C'mon, Get Happy? It's Easier Said Than Done.

The finding is of especial importance to dieters and might explain why some people go off their diets. Breaking two cookies into quarters and eating one piece on each of eight days is likely to produce no happiness at all. Better to eat two cookies at once, and then wait a week before grabbing another two.

Rule 2. Happiness often comes from what you don't know.

People generally dislike uncertainty and often go out of their way to reduce uncertainty. But in a series of experiments published in the Journal of Personality and Social Psychology in 2005, Gilbert and his colleagues showed that people who received gifts for no apparent reason felt happier than those who received identical gifts for reasons that were clear. Participants also reported more pleasure when they got a compliment without knowing who said it than when they knew who it was.

"When we have uncertainty about the nature or cause or meaning of any event, it amplifies the emotional consequences of that event," Gilbert said in an interview. "When you don't understand why a bad thing happened, it is worse. When you don't understand why a good thing happened, it's better."

While uncertainty about negative and dangerous things is unpleasant, the psychologists argued that we foolishly seek to apply the same principle to things that we know are positive. How might someone apply this idea to their lives? If you know a romantic comedy has a happy ending, Gilbert wrote in one paper, consider walking out of the theater before the movie ends.

Rule 3: Keeping your options open won't necessarily make you happier.

Given the choice, people like to keep their options open.

When researchers asked people whether they preferred to take home a poster they had to keep or take home one that could be exchanged later on, most people chose the latter. But it was people who made irrevocable choices early on who ended up happier with their posters.

Gilbert said the finding prompted him to go home and propose to the woman he had been living with: "I always thought love causes marriage, but my data said marriage causes love," he said. "When you lock yourself in something you cannot get out of, you will find ways to be happier. . . . I do love my wife more than I loved my girlfriend, and they are the same person."

Rule 4: The things you fear are not as bad as you think.

Gilbert said a number of experiments have found that people overestimate how unhappy they will be after a tragic event, and this keeps them from taking risks in life.

Torn between life choices? The experimental results suggest the worst option is usually indecision -- no matter what choice people make, they are more likely to be okay with the consequences than if they stay on the fence.

Why do we systematically fail to predict how happy and unhappy we will be? For one thing, predicting the future is inherently difficult. But even when we know what is going to happen, we base our estimates of our future happiness on the people we are today, and fail to appreciate not only that we will be different tomorrow, but that the very things we seek will change who we are.

When we seek to explain a pleasurable mystery, or to avoid risks that may bring us sorrow, or desperately to seek some symbol of success, what we fail to appreciate in advance is how quickly we will absorb such events should they come to pass -- and move on.

"For as long as anyone can remember," Gilbert once noted, "people have hungered for information about their personal futures, confident that if they knew their fates, they would also know their fortunes. Alas, knowing the future is not the same as knowing how much one will like it when one gets there."