We also studied this phenomenon in the real world. In one study, we followed 153 college students over five weeks. During stressful times, like final-exam week, students who believed that willpower was not limited reported eating less junk food and procrastinating less than students who did not share that belief. They also showed more academic growth, earning better grades that term than their “pessimistic” counterparts.

Furthermore, when we taught college students that willpower was not so limited, they showed similar increases in willpower. They reported procrastinating only once or twice a week instead of the two to three times a week reported by students in a control condition, and they cut down on excess spending, going beyond their budgets less than once a week instead of once or twice a week.

How does this happen? People who think that willpower is limited are on the lookout for signs of fatigue. When they detect fatigue, they slack off. People who get the message that willpower is not so limited may feel tired, but for them this is no sign to give up — it’s a sign to dig deeper and find more resources.

What about the glucose idea, which seems supported by so much science? Dr. Baumeister and Mr. Tierney describe studies showing that giving people glucose (in the form of a sugar drink) restores their willpower. But in our latest research we found that when people believe in willpower they don’t need sugar — they perform well whether they consume sugar or not. Sugar helps people only when they think that willpower is sharply limited. It’s not sugar we need; it’s a change in mind-set.

To be sure, willpower is not completely unlimited. Food and rest are of course necessary for functioning, and many struggles that people face are quite difficult. The question is how often we need extra sugar boosts. Messages suggesting that willpower is severely limited and that we need constant sugar boosts are bound to further inflate the American waistline and hinder our ability to achieve our goals.

At stake in this debate is not just a question about the nature of willpower. It’s also a question of what kind of people we want to be. Do we want to be a people who dismiss our weaknesses as unchangeable? When a student struggles in math, should we tell that student, “Don’t worry, you’re just not a math person”? Do we want him to give up in the name of biology? Or do we want him to work harder in the spirit of what he wants to become?