As graduate students at MIT, Dean Karlan and John Romalis didn’t just feast on food for thought, which is why, as they completed their studies in economics, they found themselves getting fat.

These pudgy scholars knew a little something about incentives, so they made a deal: each would have to lose 38 pounds in six months or forfeit half his annual income to the other. If both failed, the one who lost less would pay the other a quarter of his income. For a while nothing much happened, so they just extended the deadline—until they realized they were getting nowhere, and agreed to a clause making any further renegotiation attempts the basis of immediate forfeiture.

The new agreement worked. By 2002 both men had lost the weight, and as long as the stakes remained serious and nonnegotiable, they mostly kept it off. At one point, Karlan even collected $15,000 from his friend, whose weight had popped back up over the limit; Karlan says he had to take the money to maintain the credibility of their system.

Karlan has moved on to Yale and Romalis is at the University of Chicago, but both still struggle with their weight. Romalis reports that after ending his agreement with Karlan he has piled on the pounds. Karlan too gained weight in the absence of a possible financial loss, until he established a similar contract with his friend Ian Ayres, a Yale law professor. He’s also made a contract to exercise regularly. His weight remains under control.

Why must these disciplined intellectuals subject themselves to a Damoclean legal agreement just to control their eating? As Karlan explains it, the contracts work by counteracting the natural human tendency to prize short-term rewards—the taste of pistachio Häagen-Dazs right now—over such longer-term goals as a healthy body weight and reasonable cholesterol level. We can all make rules for ourselves; Karlan’s weight-loss contracts raise the cost of breaking them. Could he have lost the weight without a contract? Over a nice healthy salad in New Haven one sunny afternoon, he answers without hesitation: “No.”

A classic example of paternalism is the movement in business to automatically enroll employees in a 401(k) plan, with the right to opt out.

While this may sound like an arrangement only an economist could love, most of us have taken steps, at one time or another, to limit our future choices in order to guard against weakness or change of heart down the road. In fact, such arrangements are as old as the Trojan War. No less a psychologist than wily Odysseus, on the way home from sacking Troy, ordered his men to bind him to the mast of their ship (and plug their own ears) when they approached the Sirens, whose seductive song otherwise would lead them to destruction. The rest of us do just the same, even if a little less heroically, when we decide not to buy any potato chips (lest we fail to eat just one), or have the name Joan tattooed on a bicep (lest we take up instead with Barbara). Joining a gym might get you in shape; joining the Marines certainly will. Cutting up your credit cards, getting married (and wearing a big ring), or sending money to an IRA (which penalizes early withdrawals) are all examples of the same thing. So common are such acts that there’s a name for them: “precommitment,” which means binding yourself to some course of action that, later on, in the heat of temptation, you might no longer prefer.

As the economist Thomas Schelling pointed out during the Cold War, when deterrence was a hot topic, whole countries can engage in precommitment. The fledgling United States did just that more than 200 years ago when it adopted the Constitution and made it difficult for subsequent politicians and voters to change it. The Social Security system is a form of precommitment against geriatric destitution whereby Americans force one another to give money to the government in return for a pension.

Could private investors do better? Maybe, but the voters support Social Security, perhaps because they have a healthy skepticism about how much financial discipline they could muster if left to their own devices. (In precommitment circles, this makes them “sophisticated,” compared to people who assume vast willpower—and are therefore “naïve.”) Some inflation-prone Latin American nations have turned to Uncle Sam on this basis as well, adopting the dollar as their legal tender; by doing away with their local currency, they did away as well with the temptation to debase it.

What sets Karlan apart is that he realized there might be a business in this demand for precommitment. Teaming up with Ayres and Yale alum Jordan Goldberg (who doesn’t seem to have a weight problem), Karlan founded stickk.com, which might be called a precommitment store on the Internet. Despite its nerdy origins, the site has a cleverly rakish motto: “Put a contract out on yourself!”

Stickk.com (the second K is for the legal abbreviation for “contract”) lets you enter into one of several ready-made binding agreements to lose weight, quit smoking, or exercise regularly, among other things. Or you can create your own, as many of its more than 15,000 users have done. You specify the terms (say, a loss of one pound per week for 20 weeks), put up some money, and provide the name of a referee if you want one. Whenever you fail, stickk.com gives some of your money to a charity you’ve chosen. (Whether you fail or succeed, it never keeps your money for itself.) If you want a sharper incentive, you can even pick what stickk calls an anti-charity. Democrats, for instance, might find it especially motivating to know that failing to live up to a binding personal commitment on stickk.com will send some of their hard-earned money to the NRA Foundation. And rest assured, says Ayres, that “these are legally binding contracts. I’m a contracts professor.”

People use stickk.com to make commitments small and large. Several users have committed to write a novel; at least one has pledged to bring a brown-bag lunch to work. Many affirmatively commit to do something, such as pray daily or floss regularly, while others pledge to stop doing something. A lot of the pledges are aimed at what might be called compulsive behavior, including nail-biting and “self-injury.” Some, like trying to “stop calling my ex,” are even poignant.

Stickk is in the business of raising the price of bad behavior.

People on stickk.com do seem to be conscious of incentives. User “ANAJ29” vowed to give up sugar, for instance, and pay $10 for any week in which she ingested the sweet stuff. One day she reported in her “commitment journal” on a transgression: “I ate gingerbread on April 9th. I knew I was hungry, but I had company and wanted to serve fresh baked goods. I thought I could resist it, but I did not. I ony [sic] had a little bit, though. But now I’m feeling that since it cost me $10, I should have had a bigger piece.”

But stickk is no mere academic exercise in proving the effectiveness of incentives. Its founders insist it’s a real live business enterprise, one that has raised $1.2 million in initial funding and fully intends to become profitable. So how will it turn a profit? Goldberg says mainly by selling advertising, but down the road it plans to offer users some premium services for a fee. It also hopes to charge corporations who want to use stickk for employee wellness initiatives. “All stickk is doing,” says Karlan, “is raising the price of bad behavior—or lowering the cost of good behavior.”

Students of human behavior like George Loewenstein, the Herbert A. Simon Professor of Economics and Psychology at Carnegie Mellon, will tell you that having weekly or “proximal” weight-loss goals makes sense. To understand why, it’s helpful to consider a longer-term project such as writing a book. Say your publishing contract gives you three years, but you live in Hawaii and love to surf. Surfing is more fun than writing, of course, and you’re aware that a single day’s work will make little difference to a large project. So every day, the sensible choice is to hit the waves. Now, we all know that books aren’t written from atop surfboards. But a daily goal of, say, two pages makes it easier to see when you’re falling behind—and easy to obtain the satisfaction of a job well done long before the whole book is written. Just meet your daily goal. And maybe leave your surfboard with a friend, with instructions not to release it to you each day unless you show him two new pages.

Why are people better at the short run than the long run—so much better that they need to take dire measures like irrevocably limiting their own options? The economist John Maynard Keynes hit on one possibility when he suggested that, in the long run, we are all dead. But a more likely answer is evolution. The human brain developed at a time when there might not have been a lot of benefit to long-term thinking. On the contrary, in those days distractibility (“Hey, isn’t that a tiger?!”) may have paid real reproductive dividends, since it meant living long enough to have more children. Moving from cavemen to college campuses (not perhaps such a great distance after all), study after study confirms the human preference for smaller short-term rewards over greater rewards further off into the future. Psychologists call this “hyperbolic discounting,” and its persistence is the reason precommitment devices like stickk.com can be so useful.

Even some animals do it. Years ago the great George Ainslie, a legend in the quietly sizzling field of self-regulation research, gave pigeons a choice of pecking a red key for a small but immediate burst of grain, or not pecking at all and receiving a larger, later grain reward. The pigeons naturally pecked. Then Ainslie offered them a green key which, if pecked, kept the red key from appearing at all, a precommitment key, in other words. Astonishingly, a significant minority of pigeons pecked it (suggesting significant potential avian demand for stickk.com).

Study after study has confirmed the human preference for smaller short-term rewards over greater rewards further off into the future.

Writers, as you might imagine, have used all kinds of precommitment devices over the years. Coleridge reportedly paid men to keep him out of opium dens, and other scribes, more prosaically concerned with procrastination than drug abuse, have found that simply having a goal and keeping track of their daily output made a huge difference. Ernest Hemingway and Irving Wallace charted their progress on books, while the famously prolific Anthony Trollope required of himself not just a set number of daily pages but specified how many words—250—should be on each. “Nothing surely is as potent as a law that may not be disobeyed,” he wrote, suggesting he might have approved of stickk.com. “It has the force of the water-drop that hollows the stone. A small daily task, if it be really daily, will beat the labours of a spasmodic Hercules.”

Given this record, it’s not surprising that the founders of stickk.com aren’t the first to try and commercialize precommitment. In 1984 a book called The Blackmail Diet appeared. Its author, John Bear, suggested people could force themselves to lose weight by committing to some hideous consequence if they didn’t. Bear himself slimmed down by pledging to donate $5,000 to the American Nazi Party if he failed. Want to get control of your Web habit? Consider Covenant Eyes, which installs a program on your computer that logs all the websites you visit and periodically emails this list to your designated accountability partner. That could be your boss, spouse, pastor, or even your mother. (Consider the force of a Post-it on your monitor with the reminder: “What would Mom think?”)

But what’s interesting about stickk.com is that it comes along at a time of renewed interest in paternalism. A number of smart people, most prominently Richard Thaler and Cass Sunstein, have suggested lately that the time is right for institutions to help people make better choices by means of more thoughtful “choice architecture.” At company cafeterias, for instance, the fruits and vegetables might be displayed more prominently and priced more attractively with respect to desserts. The idea is not to mandate behavior, but to present choices so that the indisputably better option is more likely to be selected. It’s all about “soft paternalism.”

The human brain developed at a time when there might not have been a lot of benefit to long-term thinking.

The classic example is the movement in business to automatically enroll employees in a 401(k) plan (assuming one is offered), with the right to opt out. This is the opposite of the traditional approach, which offers employees the chance to opt in. It turns out that humans have a strong “status quo” bias, a fancy way of saying inertia is a powerful force in their lives. In a 2001 study, for instance, Brigitte Madrian and Dennis Shea found that at one company initial sign-ups rose to 86 percent from 49 percent after automatic enrollment was adopted. (Overall enrollment had been 72 percent, since many workers signed up after a while.) Reversing the default condition, which costs nothing and constrains nobody, thus significantly improved the retirement prospects of many employees.

It’s hard to object to this kind of thing. The question is the extent to which the government ought to do it as well. Advocates of this position are quick to point out that government isn’t going away, and through tax policy and other means it will have plenty of influence over the choices people make. Government policies have for years strongly encouraged the construction and ownership of single-family housing, for example.

But critics wonder whether anyone can really say which choice is indisputably better for someone else—and whether the game is likely to stop there, or proceed down what economist Mario Rizzo has called a slippery slope to much costlier constraints on personal freedom. While acknowledging that “paternalism is here to stay” and that soft paternalism is better than hard, the economist Edward Glaeser nonetheless rejects the idea on a variety of grounds. For example, government choice architects will likely be prone to error, since they’re no more rational than the rest of us yet might possess less directly relevant information than those whose choices they are influencing.

By contrast, individuals who make a commitment on stickk.com can act of their own volition to make themselves adhere to what the philosopher Harry Frankfurt has called their second-order preferences, by which he meant their preferences about preferences. You may like to smoke a cigar, for example, but you may also prefer not to have that preference. And your rational allegiance is to your second preference—the one that enables you to avoid cancer, heart disease, and the other problems of smoking.

The beauty of stickk.com, in other words, is that it lets people decide for themselves which longer-term goals they will embrace, in effect by becoming their own paternalists. And it gives them the means to enforce their own second-order desires, just as people do when they have their stomachs stapled or their jaws wired to constrain their own eating. As Vito Corleone might have put it, stickk.com wants you to make yourself an offer you can’t refuse. And what could be better than for each of us to be our own Godfather?

Daniel Akst is a writer living in New York. He previously wrote for THE AMERICAN about the business photography of Bill Wood.

Image by Martin Barraud/Getty Images.