A radical new explanation from psychologists.

Flannery O’Connor once described the contradictory desires that afflict all of us with characteristic simplicity. “Free will does not mean one will,” she wrote, “but many wills conflicting in one man.” The existence of appealing alternatives, after all, is what makes free will free: What would choice be without inner debate? We’re torn between staying faithful and that alluring man or woman across the room. We can’t resist the red velvet cake despite having sworn to keep our calories down. We buy a leather jacket on impulse, even though we know we’ll need the money for other things. Everyone is aware of such inner conflicts. But how, exactly, do we choose among them? As it turns out, science has recently shed light on the way our minds reconcile these conflicts, and the result has surprising implications for the way we think about one of society’s most intractable problems: poverty.

In the 1990s, social psychologists developed a theory of “depletable” self-control. The idea was that an individual’s capacity for exerting willpower was finite—that exerting willpower in one area makes us less able to exert it in other areas. In 1998, researchers at Case Western Reserve University published some of the young movement’s first returns. Roy Baumeister, Ellen Bratslavsky, Mark Muraven, and Dianne Tice set up a simple experiment. They had food-deprived subjects sit at a table with two types of food on it: cookies and chocolates; and radishes. Some of the subjects were instructed to eat radishes and resist the sweets, and afterwards all were put to work on unsolvable geometric puzzles. Resisting the sweets, independent of mood, made participants give up more than twice as quickly on the geometric puzzles. Resisting temptation, the researchers found, seemed to have “produced a ‘psychic cost.’”

Over the intervening 13 years, these results have been corroborated in more than 100 experiments. Researchers have found that exerting self-control on an initial task impaired self-control on subsequent tasks: Consumers became more susceptible to tempting products; chronic dieters overate; people were more likely to lie for monetary gain; and so on. As Baumeister told Teaching of Psychology in 2008, “After you exert self-control in any sphere at all, like resisting dessert, you have less self-control at the next task.”

In addition, researchers have expanded the theory to cover tradeoff decisions, not just self-control decisions. That is, any decision that requires tradeoffs seems to deplete our ability to muster willpower for future decisions. Tradeoff decisions, like choosing between more money and more leisure time, require the same conflict resolution as self-control decisions (although our impulses appear to play a smaller role). In both cases, willpower can be understood as the capacity to resolve conflicts among choices as rationally as possible, and to make the best decision in light of one’s personal goals. And, in both cases, willpower seems to be a depletable resource.

This theory of depletable willpower has its detractors, and, as in most academic topics studied across disciplinary fields, one finds plenty of disputes over the details. But this model of self-control is now one of the most prominent theories of willpower in social psychology, at the core of what E. Tory Higgins of Columbia University described in 2009 as “an explosion of scientific interest” in the topic over the last decade. Some skeptics correctly emphasize the vital role of motivation, and some emphasize instead that “attention” is limited. But the core of the breakthrough is that resolving conflicts among choices is expensive at a cognitive level and can be unpleasant. It causes mental fatigue.