Mind over Mind? Decision Fatigue Saps Willpower — if We Let It

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Are the endless decisions of modern life leading to decision fatigue, depleting our willpower to the point that we end up making increasingly poor, even self-destructive choices?

In an excerpt adapted from his forthcoming book, New York Times writer John Tierney makes the case that they do. The piece opens with a description of research on parole decisions made by Israeli judges, which found that the judges were dramatically more likely to free prisoners earlier in the day (before the judges had made any big decisions) or right after lunch (when they were rested and replenished), compared with other times.

Tierney writes:

The judges’ erratic judgment was due to the occupational hazard of being, as George W. Bush once put it, “the decider.” The mental work of ruling on case after case, whatever the individual merits, wore them down. This sort of decision fatigue can make quarterbacks prone to dubious choices late in the game and C.F.O.’s prone to disastrous dalliances late in the evening. It routinely warps the judgment of everyone, executive and nonexecutive, rich and poor — in fact, it can take a special toll on the poor. Yet few people are even aware of it, and researchers are only beginning to understand why it happens and how to counteract it.

Decision fatigue helps explain why ordinarily sensible people get angry at colleagues and families, splurge on clothes, buy junk food at the supermarket and can’t resist the dealer’s offer to rustproof their new car. No matter how rational and high-minded you try to be, you can’t make decision after decision without paying a biological price.

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But Stanford psychology professor Carol Dweck has found an interesting twist. Her research shows that while decision fatigue does occur, it primarily affects those who believe that willpower runs out quickly.

“We find that people get fatigued or depleted after a taxing task only when they believe that willpower is a limited resource, but not when they believe it’s not so limited,” says Dweck. Because the majority of people tend to view their stores of willpower as being scarce, the effect appears universal in studies that don’t distinguish between people holding various beliefs.

“If you look over our studies, we get effects that look like overall fatigue. But when you break it down into the people who believe willpower is limited and those who don’t, you have two separate groups,” says Dweck. “In fact, in some cases, the people who believe that willpower is not so limited actually perform better after a taxing task.”

In the Times piece, Tierney cites research showing that consumption of sugar restores depleted willpower. One study involved giving people either thick, tasty milkshakes or bland, low-fat glop after a series of decision-based tasks had reduced their performance.

As long as the food contained glucose, it didn’t matter whether the stuff was rewardingly tasty. Although the researchers initially thought that indulging in something pleasurable (i.e., a delicious milkshake) would reduce participants’ decision fatigue, it turned out that it was the glucose, not the pleasure, that actually boosted brain performance.

Other studies have shown that, similarly, sugar consumption reduces people’s expressions of prejudice, presumably by helping them maintain self-restraint.

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Dweck has tested how this idea fits with her theory on willpower: “We find that sugar improves self-control only for the people who believe in limited willpower,” she says. “We think that people who believe in limited willpower are always checking to see how fatigued they are. If they feel fatigued, they show a deficit. If you give them sugar and they get a surge of energy, they don’t show a deficit.”

Dweck has also studied how changing people’s beliefs about the boundaries of willpower affects their performance. In one study, a group of college students were taught that willpower isn’t particularly limited and were asked to write essays convincing others of this belief; another group of students were given instruction on time management and asked to write essays about that topic.

Dweck followed the two groups early in the semester, when classes were less stressful, and around final-exam time, when tensions ran higher. “During final exams, the two groups pulled apart,” Dweck says. “The group that got the [willpower] training ate less unhealthy food, procrastinated less and engaged in less reckless spending.”

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But even Dweck’s best-performing participants don’t believe that willpower is infinite. Rather, “these people believe in momentum. Once you’re working hard, you get energized; once you resist something tempting, you’re better able to keep on resisting,” she says. “They don’t think it’s infinite, and neither do we. But it seems to be far less limited than some research suggests.”

Overall, the body of evidence suggests that it’s best to view willpower as something that can be strengthened like a muscle: hard work will improve your endurance and discipline. Believing in your ability to fortify your willpower will in turn help you persist.

Similarly, however, both your willpower and muscles can be overloaded if you don’t take breaks and build strength over time. And a little sugar doesn’t hurt either.

Maia Szalavitz is a health writer for TIME.com. Find her on Twitter at @maiasz. You can also continue the discussion on TIME Healthland‘s Facebook page and on Twitter at @TIMEHealthland.