James Smith, a health economist at the RAND Corporation, has heard a variety of hypotheses about what it takes to live a long life money, lack of stress, a loving family, lots of friends. But he has been a skeptic.
The New Age
Piece in a PuzzleThis is last of four articles looking at the science of aging. The articles will remain online at nytimes.com/aging.
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Yes, he says, it is clear that on average some groups in every society live longer than others. The rich live longer than the poor, whites live longer than blacks in the United States. Longevity, in general, is not evenly distributed in the population. But what, he asks, is cause and what is effect? And how can they be disentangled?
He is venturing, of course, into one of the prevailing mysteries of aging, the persistent differences seen in the life spans of large groups. In every country, there is an average life span for the nation as a whole and there are average life spans for different subsets, based on race, geography, education and even churchgoing.
But the questions for researchers like Dr. Smith are why? And what really matters?
The answers, he and others say, have been a surprise. The one social factor that researchers agree is consistently linked to longer lives in every country where it has been studied is education. It is more important than race; it obliterates any effects of income.
Year after year, in study after study, says Richard Hodes, director of the National Institute on Aging, education keeps coming up.
And, health economists say, those factors that are popularly believed to be crucial money and health insurance, for example, pale in comparison.
Dr. Smith explains: Giving people more Social Security income, or less for that matter, will not really affect peoples health. It is a good thing to do for other reasons but not for health.
Health insurance, too, he says, is vastly overrated in the policy debate.
Instead, Dr. Smith and others say, what may make the biggest difference is keeping young people in school. A few extra years of school is associated with extra years of life and vastly improved health decades later, in old age.
It is not the only factor, of course.
There is smoking, which sharply curtails life span. There is a connection between having a network of friends and family and living a long and healthy life. And there is evidence that people with more powerful jobs and, presumably, with more control over their work lives, are healthier and longer lived.
But there is little dispute about the primacy of education.
If you were to ask me what affects health and longevity, says Michael Grossman, a health economist at the City University of New York, I would put education at the top of my list.
Graduate Student Finds Answer
The first rigorous effort to decide whether education really changes people so they live longer began in a most inauspicious way.
It was 1999 and a Columbia University graduate student, Adriana Lleras-Muney, was casting about for a topic for her doctoral dissertation in economics. She found an idea in a paper published in 1969. Three economists noted the correlation between education and health and gave some advice: If you want to improve health, you will get more return by investing in education than by investing in medical care.
It had been an inflammatory statement, Dr. Lleras-Muney says. And for good reason. It could only be true if education in and of itself caused good health.
But there were at least two other possibilities.
Maybe sick children did not go to school, or dropped out early because they were ill. Or maybe education was a proxy for wealth, and it was wealth that led to health. It could be that richer parents who gave their children everything, including better nutrition, better medical care and a better education, had children who, by virtue of being wealthy, lived longer.
How, she asked herself, could she sort out causes and effects? It was the chicken-and-egg problem that plagues such research.
The answer came one day when Dr. Lleras-Muney was reading another economics paper. It indicated that about 100 years ago, different states started passing laws forcing children to go to school for longer periods. She knew what to do.
The idea was, when a state changed compulsory schooling from, say, six years to seven years, would the people who were forced