Self-control separates us from our ancient ancestors and the rest of the animal kingdom, thanks to our large prefrontal cortex. It is the ability to subdue our impulses in order to achieve longer-term goals. Rather than responding to immediate impulses, we can plan, evaluate alternative actions, and, often enough, avoid doing things we'll later regret. The ability to exert self-control is typically called willpower. It is what allows us to direct our attention, and it underlies all kinds of achievement. There is significant debate in science as to whether or not willpower is a finite resource. Studies demonstrate that exercising willpower makes heavy demands on mental energy, notably on reserves of glucose, the brain's preferred fuel, creating ego depletion. It's one reason we're more apt to reach for that chocolate chip cookie when we're feeling stressed than when we're feeling on top of the world. Recently, scientists have failed to replicate some of the studies underlying the concept of ego depletion, and more research is underway.