By Ana Swanson

In a study published in the journal Health Economics in January, researchers carried out a field experiment to try to motivate 8,000 children in different schools to eat healthier. They found that giving the students a small incentive for eating healthy — in this case, a 25 cent token the kids could spend at the school store, carnival or book fair — doubled the fraction of kids eating at least one serving of fruits or vegetables.

The researchers also found that the effects lingered after the experiment ended, though they did subside somewhat. Two months after the end of the experiment, kids who had been rewarded for their health behavior for a period of five weeks were still eating 44 percent more fruit and vegetables than they had before the experiment begun.

The authors argue that the study provides evidence that short-run incentives can help form lasting behaviors and that longer periods of “interventions” are more effective at changing behavior than shorter ones.

There’s an obvious downside to these practices. Psychological studies have shown that, while external rewards like bribes are effective at getting people to adopt certain behaviors, they can undermine what’s called intrinsic motivation — a person’s internal drive to do a task, for example because it makes them feel more autonomous or competent.

In a 1999 analysis of 128 previous studies, researchers found that giving people rewards did significantly reduce their motivation to pursue the activity when they were not given a reward. In simpler terms that are probably obvious to any parent — bribing kids can spoil them.

Even when this is true, however, there’s often another force at work that may be even more important in changing people’s behaviors in the long-term: healthy life habits. Once people do something over and over again, it can become a subconscious, almost involuntary, inclination. And recent research suggests that those healthy habits have more staying power than the incentive of a bribe or reward.

Habits are powerful things. Most people have certain cues — like certain emotions, places, friends or times of day — that trigger a powerful urge to do a habit, whether it biting nails, smoking a cigarette, or eating a cookie. But habits can also be incredibly useful, if people can harness their power to shake bad behaviors in the longer run.

Another study of more than 1,500 kids in Chicago published in 2014, which was the subject of a Freakonomics podcast, supports the idea that they can. By giving kids who ate a healthy a small reward — a pen, a rubber bracelet or a little plastic trophy — the researchers raised the proportion of children who chose a healthy snack from 17 percent to about 75 percent. But when the researchers stopped doling out the rewards, they found that more kids continued to choose the healthy option — suggesting that intervention might have helped to form a healthy habit.

The study included some other interesting findings: For one, education alone wasn’t enough to change kids’ behaviors. After extolling the virtues of eating healthy, about 80 percent of kids still chose a cookie over fruit. But the researchers found that the combination of education and incentives was powerful.

Past experiments have produced mixed results on how long these effects last, with some finding that beneficial effects taper off relatively quickly. But in some cases, getting over the initial hurdle to change one’s behavior is enough. Just a brief change of habit can convince people that they actually do like the taste of squash, that they like the way exercise makes them feel, or that they have the willpower to resist cigarettes. In one 2009 study, for example, incentives helped some adults quit smoking for 12 months, and that period of cessation made it easier for people to stay smoke-free in the longer term, even without incentives.

In all of these studies, the rewards didn’t always work. For adults as well as kids, habits can be extremely hard to change. But offering an incentive appears to be the most effective way to change them. That’s because people are psychologically inclined to favor short-term rewards, like goofing off or eating tasty food, over long-term ones, like being healthy or saving for retirement. Giving people rewards for healthy behaviors just helps align their short-term incentives with their long-term ones.

Among parents, a lot of the debate over how to influence behavior comes down to exactly what you do and how you do it. Obviously, it may be preferable to just establish eating fruit and vegetables as normal behavior, and not give kids another option. But if that doesn’t work, many parents feel that offering their kids a trip to the zoo or the chance to pick the movie that night is a healthy way to reward good behavior. Others feel that, while giving a kid a dollar per carrot is a slippery slope, rewarding hard work with an allowance or a sought-after possession teaches kids what it’s like to work and have a job.

The debate often comes down to the language people use — whether they call it bribery, a reward or an incentive.