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Why does willpower often seem to fail us, just when we need it most?

Some researchers argue that willpower is a limited resource that wears out, like a muscle exhausted by overuse. Other experts say that our will may falter only if we think it’s fallible: if we believe we have unlimited self-control, we do. The answer matters, of course, because the distinctly human capacity to temper one’s impulses is essential to virtually all aspects of success.

Research has offered evidence in favor of both theories. Early studies suggested that willpower is a finite resource, finding that when people are repeatedly faced with exercising their self-control while making choices, their ability to make good decisions eventually declines. One of the earliest such studies found that when people first resist freshly baked cookies or luscious chocolates, it reduces their later ability to persevere in a mentally challenging task. Since that early study, more than 100 other experiments have reached similar conclusions, finding that decision fatigue reduces people’s control over everything from aggression to tolerance to pain. Obviously, as the number of decisions we face rises — from trivial, tedious ones like selecting which toothpaste to buy to major life-changing decisions like choosing a job or partner — these findings have important implications for how we cope.

In contrast, more recent research has challenged the idea that willpower is necessarily limited. Indeed, several studies have shown that reduced willpower is seen only in people who believe that their self-control is limited; people who think self-discipline is a function that can be maintained over time don’t experience the same depletion. Moreover, studies have also found that if you teach people that they have unlimited self-control, it eliminates the decline in discipline; teaching them that willpower is limited, however, has the opposite effect.

(MORE: Mind over Mind? Decision Fatigue Saps Willpower — if We Let It)

Now, new research, published in the Journal of Experimental Social Psychology, offers support for both perspectives, while clarifying the picture overall. In a series of experiments involving some 300 undergraduates, researchers led by Roy Baumeister, a professor of psychology at Florida State University, showed that believing in unlimited willpower helps shore up your resolve when depletion is mild, but not if you’re already running close to empty. “When people are more seriously depleted, belief in unlimited willpower actually made things worse,” says Baumeister.

In one experiment, Baumeister and his colleagues compared undergraduates who were given information that suggested that willpower was either limited or unlimited. The undergraduates were then asked to perform tasks designed to deplete their self-control: one group had to do two such tasks while another had to do four, and a third group did no depleting tasks.

Those in the two-task group were assigned to view multiple products and choose between them; later, they were also asked to look at words on a computer and type the first letter of the color of the word’s font — the catch was that the word itself spelled out a different color than the font. Those who were asked to do four tasks completed both of the assignments given to the other group; in addition, they were asked to watch an Eddie Murphy video while suppressing their laughter, and then, as their final task, they had to peruse a document and cross out every letter e, a task that was then made even harder as participants later had to cross out e‘s only in cases where a vowel occurred immediately after or two letters before it.

Following these challenges, researchers assessed participants’ self-control on two measures, including one in which they measured volunteers’ preference for a small immediate reward versus a larger delayed one. Among those who had completed two willpower-depleting tasks, their beliefs about willpower — whether it was limited or infinite — predicted their performance on the self-control tests. Those who had been primed to believe that willpower was unlimited did much better. But among the participants who had to perform four depleting tasks, the opposite was true: those who believed their willpower was fixed did just as well or significantly better on the self-control tests.

“It’s analogous to physical tiredness,” says Baumeister. “When you are just starting [to get tired], believing in unlimited strength or being Superman can help you continue to perform well, but at some point, it really does catch up with you.”

As for why faith in your ability to overcome may backfire when your task load is greatest, Baumeister suggests that it’s a matter of resource allocation. Like running a marathon, you need to pace yourself: if you don’t, and instead expend all your energy speeding through the first part of the race, positive thinking may not be enough to get you to the finish line.

(MORE: The Secrets of Self-Control: The Marshmallow Test 40 Years Later)

In another new paper, researchers at the University of Toronto suggest that depletion of willpower is more complicated. “There’s got to be something else going on,” says Michael Inzlicht, lead author of the research, which was published in Perspectives on Psychological Science. “The basic idea that we propose is that what looks like depletion are actually shifts in motivation and attention.”

For example, if you’ve recently exercised a lot of self-control, you might feel like you deserve a reward. “[Maybe] people just don’t feel like controlling themselves anymore,” says Inzlicht. “It’s not that they can’t, but they just don’t feel like it. They just worked hard, now they feel like they deserve a break.”

This motivational balance can be manipulated by various factors — including the belief that willpower is unlimited. Baumeister notes that in his study, he found that students who were prompted to see the study itself as making an important contribution showed less of a depletion effect.

“When you’re depleted, it’s not that you cannot; it’s that you don’t feel like engaging in self-control,” says Inzlicht. “If someone puts a gun to your head, no matter how tired you are, you’re going to try to do what they say. That shifts the question to, Why don’t you feel like doing it? If I offer a strong enough incentive, you can overcome that.”

Inzlicht also thinks attention plays an important role. “People may [stop] attending to cues that say, Hey, control is appropriate right now, and start paying more attention to, Hey, look, there’s a rewarding thing,” he says.

(MORE: To Keep Willpower from Flagging, Remember the F-word: Fun)

That means that to improve your willpower, you need to keep many factors in balance. When you are facing short-term challenges, believing that your will is indomitable helps. For longer or more stressful battles, knowing your actual limits can help you pace yourself and allow you to avoid temptations when you’re most vulnerable to them. Finally, being mindful of your attention, recognizing when you need a break — and allowing small treats that don’t actually get in the way of your goals — can also help.

In fact, studies have found that those who have the most self-discipline don’t spend all their time resisting temptation, but instead create environments that limit their exposure to anything that would distract or drain them. As any office worker with an Internet connection knows, that’s not easy, but hey, I did manage to finish this post.

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.