As Gilbert points out, this glitch is also significant when it comes to negative events like losing a job or the death of someone we love, in response to which we project a permanently inconsolable future. ''The thing I'm most interested in, that I've spent the most time studying, is our failure to recognize how powerful psychological defenses are once they're activated,'' Gilbert says. ''We've used the metaphor of the 'psychological immune system' -- it's just a metaphor, but not a bad one for that system of defenses that helps you feel better when bad things happen. Observers of the human condition since Aristotle have known that people have these defenses. Freud spent his life, and his daughter Anna spent her life, worrying about these defenses. What's surprising is that people don't seem to recognize that they have these defenses, and that these defenses will be triggered by negative events.'' During the course of my interviews with Gilbert, a close friend of his died. ''I am like everyone in thinking, I'll never get over this and life will never be good again,'' he wrote to me in an e-mail message as he planned a trip to Texas for the funeral. ''But because of my work, there is always a voice in the back of my head -- a voice that wears a lab coat and has a lot of data tucked under its arm -- that says, 'Yes, you will, and yes, it will.' And I know that voice is right.''

Still, the argument that we imperfectly imagine what we want and how we will cope is nevertheless disorienting. On the one hand, it can cast a shadow of regret on some life decisions. Why did I decide that working 100 hours a week to earn more would make me happy? Why did I think retiring to Sun City, Ariz., would please me? On the other hand, it can be enlightening. No wonder this teak patio set hasn't made me as happy as I expected. Even if she dumps me, I'll be O.K. Either way, predicting how things will feel to us over the long term is mystifying. A large body of research on well-being seems to suggest that wealth above middle-class comfort makes little difference to our happiness, for example, or that having children does nothing to improve well-being -- even as it drives marital satisfaction dramatically down. We often yearn for a roomy, isolated home (a thing we easily adapt to), when, in fact, it will probably compromise our happiness by distancing us from neighbors. (Social interaction and friendships have been shown to give lasting pleasure.) The big isolated home is what Loewenstein, 48, himself bought. ''I fell into a trap I never should have fallen into,'' he told me.

Loewenstein's office is up a narrow stairway in a hidden corner of an enormous, worn brick building on the edge of the Carnegie-Mellon campus in Pittsburgh. He and Gilbert make for an interesting contrast. Gilbert is garrulous, theatrical, dazzling in his speech and writing; he fills a room. Loewenstein is soft-spoken, given to abstraction and lithe in the way of a hard-core athlete; he seems to float around a room. Both men profess tremendous admiration for the other, and their different disciplines -- psychology and economics -- have made their overlapping interests in affective forecasting more complementary than fraught. While Gilbert's most notable contribution to affective forecasting is the impact bias, Loewenstein's is something called the ''empathy gap.''

Here's how it expresses itself. In a recent experiment, Loewenstein tried to find out how likely people might be to dance alone to Rick James's ''Super Freak'' in front of a large audience. Many agreed to do so for a certain amount of money a week in advance, only to renege when the day came to take the stage. This sounds like a goof, but it gets at the fundamental difference between how we behave in ''hot'' states (those of anxiety, courage, fear, drug craving, sexual excitation and the like) and ''cold'' states of rational calm. This empathy gap in thought and behavior -- we cannot seem to predict how we will behave in a hot state when we are in a cold state -- affects happiness in an important but somewhat less consistent way than the impact bias. ''So much of our lives involves making decisions that have consequences for the future,'' Loewenstein says. ''And if our decision making is influenced by these transient emotional and psychological states, then we know we're not making decisions with an eye toward future consequences.'' This may be as simple as an unfortunate proclamation of love in a moment of lust, Loewenstein explains, or something darker, like an act of road rage or of suicide.

Among other things, this line of inquiry has led Loewenstein to collaborate with health experts looking into why people engage in unprotected sex when they would never agree to do so in moments of cool calculation. Data from tests in which volunteers are asked how they would behave in various ''heat of the moment'' situations -- whether they would have sex with a minor, for instance, or act forcefully with a partner who asks them to stop -- have consistently shown that different states of arousal can alter answers by astonishing margins. ''These kinds of states have the ability to change us so profoundly that we're more different from ourselves in different states than we are from another person,'' Loewenstein says.

Part of Loewenstein's curiosity about hot and cold states comes from situations in which his emotions have been pitted against his intellect. When he's not teaching, he treks around the world, making sure to get to Alaska to hike or kayak at least once a year. A scholar of mountaineering literature, he once wrote a paper that examined why climbers have a poor memory for pain and usually ignore turn-back times at great peril. But he has done the same thing himself many times. He almost died in a whitewater canoeing accident and vowed afterward that he never wanted to see his runaway canoe again. (A couple of hours later, he went looking for it.) The same goes for his climbing pursuits. ''You establish your turn-back time, and then you find yourself still far from the peak,'' he says. ''So you push on. You haven't brought enough food or clothes, and then as a result, you're stuck at 13,000 feet, and you have to just sit there and shiver all night without a sleeping bag or warm clothes. When the sun comes up, you're half-frozen, and you say, 'Never again.' Then you get back and immediately start craving getting out again.'' He pushes the point: ''I have tried to train my emotions.'' But he admits that he may make the same mistakes on his next trip.

Would a world without forecasting errors be a better world? Would a life lived without forecasting errors be a richer life? Among the academics who study affective forecasting, there seems little doubt that these sorts of questions will ultimately jump from the academy to the real world. ''If people do not know what is going to make them better off or give them pleasure,'' Daniel Kahneman says, ''then the idea that you can trust people to do what will give them pleasure becomes questionable.'' To Kahneman, who did some of the first experiments in the area in the early 1990's, affective forecasting could greatly influence retirement planning, for example, where mistakes in prediction (how much we save, how much we spend, how we choose a community we think we'll enjoy) can prove irreversible. He sees a role for affective forecasting in consumer spending, where a ''cooling off'' period might remedy buyer's remorse. Most important, he sees vital applications in health care, especially when it comes to informed consent. ''We consider people capable of giving informed consent once they are told of the objective effects of a treatment,'' Kahneman says. ''But can people anticipate how they and other people will react to a colostomy or to the removal of their vocal cords? The research on affective forecasting suggests that people may have little ability to anticipate their adaptation beyond the early stages.'' Loewenstein, along with his collaborator Dr. Peter Ubel, has done a great deal of work showing that nonpatients overestimate the displeasure of living with the loss of a limb, for instance, or paraplegia. To use affective forecasting to prove that people adapt to serious physical challenges far better and will be happier than they imagine, Loewenstein says, could prove invaluable.