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Three days and nearly 2,000 sets of twins
01:03 - Source: CNN

Identical twins account for just four in every 1,000 births in America—but are these uncommon siblings so different from the rest of us? Lisa Ling learns just how complicated that answer can be on “This Is Life” Sunday at 10 p.m. ET/PT.

CNN  — 

One of the broadest studies of twins in the United States suggests that our genes tend to influence the diseases that afflict us more than where we live, according to research published Monday in the journal Nature Genetics.

Using insurance claims data, researchers identified more than 56,000 pairs of twins and estimated the heritability of 560 diseases, finding that nearly a third of the variation in these conditions could be attributed to genetics, on average. Where people grew up was less contributory on the whole.

But that still means the majority of the variation we see in human disease is unaccounted for by genetic or ZIP codes.

“The relationship between genetics and environment in disease is incredibly nuanced,” said study author Chirag Patel, assistant professor of biomedical informatics at Harvard Medical School.

Of the diseases the researchers looked at, 40% had a significant genetic component, and about 25% had an environmental one – though the strength of that relationship could be different for any given disease. For example, morbid obesity was found to be strongly influenced by genetics and the environment, Patel said.

“You get people who are very deterministic, who say ‘it’s genes’ or ‘it’s environment.’ This shows it’s a mixture,” said Jeffrey M. Craig, associate professor at the Deakin University School of Medicine and deputy director of Twins Research Australia. Craig was not involved in the new study.

As twin registries merge and incorporate big data, as in the new study, experts like Craig look forward to new stages of twins enriching science.

“That’s one of the ways twin research is growing,” he said.

Dynamic duos

Scientists love to study twins because they help answer age-old questions about nature vs. nurture. Identical twins share the same genes, so any differences between them – say, more wrinkled versus less wrinkled skin – could be the result of their environment.

“Studies of human health are plagued by these confounding factors,” Craig said, including when someone was born, where they were born and their socioeconomic status, ethnicity and gender. Often, twins are perfect controls, he said.

About 33 in every 1,000 human births in the United States are twins, a rate that has climbed in recent decades as more women marry later and take fertility drugs or employ in vitro fertilization, factors that are known to increase the likelihood of multiple births. Identical twins are an even more exclusive club: roughly four in every 1,000 births. They are formed when a single fertilized egg splits in two, creating two embryos with the same DNA.

Attempts to put twins under the proverbial microscope go as far back as 1875, when English scientist Francis Galton, who was also Charles Darwin’s half-cousin, “first came upon the twin research design, which is really a very simple and very elegant way of looking at behavior,” said Nancy Segal, professor of psychology and director of the Twin Studies Center at California State University, Fullerton. Segal is a fraternal twin herself.

According to a paper in the International Journal of Epidemiology, Galton published “the first detailed attempt to use the phenomenon of twinning to estimate the relative powers of nature and nurture.” But he took the research to a troubling conclusion and “argued repeatedly for the need for the state or its wiser citizens to seek to encourage the best to outbreed the rest,” ultimately coining the term “eugenics.”

In more recent years, however, twins have revealed a genetic component to a number of outcomes such as epilepsy, religiosity, autism and mental health, according to experts. NASA even conducted its own twin study on how astronaut Scott Kelly’s gene expression changed after a year on the International Space Station, relative to that of his identical twin, Mark, who remained on Earth.

Twins have also suggested that something outside the genetic code can explain why one identical twin might develop Type 1 diabetes or Parkinson’s disease and the other doesn’t.

Four decades ago, “when I was coming up, there was very little acknowledgment of genetic influences on children’s mental health,” said Judy Silberg, scientific director of the largest twin registry in the United States, the Mid-Atlantic Twin Registry. “Everything was pretty much due to poor parenting, specifically mothers.”

But now, researchers understand that there are varying levels of heritability to things like attention deficit hyperactivity disorder, schizophrenia, post-traumatic stress disorder and depression – and life events may cause those genes to “come online,” said Silberg, who is also an associate professor of human and molecular genetics and psychiatry at Virginia Commonwealth University.