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.