Credit cards have been extremely profitable to banks. They're profitable not from the fees they collect from the retailers that use the credit cards, that pays the bills, but the real profits come from the interest payments and the charges to users that are unexpected.

I think one lesson we have to learn is that there's a lot more risk than we're giving credit to, a lot more what economist calls systematic risk.

It turns out, that men, when they're taking care of their business, they're not fully attending to the task at hand, but, I'm sure there's an evolutionary explanation for this, if you give them a target, they will aim.

Is there a market for somebody selling a credit card that helps people pay down their balances? I think the question is yes. But it would have to be sold by a bank that's really willing to invest in being a trusted partner with its consumers, because they will make less money on each consumer.

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.

The same with the mortgage brokers that were selling people mortgages they couldn't afford. We shouldn't pay them on each mortgage they write. They should have what they call "skin in the game," where they've got to reimburse us if the guy who sold the mortgage defaults.

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.