Willpower, like a bicep, can only exert itself so long before it gives out; it's an extremely limited mental resource.

Given its limitations, New Year's resolutions are exactly the wrong way to change our behavior. It makes no sense to try to quit smoking and lose weight at the same time, or to clean the apartment and give up wine in the same month. Instead, we should respect the feebleness of self-control, and spread our resolutions out over the entire year. Human routines are stubborn things, which helps explain why 88% of...