The central role of prospection has emerged in recent studies of both conscious and unconscious mental processes, like one in Chicago that pinged nearly 500 adults during the day to record their immediate thoughts and moods. If traditional psychological theory had been correct, these people would have spent a lot of time ruminating. But they actually thought about the future three times more often than the past, and even those few thoughts about a past event typically involved consideration of its future implications.

When making plans, they reported higher levels of happiness and lower levels of stress than at other times, presumably because planning turns a chaotic mass of concerns into an organized sequence. Although they sometimes feared what might go wrong, on average there were twice as many thoughts of what they hoped would happen.

While most people tend to be optimistic, those suffering from depression and anxiety have a bleak view of the future — and that in fact seems to be the chief cause of their problems, not their past traumas nor their view of the present. While traumas do have a lasting impact, most people actually emerge stronger afterward. Others continue struggling because they over-predict failure and rejection. Studies have shown depressed people are distinguished from the norm by their tendency to imagine fewer positive scenarios while overestimating future risks.

They withdraw socially and become paralyzed by exaggerated self-doubt. A bright and accomplished student imagines: If I flunk the next test, then I’ll let everyone down and show what a failure I really am. Researchers have begun successfully testing therapies designed to break this pattern by training sufferers to envision positive outcomes (imagine passing the test) and to see future risks more realistically (think of the possibilities remaining even if you flunk the test).

Most prospection occurs at the unconscious level as the brain sifts information to generate predictions. Our systems of vision and hearing, like those of animals, would be overwhelmed if we had to process every pixel in a scene or every sound around us. Perception is manageable because the brain generates its own scene, so that the world remains stable even though your eyes move three times a second. This frees the perceptual system to heed features it didn’t predict, which is why you’re not aware of a ticking clock unless it stops. It’s also why you don’t laugh when you tickle yourself: You already know what’s coming next.

Behaviorists used to explain learning as the ingraining of habits by repetition and reinforcement, but their theory couldn’t explain why animals were more interested in unfamiliar experiences than familiar ones. It turned out that even the behaviorists’ rats, far from being creatures of habit, paid special attention to unexpected novelties because that was how they learned to avoid punishment and win rewards.