When should we nudge and when should we shove, I think, it's a political judgment. Obviously in some situations we need shoves, we need laws. Fraud is against the law, murder is against the law, drunk-driving is against the law. We don't need just nudges.

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

It would be much more consumer friendly for them to beep you when you swipe your card that says, uh-oh you're over your limit, are you sure you want to use that?

There's a second component of a good savings plan, which is something that a colleague of mine called Schlomo Benartzi and I developed many years ago, that we call "save more tomorrow."

The assumption that everybody will figure out how much they have to save and then will just implement that plan is obviously preposterous.

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.

"Save more tomorrow" is a nudge to help people do what they know they want to do, which is save more, but they can't bring themselves to save more now. Just like many of us are planning to go on diets next month, or maybe in two months, certainly not tonight.