The secret to a long life has been much studied. The health economist James Smith found that the answer was education. Stay in school. This is no doubt true. But his findings don’t necessarily conflict with a study conducted by Howard S. Friedman and Leslie R. Martin and detailed in their book, “The Longevity Project.” Their effort was unique in that it followed a single set of participants for eight decades, from childhood to death.

Their key to long life: conscientiousness - which is, after all, what keeps people in school.

Many assume biology is the critical factor in longevity. If your parents lived to be 85, you probably will, too. Not so, Dr. Friedman said. “Genes constitute about one-third of the factors leading to long life,” he said. “The other two-thirds have to do with lifestyles and chance.”

As an example of chance, he cited veterans of World War II. “A disproportionate number of those sent overseas, especially to the Pacific, died at a greater rate after the war than the men who had been deployed at home,” he said. Men sent overseas were more than one and a half times as likely to die, compared with their peers who had stayed home.

There are three explanations for the dominant role of conscientiousness.

The first and most obvious is that conscientious people are more likely to live healthy lifestyles, to not smoke or drink to excess, wear seat belts, follow doctors’ orders and take medication as prescribed. Second, conscientious people tend to find themselves not only in healthier situations but also in healthier relationships: happier marriages, better friendships, healthier work situations.

The third explanation for the link between conscientiousness and longevity is the most intriguing. “We thought it must be something biological,” Dr. Friedman said. “We ruled out every other factor.”

He and other researchers found that some people are biologically predisposed to be not only more conscientious but also healthier. “Not only do they tend to avoid violent deaths and illnesses linked to smoking and drinking,” they write, “but conscientious individuals are less prone to a whole host of diseases, not just those caused by dangerous habits.”

The precise physiological explanation is unknown but seems to have to do with levels of chemicals like serotonin in the brain.

As for optimism, it has its downside. “If you’re cheerful, very optimistic, especially in the face of illness and recovery, if you don’t consider the possibility that you might have setbacks, then those setbacks are harder to deal with,” Dr. Martin said. “If you’re one of those people who think everything’s fine - ‘no need to back up those computer files’ - the stress of failure, because you haven’t been more careful, is harmful. You almost set yourself up for more problems.”

How about exercise? Dr. Martin once ran the Marathon des Sables, a six-day race across the Moroccan desert . But extreme exercise is not a predictable indicator of longevity (though the organization and persistence required to get there probably are).

Spend your time working at a job you like instead. “There’s a misconception about stress,” Dr. Friedman said. “People think everyone should take it easy.”

Rather, he said, “a hard job that is also stressful can be associated with longevity. Challenges, even if stressful, are also a link.” In the end, he said, “if people were involved, working hard, succeeded, were responsible - no matter what field they were in - they were more likely to live longer.”

Many people, of course, have to stay in a job they don’t like or don’t do well in. That’s bad stress, and they found that those people were more likely to die young.

Marriage , adding together the husband’s and the wife’s happiness, was a good predictor of future health and longevity. But more interestingly, it was the man’s happiness that was the better predictor of health and well-being - for both the husband and the wife. Her own happiness mattered much less to her future well-being.

And the single strongest social predictor of long life? A strong social network. Women’s tend to be stronger.

The project is not finished. Dr. Martin wants to look into other variables , like sleep patterns.

Dr. Friedman said he thought the most important unanswered question was about work, “retirement kinds of issues,” he said. “We know it’s not good to retire and go to the beach.”

But it’s also not good to stay in a stressful boring job. “We need to think about negotiating these transitions in a healthy way,” he added.


KATHERINE BOUTON