We are surrounded by temptation, whether it’s reaching for an unhealthful snack… making an impulse purchase… or lighting a cigarette. We like to think that we can resist these daily temptations simply by summoning our willpower. But that’s harder than you think, say Northwestern University Kellogg School of Management researchers in Evanston, Illinois, who found that people think that they have more willpower than they actually do.

The researchers examined college students’ belief in their ability to control their impulses and behavior. They found that participants who overestimated their capacity to resist temptation tended to expose themselves to more temptation than they were able to handle. As a result, these students were actually more likely to make poor choices and to abandon self-restraint — for example, choosing an unhealthful snack or breaking their resolve to refrain from smoking — than their counterparts who had lower expectations of their capacity to resist temptation.

These results may help explain why many people who are fighting addiction, obesity and other unhealthful lifestyle habits have so much difficulty making positive changes—despite their best intentions. Make a plan to avoid excessive contact with objects that tempt you—and avoid situations where temptation abounds.