bodies that are not caused by changes in DNA. After nature and nuture, this is a third way of thinking about how different illnesses or habits develop. The study of epigenetics includes the possibility that our cells change based on their exposure to chemicals, our diet and our lifestyle.

“For many years, the way to prove that something is genetic was to find proof in identical twins, but we are finding there’s more to the story,” Griffith says.
For example, 20 countries conducted studies on post-partum depression by investigating twin women who had been pregnant. Through looking at the factors, associated researchers may be able to find out if there is a genetic link.

Early epigenetic research indicates autoimmune disorders are less likely to be shared by twins, which may indicate that it is activated by something in the environment. On the other hand, neurological conditions, like depression, tend to be more likely in identical twins and may be genetically predisposed.

There are two primary ways to study twins: Cross sectional (studying twins across the population) and longitudinal (studying twins over the course of their lives.) Besides the twins themselves, children of twins may be studied to understand the social and genetic transmission from parents to children. Spouses of twins may also be involved,
to analyze the role of mate selection.

Identical Twins Growing More Rare

Did you know that only about one-third of twins are identical? Today, according to the Centers for Disease Control, 1 in 30 births is a set of twins. This means more twins are being born, but identical twins remain scarce. Because older women are more likely to release more than one egg per cycle, this increases the number of fraternal twins. The rate of identical twin births does not change with the age of the mother. Studying fraternal twins is slightly more advantageous than studying siblings, but doesn’t have exact genetic copies to compare.

Some twins don’t know if they are identical, believe it or not. Relying on looks alone isn’t enough—only a genetic test can reveal it. The test, called a “clinical zygosity determination,” compares the twins’ DNA to see how alike they are genetically. This can be done using a saliva sample sent in by mail to a lab that specializes in this sort of testing.

“If the twins are only of normal family likeness, in other words they look as alike as ordinary siblings, they are probably fraternal. If they look more alike than normal siblings, then they might be identical,” Lilley says.

Newport News twins Shannon Edwards and Christen Campbell use their “twinness” to their advantage as real estate agents, featuring their photos on their business cards.

But, they don’t know if they are identical. Shannon broke her jaw at the age of 3 and some of the differences in their face shapes have been attributed to that. They are considering a genetic test now that the costs have come down.

When Shannon and Christen were born, they weren’t the first twins in their family. “Our mom’s brothers are twins, and our mom’s cousins, too,” says Campbell. Edwards has two daughters, Campbell has two sons—but they say their kids never get them confused with their aunt.

Why Do Twins Run in Families?

It’s another question that science is trying to answer. It’s long been thought to be inherited by women—with an egg more likely to split—but now there’s a suspicion that men may contribute sperm that causes the split. Since we don’t actually know why identical twins occur, it makes it even more interesting to study.

Caleb and Joshua Hammond, 20-year-old twins from Fredericksburg, Virginia, are identical but have different allergies. Josh is allergic to poison ivy, and Caleb isn’t, but Caleb has food allergies that his brother doesn’t. They haven’t participated in any studies yet, but it may just be a matter of time!

“I would say overall that twins are typically very willing to participate in research. Being a twin is unique in and of itself, and add to that the fact that studying twins is a very unique way for researchers to increase our knowledge of human health and behaviors—you can see how twins really get excited about being a part of research! And of course, a lot of times they can share the experience with their best friend, and that is pretty neat,” says Lilley.

The biggest challenge doesn’t seem to be convincing twins to participate, but finding them and keeping track of them. “Keeping current contact information for a registry is a challenge—you would be surprised how often people move. We always encourage our twins to keep us updated with their current contact information so we can get in touch with them easily,” says Lilley.

If you are a twin or the parent of twins and you want to participate in studies, you can register with the NIH Twin Registry, as well as the Mid-Atlantic Twin Registry run through Virginia Commonwealth University in Richmond. Or, even the International Society of Twin Studies which presents results of their studies at meetings all over the world.

About the author

Natalie Miller Moore

Natalie runs Moore than Words, a health communications consulting firm in Williamsburg. She loves to learn and write about health, particularly relating to patient experience and research.