Ed Diener, alumni professor of psychology at the University of Illinois, has been studying happiness -- or "subjective well-being," as he calls it -- for 20 years. One of the questions he hears most often is whether being happy all the time is really such a good idea. Certainly critical, negative thinking has its place. As Positive Psychology advocate Martin Seligman points out, you probably wouldn't want to fly during a snowstorm with a pilot who didn't de-ice the plane's wings because he felt really good about his piloting skills. "If we are happy, might we achieve less, be less good citizens, or be just plain dumb?" asks Diener on his extensive Web site on the subject (www.psych.uiuc.edu/~ediener/). Here's his response:

Happy people:

* Have stronger immune systems, and there is some evidence they live longer.

* Are more creative, at least according to lab tests, than unhappy people.

* Are better citizens at work -- they tend to help others more and skip work less.

* Are more successful -- they earn more, have better marriages, get more job interviews.

* Do better in social relationships.

* Are better able to cope with difficult situations.

* Like themselves and other people more, and others like them in return.

* Are more helpful and altruistic.

But pessimists have it all over their happy brethren in the area of good judgment and decision-making, Diener says. "Those in a positive mood have been found in lab studies to use stereotypes more, to be less logical and to be more biased in their judgments," he writes.