For years, public health experts have been arguing that where people live matters crucially to their health and life span, even though factors such as genetics or access to health insurance typically get more attention.

Health systems and governments have embarked on massive projects to decode people’s DNA, in the hopes that the information will lead to a new era of tailored treatments and personalized prevention.

It’s the next chapter in the nature-nurture debate: To keep people healthy, is it better to focus on people’s Zip codes or their genetic codes?

A new study in Nature Genetics examined 56,000 pairs of twins from a database of 45 million people insured through Aetna to try to answer the question and found — as might be expected — a mixed picture. Of 560 diseases and conditions studied, 40 percent had some genetic contribution, while a quarter were influenced by shared environment. Cognitive conditions such as attention-deficit/hyperactivity disorder had the strongest genetic influence, while eye disorders and respiratory diseases such as sinusitis or hyperventilation were more influenced by the environment.

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The relative influence of Zip code or genetic code “is incredibly nuanced. It depends on what disease you’re interested in,” said Chirag Patel, a data scientist at Harvard Medical School who oversaw the work that was led by Chirag Lakhani, a postdoctoral researcher.

The study design was clever, constructing a massive review of twins out of an insurance database, but it had a limitation — researchers did not have information on which twins were identical, sharing 100 percent of their DNA, and which were fraternal, sharing only 50 percent of their DNA. They had to infer which twins were identical by relying on a statistical method.

But their results were broadly in line with the findings of decades of twin studies, which have typically examined the question of genes vs. environment in individual diseases, although the new study found the diseases were a little less influenced by genes and a little more influenced by environment, on average.. The unsatisfying answer to the debate over whether genes or environment are more important may simply be that both matter, and the contributions of each will vary depending on the disease.

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“It’s nature via nurture; it’s a clear interaction,” said Tim Spector, a professor of genetic epidemiology at King’s College London who specializes in twin studies and was not involved in the work.

Spector said the new study, the largest known review of American twins, would be a valuable contribution to research on the subject because of its size. But he pointed out another limitation: Because of the structure of American health insurance, only twins younger than 24 could be studied. They had to be on their parents’ insurance and covered for three years to be included in the database and identifiable as twins. He noted that even though some diseases have a strong genetic component, people with identical DNA die at different ages and of different diseases.

“I think that’s what people need to realize — you have this overall genetic soup that predisposes you to things, but life will throw things at you that sometimes you can’t measure, that will give a different end result,” Spector said.

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On average, across the conditions the researchers examined, diseases were 31 percent heritable and 9 percent determined by the twins’ shared environment. That leaves the majority of the difference unexplained, which could be attributable to individual contrasts between twins’ experiences, randomness or data that is erroneous or incomplete.

Melody Goodman, a biostatistician at New York University’s College of Global Public Health, said the study was a great use of the data set, but because it represented a commercially insured population — who mostly received coverage through their employers — it was unlikely to fully capture one of the biggest determinants of health: poverty.

Goodman said genetic tools would be valuable, especially as studies recruit increasingly diverse subjects, but many factors that influence people’s health were already known.

“Even if we can fix all these genetic things,” Goodman said, “it won’t take away the impact of poverty on people’s health.”

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