So, what's a nudge? A nudge is some small feature of the environment that attracts our attention and alters our behavior.

When an economist says the evidence is "mixed," he or she means that theory says one thing and data says the opposite.

A good rule of thumb is to assume that everything matters.

he card companies will often, as a courtesy, honor that credit card, but hit you with a penalty. And you keep swiping your card for $3 at Starbucks for your latté, and you're getting hit with a $25 penalty because it's over your credit limit.

Most people start claiming benefits within a year of when they become eligible, although benefits increase substantially if they wait.

Retirement savings is probably behavioral economists' greatest success story. It is a prototypical behavioral-economics problem because saving for retirement is cognitively hard - figuring out how much to save - and requires self-control.

People worry that if they buy an annuity and then die before the policy starts to pay off, their heirs will lose out. I tell them, "What you should be more worried about is if you outlive your money, you will have to move in with your kids. Ask your kids which of these outcomes they are more worried about."