But recent long-term studies have revealed that the happiness thermostat is more malleable than the popular theory maintained, at least in its extreme form. "Set-point is not destiny," says psychologist Ed Diener of the University of Illinois.

One new study showing change in happiness levels followed thousands of Germans for 17 years. It found that about a quarter changed significantly over that time in their basic level of satisfaction with life. (That's a popular happiness measure; some studies sample how one feels through the day instead.) Nearly a tenth of the German participants changed by three points or more on a 10-point scale.

Other studies show an effect of specific life events, though of course the results are averages and can't predict what will happen to particular individuals. Results show long-lasting shadows associated with events like serious disability, divorce, widowhood, and getting laid off.

The boost from getting married, on the other hand, seems to dissipate after about two years, says psychologist Richard E. Lucas of Michigan State University.

What about the joys of having children? Parents recall those years with fondness, but studies show childrearing takes a toll on marital satisfaction, Harvard psychologist Daniel Gilbert notes in his recent book, "Stumbling on Happiness." Parents gain in satisfaction as their kids leave home, he said.

"Despite what we read in the popular press," he writes, "the only known symptom of 'empty nest syndrome' is increased smiling."

Gilbert says people are awful at predicting what will make them happy. Yet, Lucas says, "most people are happy most of the time." That is, in a group of people who have reasonably good health and income, most will probably rate a 7.5 or so on a happiness scale of zero to 10, he says.

Still, many people want to be happier. What can they do? That's where research by Lyubomirsky, Seligman and others comes in.

The think-of-three-good- things exercise that Miller, the motivational speaker, found so simplistic at first is among those being tested by Seligman's group at the University of Pennsylvania. People keep doing it on their own because it's immediately rewarding, said Seligman colleague Acacia Parks. It makes people focus more on good things that happen, which might otherwise be forgotten because of daily disappointments, she said.

Miller said the exercise made her notice more good things in her day, and that now she routinely lists 10 or 20 of them rather than just three.

A second approach that has shown promise in Seligman's group has people discover their personal strengths through a specialized questionnaire and choose the five most prominent ones. Then, every day for a week, they are to apply one or more of their strengths in a new way.