How does motivation work? What does it take to get you to go the extra mile? Is it a social or market transaction that presses your buttons?

Here’s a fascinating video and excerpt from Dan Ariely’s  book, Predictably Irrational (Barnes and Noble).

You are at your mother-in-law’s house for Thanksgiving dinner, and what a sumptuous spread she has put on the table for you! The turkey is roasted to a golden brown; the stuffing is homemade and exactly the way you like it. Your kids are delighted: the sweet potatoes are crowned with marshmallows. And your wife is flattered: her favorite recipe for pumpkin pie has been chosen for dessert.

The festivities continue into the late afternoon. You loosen your belt and sip a glass of wine. Gazing fondly across the table at your mother- in- law, you rise to your feet and pull out your wallet. “Mom, for all the love you’ve put into this, how much do I owe you?” you say sincerely. As silence descends on the gathering, you wave a handful of bills. “Do you think three hundred dollars will do it? No, wait, I should give you four hundred!”

This is not a picture that Norman Rockwell would have painted. A glass of wine falls over; your mother- in- law stands up red- faced; your sister- in- law shoots you an angry look; and your niece bursts into tears. Next year’s Thanksgiving celebration, it seems, may be a frozen dinner in front of the television set.

What’s going on here? Why does an offer for direct payment put such a damper on the party? As Margaret Clark, Judson Mills, and Alan Fiske suggested a long time ago, the answer is that we live simultaneously in two different worlds-?one where social norms prevail, and the other where market norms make the rules. The social norms include the friendly requests that people make of one another. Could you help me move this couch? Could you help me change this tire? Social norms are wrapped up in our social nature and our need for community. They are usually warm and fuzzy. Instant paybacks are not required: you may help move your neighbor’s couch, but this doesn’t mean he has to come right over and move yours. It’s like opening a door for someone: it provides pleasure for both of you, and reciprocity is not immediately required.

The second world, the one governed by market norms, is very different. There’s nothing warm and fuzzy about it. The exchanges are sharp- edged: wages, prices, rents, interest, and costs- and- benefits. Such market relationships are not necessarily evil or mean-in fact, they also include self- reliance, inventiveness, and individualism-but they do imply comparable benefits and prompt payments. When you are in the domain of market norms, you get what you pay for-that’s just the way it is.

When we keep social norms and market norms on their separate paths, life hums along pretty well. Take sex, for in stance. We may have it free in the social context, where it is, we hope, warm and emotionally nourishing. But there’s also market sex, sex that is on demand and that costs money. This seems pretty straightforward. We don’t have husbands (or wives) coming home asking for a $50?trick; nor do we have prostitutes hoping for everlasting love.

When social and market norms collide, trouble sets in. Take sex again. A guy takes a girl out for dinner and a movie, and he pays the bills. They go out again, and he pays the bills once more. They go out a third time, and he’s still springing for the meal and the entertainment. At this point, he’s hoping for at least a passionate kiss at the front door. His wallet is getting perilously thin, but worse is what’s going on in his head: he’s having trouble reconciling the social norm (courtship) with the market norm (money for sex). On the fourth date he casually mentions how much this romance is costing him. Now he’s crossed the line. Violation! She calls him a beast and storms off. He should have known that one can’t mix social and market norms-especially in this case-without implying that the lady is a tramp. He should also have remembered the immortal words of Woody Allen:

“The most expensive sex is free sex.”