Over the last decade a number of studies and books have likened willpower to a single muscle. According to this model, all of our choices and temptations are tugging on the same “muscle” in the brain. So the strain of resisting a luscious chocolate chip cookie in front of us may leave us with less willpower to stick with a difficult test afterward. The model is useful in understanding, for example, why even health-conscious people may find themselves making grossly unhealthy choices when driving through McDonald’s. You are in a hurry; you are hungry; you are confronted by a dizzying menu of items, combinations, and prices — and prodded with more choices by voice prompts. You leave feeling vaguely guilty, asking yourself, “Why did I agree to be supersized?” And the answer is that the drive-through experience was designed to stretch your willpower “muscle” to the breaking point — and it worked. Now, researchers from Kellogg School of Management at Northwestern University are proposing a similar model to explain more complex behaviors, such as what happens when we lose our inhibitions, either …