Those looking for a magic elixir to keep them healthy and happy need look no further than their bank account. Wealth and, more broadly, socioeconomic status play powerful roles in determining how long we live.

"It's clear that those who have less wealth will have fewer years to live than those with more wealth," says James Smith, senior economist at the research group Rand. The connection is so widely accepted that researchers have given it a name: "the wealth gradient in mortality." What's far more complicated to understand is why the connection exists, and whether wealth causes better health, or vice versa.

The longest-running longitudinal study of health, run by George Vaillant, a professor of psychiatry at Harvard Medical School, found education to be one of the biggest determinants of longevity, along with behavioral factors -- excessive drinkers were more likely to die young, for example. Out of the 500-plus Harvard students and inner-city Boston men the study has followed since 1937, the Harvard students have lived an average of 10 years longer than the inner-city men, says Vaillant. In fact, three in 10 of the Harvard students reached age 90, compared with the 3% to 5% one would expect.

Among the inner-city men who attended college, health was just as good as that of Harvard students who attended college but not graduate school, says Vaillant. "(The Boston men) went to terrible colleges by Harvard standards, but they did get 16 years of education, and that absolutely evened the playing field," says Vaillant. People who go to college tend to drink less, smoke less, and are less likely to be obese, he adds, all factors that contribute to longevity. In fact, after controlling for education and other factors, Vaillant found that income alone had little effect on longevity.

People who pursue higher education, explains Vaillant, tend be more focused on the future, which probably also helps them make healthier choices. "In order to get an education, especially if you're poor, you have to think you have a future," he says.

Indeed, says Smith, one hypothesis is that "more-educated people are more forward-looking, and when they make decisions, they take into account the future more than uneducated people. A lot of things you might do don't have an immediate negative impact -- excessive drinking, smoking and doing drugs can (feel good in the short-term) -- but the fact is, it's going to kill you in the future." Another possibility is that people with higher levels of education are more likely to maintain their health, have better access to health care, and follow doctors' directions when it comes to taking pills or other instructions.

Smith's research also suggests that causality doesn't just run one way; health contributes to wealth, as well. "Because you are healthy and able to work, you are wealthier," he explains. At the same time, poor health often takes a toll on a person's wealth, either because it prevents one from working or because of expensive medical treatments. Taken together, researchers at the University of Chicago estimate that the gains in life expectancy between 1970 and 2000 resulted in an additional $3.2 trillion a year in national wealth.

Meanwhile, as income disparities continue to grow in this country, so do life expectancy disparities. According to an analysis by the Social Security Administration, life expectancy for 65-year-old men in the top half of the earnings distribution has increased by five years, to 21.5 more years. For those in the bottom half of the earnings distribution, life expectancy has increased just over one year, to 16.1 more years.

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Do rich people live longer?