Typically, people who exercise start eating better and becoming more productive at work. They smoke less and show more patience with colleagues and family. They use their credit cards less frequently and say they feel less stressed. It's not completely clear why. But for many people, exercise is a keystone habit that triggers widespread change.

If you want to do something that requires willpower - like going for a run after work - you have to conserve your willpower muscle during the day.

When people have a willpower failure, it's because they haven't anticipated a situation that's going to come along.

Once people learned how to believe in something, that skill started spilling over to other parts of their lives, until they started believing they could change. Belief was the ingredient that made a reworked habit loop into a permanent behavior.

If you believe you can change - if you make it a habit - the change becomes real.

What we know from lab studies is that it's never too late to break a habit. Habits are malleable throughout your entire life. But we also know that the best way to change a habit is to understand its structure - that once you tell people about the cue and the reward and you force them to recognize what those factors are in a behavior, it becomes much, much easier to change.

The same process that makes AA so effective—the power of a group to teach individuals how to believe—happens whenever people come together to help one another change. Belief is easier when it occurs within a community.