ROBERT SIEGEL, host:

From NPR News, it's ALL THINGS CONSIDERED. I'm Robert Siegel.

MELISSA BLOCK, host:

And I'm Melissa Block.

This week, the South Carolina GOP censured Governor Mark Sanford for his extramarital affair. It appears the infidelity may not cost him his job, but it certainly was a costly choice. So why would he make it? This is the kind of human decision economists love to study. So Chana Joffe-Walt from our Planet Money team gave them a challenge: explain affairs.

CHANA JOFFE-WALT: Tim Harford says economics can help. He is - surprise - an economist. And he says there's a model already that applies here. It's called "An Economic Theory of Extramarital Affairs" by Ray Fair.

Mr. TIM HARFORD (Economist): What he focuses on is the amount of time it takes to have an affair. This must be one of the things that weighs on your mind a lot. It's awfully time consuming. And if you're a busy person, like, I don't know, the governor of South Carolina, for instance, it's going to be very difficult to clear your schedule and make time for some quality, you know, affair.

Mr. CHRIS PROCTOR (Marketing Representative): Oh, sure. Yeah, when you're busy, you know, as I am, it is, absolutely - difficult to find the time. And that's one of the considerations is, you know, where do you have the time?

JOFFE-WALT: This is not Governor Mark Sanford. Sorry, that was tricky. This is a different married guy who had an affair, Chris Proctor. He's a marketing representative in St. Louis and he met a woman from Virginia at this weeklong gathering. They carried on a nine month affair. And looking back on it, it was expensive.

Mr. PROCTOR: The cell phone plan that I hadn't planned on, so that was 50, 60 bucks a month or whatever, by the time you did text messaging, the phone calls on top of that. And trips, those were, I figure probably, you know, nine months probably cost me between three and $5,000. So it wasn't cheap.

JOFFE-WALT: Proctor started to add in other costs: a potential divorce, the cost of his kids not growing up with two married parents and he decided the affair was not worth it to him. Or, as Harford would say, the utility Proctor was receiving from the affair was not worth the money and the opportunity cost - what he was giving up in order to have it.

Mr. HARTFORD: The way economists tend to think about any problem, any choice, is you have preferences. You can't satisfy all the preferences all at once 'cause you've got a budget constraint, you've got a time constraint. And so, given all your different preferences, you maximize your happiness. You choose rationally the best option for you.

Ms. KATHERINE JOHNSON: Actually, I met him at the gym. We both had lived in Indiana and he did it, you know, that kind of thing. So he was, like, oh, well, you should come to lunch with me. And so we went out to lunch.

JOFFE-WALT: This is Katherine Johnson. She works in a law firm in D.C. and she's not married, but this gym guy was. After lunch, there's another lunch, there was lunch every day and other stuff, you know. Johnson wasn't betraying a personal commitment. The affair didn't cost her a lot of money or anything, but she did factor in other costs.

Ms. JOHNSON: You can't actually go out on a date and can't really tell my mom about this. My own integrity was at stake.

JOFFE-WALT: And what about the benefits?

Ms. JOHNSON: It's exciting. The thought of it is definitely very exciting. For me, the benefit was like the companionship and the camaraderie and having somebody to talk to throughout the day.

JOFFE-WALT: Those benefits outweighed the costs for Johnson for almost a year. And then they didn't anymore, and she ended it.

So, we've got the cheater, we've got the person he is cheating with and of course there is one last person with the wrong cost and benefits - the one who is being cheated on. This is the person who writes Tim Hartford a lot, the British economist. He has an advice column in the Financial Times. He plays this nutty guy who only reads economics papers and that is all he knows.

Here's a reading of one of the letters that came in.

Unidentified Woman: Dear Economist, I am starting to suspect that my husband is having an affair. How can I find out? Yours sincerely, Mrs. F. in Oxford.

JOFFE-WALT: Harford says, ah, yes, typical information problem. Mrs. F. probably knows people who know whether or not her husband is having an affair, but they don't want to be the one to tell her. Thus, she must set up an information market: have her friends and acquaintances make bets on her husband.

Mr. HARTFORD: She should issue a bond that would pay money if her husband was caught having an affair by a certain date. Another bond that would pay money if her husband was not caught having an affair by a certain date. So by looking at the price of these two bonds, she gets a sense of whether there are people out there who think that her husband is betraying her.

JOFFE-WALT: Obviously this is crazy. It's also maybe brilliant. It seems to hit people one or the other. Economics is helpful in lining up the rational, measurable benefits of cheating. And sure, Chris Proctor says if he had listened to his internal economist, he probably would have decided that $5,000 was more than he wanted to pay for an affair. But he didn't listen.

For politicians, the costs are obviously huge, seem so clearly to outweigh the benefits. But every few months, another politician and another apology. People screw up these models. They are so undependable. Maybe if we just work the passion of X into the utility function - forget it.

Chana Joffe-Walt, NPR News.

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