Most of the contributors to the new book agree that this peculiar irrationality stems from our relationship to time—in particular, from a tendency that economists call “hyperbolic discounting.” A two-stage experiment provides a classic illustration: In the first stage, people are offered the choice between a hundred dollars today or a hundred and ten dollars tomorrow; in the second stage, they choose between a hundred dollars a month from now or a hundred and ten dollars a month and a day from now. In substance, the two choices are identical: wait an extra day, get an extra ten bucks. Yet, in the first stage many people choose to take the smaller sum immediately, whereas in the second they prefer to wait one more day and get the extra ten bucks. In other words, hyperbolic discounters are able to make the rational choice when they’re thinking about the future, but, as the present gets closer, short-term considerations overwhelm their long-term goals.