"What I have in mind is an act or decision that a person takes decisively at some particular point in time, about which the person's preferences differ from what they were earlier, when the prospect was contemplated but the decision was still in the future," he wrote in "Ethics, Law and the Exercise of Self-Command," (available online at http://www.tannerlectures.utah.edu/lectures/schelling83.pdf). "If the person could make the final decision about that action at the earlier time, precluding a later change in mind, he would make a different choice from what he knows will be his choice on that later occasion."

New Year's resolutions help the earlier self overrule the later one by raising the cost of straying. "More is threatened by failure than just the substance of the resolution: one's personal constitution is violated, confidence demoralized, and the whole year spoiled. At least one can try to make it so," wrote Professor Schelling in "The Intimate Contest for Self-Command," a 1980 essay in his book "Choice and Consequence: Perspectives of an Errant Economist" (Harvard University Press, 1984).

As many a broken resolution demonstrates, those consequences often are not a big enough deterrent. To make success more likely, Professor Schelling's work suggests a few additional strategies.

One is a mild precommitment: not keeping sweets or tobacco in the house, for instance. At the very least, this step forces you to delay indulgence until you can go to the store -- and allows time to recover your resolve.

Another approach is to use bright-line rules, which make it harder to cheat through clever reinterpretation. That may explain why many people find it easier to eliminate whole categories of food, like carbohydrates, rather than simply to cut back on calories.

"Just as it may be easier to ban nuclear weapons from the battlefield in toto than through carefully graduated specifications on their use, zero is a more enforceable limit on cigarettes or chewing gum than some flexible quantitative ration," Professor Schelling wrote.

He once resolved to smoke "only after the 'evening meal.' " That rule "led to tortured reasoning Thanksgiving afternoon, or flying west across the Atlantic with perpetual afternoon, and it stimulated lots of token sandwiches on leaving the ski slopes to drive home."