Most of the time we are pretty sure that we will not die in minutes. But we all die sometime, and we all risk forgoing the opportunity to enjoy something if we defer it too long. In our ancestors' nomadic lifestyle, without an ability to accumulate possessions or to count on long-lived social institutions like depositors' insurance, the payoffs for consumption must have been even higher. But even if they were not, some urge to indulge now had to be built into our emotions. Most likely, we evolved a mechanism to estimate our longevity and the opportunities and risks posed by difference choices (eating now or later, setting up camp or pushing on) and to tune the emotions accordingly.

The political scientist James Q. Wilson and the psychologist Richard Hermstein have pointed out that many criminals act as if they discount the future steeply. A crime is a gamble whose payoff is immediate and whose possible cost comes later. They attributed the discounting to low intelligence. The psychologists Martin Daly and Margo Wilson have a different explanation. In the American inner cities, life expectancy for young males is low, and they know it. (In Hoop Dreams, the documentary about aspiring basketball players in a Chicago ghetto, there is an arresting scene in which the mother of one of the boys rejoices that he is alive on his eighteenth birthday). Moreover, the social order and long-term ownership rights which would guarantee that investments are repaid are tenuous. These are precisely the circumstances in which steeply discounting the future - taking risks, consuming rather than investing - is adaptive.

More puzzling is myopic discounting: the tendency in all of us to prefer a large late reward to a small early one, but then to flip our preferences as time passes and both rewards draw nearer. A familiar example is deciding before dinner to skip dessert (a small early reward) in order to lose weight (a large late one), but succumbing to temptation when the waiter takes the dessert orders. Myopic discounting is easy to produce in the lab: give people (or pigeons, for that matter) two buttons, one delivering a small reward now, the other delivering a large reward later, and the subject will flip from choosing the large reward to choosing the small reward as the small one becomes imminent. The weakness of the will is an unsolved problem in economics and psychology alike.