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Britain's Conservative Party Prime Minister David Cameron looks on during a press conference inside 10 Downing Street to mark the half-way point in the five-year term of the coalition government in London on January 7, 2013. (AFP PHOTO/POOL/Alastair Grant)

If you want to deter crime, it seems that you'd want to lengthen prison sentences so that criminals would face steeper costs for breaking the law. In fact, research shows that increases in prison terms have done nothing to deter crime. Criminals, like the rest of us, aren't much influenced by things they might have to experience far in the future.

If a police officer witnesses the death of his partner, it seems that you'd want to quickly send in a grief counselor. In fact, this sort of immediate counseling freezes and fortifies memories of the trauma, making the aftershocks more damaging.

If you want to get people to vote more, it seems you'd want to tell them what a problem low turnout is. In fact, if you want people to vote, tell them everybody else is already voting and they should join the club. Voting is mostly about social membership and personal expression.

These are three examples of policies and practices that are based on bad psychology. The list of examples could fill this page. That's because we spend trillions putting policies and practices into place, but most of these efforts are based on the crudest possible psychological guesswork.

Fortunately, people in the behavioral sciences are putting policies to the test. I know of groups at Duke and Penn that are applying behavioral research findings to policy issues. Eldar Shafir of Princeton has edited a new book, "The Behavioral Foundations of Public Policy," a master compendium of what we know.

One of the things we know is that seemingly trivial changes can have big effects. People who are presented with a wide variety of choices of, say, yogurt, will eat more than people who are presented with a small array of choices or no choice. People who were randomly given a short, wide 22-ounce glass, poured 88 percent more juice or soda into it than people who were offered a tall, narrow 22-ounce glass, but they believed they only poured in half as much as they actually did.

Sometimes the behavioral research leads us to completely change how we think about an issue. For example, many of our anti-discrimination policies focus on finding the bad apples who are explicitly prejudiced. In fact, the serious discrimination is implicit, subtle and nearly universal. In computer shooting simulations, both black and white participants were more likely to think black figures were armed. In emergency rooms, whites are pervasively given stronger painkillers than blacks or Hispanics. Clearly, we should spend more effort rigging situations to reduce universal, unconscious racism.

The research is also leading to new policy approaches. In one clever program, dieters were told to phone in their weight to a nurse daily. Every day they called, they got an encouraging text and a lottery ticket, with a chance of winning a small amount. These dieters lost three times more weight than people who didn't get tickets. Another ingenious program automatically diverts some money into your savings account every time you buy a state lottery ticket.

Prime Minister David Cameron's government in Britain has gone furthest in implementing these sorts of programs. Personalized text messages were found to be six times more effective in getting people to pay fines than warning letters. If you tell people what percentage of their neighbors has already paid their taxes, you are more likely to get late filers to actually pay than if you nag them another way.

My problem with these efforts is that they are still so modest. What about the big problems? How do we get people to restrain government commitments now so that debt down the road won't be so ruinous? How do we calculate the multiplier effects of tax cuts or spending increases among different subgroups of the population, or under different emotional conditions? How do we rig the context of budget negotiations so participants can actually come to a deal? How are people in different cultures likely to react to drone strikes?

These are the big questions, and most of our policies rely on crude folk psychology from a few politicians. But there's hope. As Brian Wansink notes in Eldar Shafir's volume, the 20th century saw great gains in sanitation and public health. The 21st century could be a great period for behavior change.

David Brooks writes for the New York Times.