Increasingly, people who are desperate to change their behaviour are putting their money on the line to motivate them towards success. But can placing a public bet really help people to lose weight, quit smoking, or exercise more? Could this type of commitment contract be a valuable tool for promoting public health?

Over 20 000 people have publicly signed up to change their behaviour at the online commitment store StickK (pronounced stick—the silent second letter k refers to the legal shorthand for contract) since the website launched in January 2008. Of these, about a third have placed a financial stake—promising to hand over a total of $1.28m (£870 000; €960 000) if they fail to meet their goals.

It may seem surprising that people chose to put their money at risk in this way, especially since many would already have tried to change their behaviour and failed. But Yale economics professor and StickK cofounder, Dean Karlan, thinks the explanation is clear. “Most people have something in their life that they would like to change. That’s why we make New Year resolutions—yet most people can think back to their previous resolutions and realise that they didn’t succeed. So it makes sense that offering someone a new way to meet their goal should generate very high take-up rates,” he explains.

The self made contracts at www.stickK.com and similar websites such as www.fatbet.net and www.makemoneylosingweight.com are examples of commitment contracts. The idea behind a commitment contract is that people choose a goal for changing an aspect of their behaviour, make a public commitment to change, and in some cases put up a financial stake that they are …