If you want to live a long, rich life, one of the best things you can do is to fill that life with happy feelings.

Such a sentiment is, of course, easier said than done. And that's why it's a topic psychologists are tackling with zeal, many spurred by a landmark study of elderly nuns. That report, by Deborah Danner, a psychologist at the University of Kentucky, found that nuns who'd expressed the most positive emotions in early adulthood—using words like "thankful" and "joy" in diary entries—lived about ten years longer than those who'd shown the fewest good feelings.

Barbara Fredrickson, a University of Michigan psychologist, is consumed with teasing out the causes of happy people's longer lives. Building on her studies of hostility and heart disease, Fredrickson believes a piece of the puzzle is how individuals cope with stress: Does a person deal with trouble head-on, or shy away?

An intriguing finding came from her research on the Sept. 11 terrorist attacks. A few months before the attacks, Fredrickson had studied a group of students to determine how quickly they bounced back from stress. Within two weeks of the attacks, she contacted them to see how they had fared during the crisis.

Not surprisingly, she discovered that the more resilient students had fewer signs of depression. But what she did not anticipate was that they had shown emotional growth in the months since they'd first been tested, and were now more optimistic and more satisfied with life than before. In rebounding from the crisis, they had counted their blessings, embraced love and friendship, and watched the news attentively but without fear.

Fredrickson believes that these people had harnessed the "undo" effect of positive emotions. In prior experiments, she and Michele Tugade, of Boston College, subjected students to a stressor by telling them they had only a few minutes to prepare a speech that would be critiqued by experts. Under such circumstances, almost everyone sees changes in "cardiovascular reactivity"—the cardiac equivalent of nervousness.

But when the stress was removed by telling the students they wouldn't actually have to give their speeches, people who viewed the test with amusement, interest and excitement saw their quick pulses "undone" more quickly than those who were angry about being fooled. This is important because high cardiac reactivity has been linked to heart disease.

Fredrickson also found that it is possible to speed the recovery of emotionally negative people by coaching them to view the experiment as a challenge, rather than a threat, and asking them to think of themselves as people who are capable of meeting that challenge.

One way to encourage such a change in mind-set is by therapy. "Therapists have an arsenal of tools to eliminate negative behaviors," she says. "The same methods could be used to train people to find more positive meaning and to build skills so that they are automatically optimistic."

Many people may also be able to adjust their attitudes on their own. For example, Fredrickson says, if she has to walk across campus, she could view that negatively—as a waste of time—or positively, as a chance to enjoy the outdoors, people-watch and get a bit of exercise. What's important, she believes, is not just pushing negative thoughts out of mind but reorienting to the positive.