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There are few things people won't do for money. That's the thinking behind a new weight-loss study published by behavioral economists Kevin Volpp of the University of Pennsylvania School of Medicine and the Wharton School and George Lowenstein of Carnegie Mellon University. With a shocking 71% of Americans considered overweight or obese and most weight-reduction plans proving helpful at getting pounds off but far less so at keeping them off, Volpp and Lowenstein decided it was time to quit fooling around. Never mind fad diets and you-can-do-it affirmations. Better to just reward successful dieters with something even more lip-smacking than food: cash.

Volpp and Lowenstein designed a program in which a group of volunteers enrolled in a 16-week diet program with monthly weigh-ins and an overall goal of shedding 16 lb. The volunteers were then divided into three groups. The first group participated in a lottery program in which those who came closest to or exceeded the weight-loss goal received a variable cash prize determined by how many pounds they shed. The second group agreed to a deposit contract in which they anted up some of their own money as part of a pool. Those who lost the most weight split the pot; those who didn't drop enough weight lost their investment. The last group was given no financial incentive and relied on willpower and commitment alone. (See pictures of what makes you eat more food.)

No surprise who did best. At the end of the 16-week trial, the lottery group dropped an average of 13.1 lb., with 53% reaching the goal. In the deposit-contract group, the average weight loss was 14 lb., with 47% taking off 16 lb. or more. People in the no-incentive group logged an average weight loss of just 3.9 lb., with only 11% of them reaching the 16-lb. goal. As for the payouts? The winning lottery players walked away with an average of $272.80. The most successful members of the deposit group pocketed a cool $378.49, on average. The final group, of course, got skunked.

"Lotteries have the powerful incentive of providing a variable reinforcement, with big payouts sometimes and smaller ones others," says Volpp. "For the deposit contract, the idea is that people are very averse to losing money. It's good to have some skin in the game."

Volpp and Lowenstein concede that when the dividends stopped, so did the weight loss. About three months after the study ended, both incentive groups had put some pounds back on, though none of the subjects had returned to their original weight. (Read TIME's story on exercise tips.)

Still, the cash-for-fat idea proved powerful, and Volpp envisions ways it could be improved so that the weight loss sticks. If you run the programs for 12 to 18 months, for example, subjects start to reap the physical rewards of trimming down. "Their knees and back no longer ache. They start to look better in clothes," he says. "Those benefits become their own reinforcements, so you can turn off the incentive program." Employers and insurers could adopt a similar program, with monthly premiums falling along with the numbers on the scale. "We all tend to discount amorphous health benefits that will come sometime in the future," Volpp says. "So the key is to provide some immediate gratification too."

Dieting is not the only discipline that can be encouraged with the promise of cash. Volpp, who is affiliated with the Philadelphia Veterans Affairs Association and Penn's Center for Health Incentives, is exploring similar pay-to-play strategies for such challenges as quitting smoking and adhering to medication regimens.

Not lucky enough to be part of one of Volpp's studies or have an enlightened insurer or employer? Not a concern. Spouses, parents and friends can set up lotteries or deposit contracts for one another too. Or you can make it even simpler: give a trusted person $100 or so, with instructions that if you cheat on your diet or no-smoking pledge, the money be donated — in your name — to a political candidate or group you find objectionable (Greenpeace if you work for an oil company, for example, or an SUV club if you work for Greenpeace). Whatever the strategy, the person holding the cash has to adhere to the rules, no matter what — which is not always easy. A behavioral economist, after all, doesn't have to live with the glowering family member who's fallen off the wagon and is demanding a refund.

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• Jeffrey Kluger
• You can pay people to do pretty much anything. So why not do it to get them to lose weight?