Passage Three When trying to understand why some people have trouble living within their means, we tend to blame factors such as high interest rates and irresponsible spending. Now researchers have found another possible factor to add to the list: a gene linked to credit-card debt. Earlier work has shown that genetics plays a role in how we handle money. But a recent study was the first to show that a particular gene affects financial behavior outside the lab. Researchers at the University of California, San Diego, and the London School of Economics looked at genetic data and questionnaires already collected from more than 2,000 young s aged 18 to 26 as part of the National Longitudinal Study of Adolescent Health. In particular, they looked at whether these young s said they had any credit-card debt and what version of the MAOA gene they had. Monoamine oxidase A (MAOA) (甲型单胺氧化酶) is an enzyme that breaks down the signaling chemicals called neuro-transmitters in the brain. Previous studies have linked the low-efficiency versions of the MAOA gene - the variants that cause less MAOA to be produced by brain cells - to impulsiveness. In the new study, people with one “low” MAOA gene and one “high” MAOA gene reported having credit-card debt 7.8 percent more often than did people with two “high” versions, the researchers found, even when they controlled for factors such as education and socioeconomic status. For people with two “low” versions of the gene, that number jumped to 15.9 percent. The researchers were surprised by the magnitude of the difference. “The effect is almost as big as financial literacy,” meaning people’ s ability to digest complicated financial information, says Jan-Emmanuel de Neve, an author of the study. But, de Neve cautions, an individual’ s version of the MAOA gene does not predict whether he or she is carrying debt. The gene affects credit-card debt the way other genes have been found to play a role in breast cancer: a particular version of the gene increases risk, but many other genetic and environmental factors are important, too. Questions 11-15 are based on Passage Three. What have earlier studies proved