School lunches are getting healthier, or so it would seem. New guidelines call for a serving of fruit or vegetables on every student's tray, but the problem is, many students simply throw the food away.

Two studies explore some of the problems associated with the new guidelines and offer one possible remedy — bribery.

When researchers looked into whether making students put fruit or vegetables on their tray made it more likely they'd eat better, they found that it had essentially no effect. The reason is simple, according to a study of children's eating habits: students don't put the nutritious food in their stomachs, .

You can lead a kid to vegetables, but you can't make them eat them.

In schools requiring that kids be given a serving of fruit or vegetables at lunch, 35% of the students ate them, which was only a slight improvement over the 33% of kids who took fruit or veggies voluntarily at schools with no requirement. Worse, there was nearly three times the amount of fruit and vegetables in the trash in the schools where taking a serving was mandatory.

Before the incentives were offered, about a third (33.6%) of the students ate a serving of fruit or veggies with lunch. After being offered any of the incentives, this nearly doubled.

Results were similarly disappointing when researchers looked at how lunchroom fruit and veggie consumption changed among five schools which went from having no food requirements to requiring kids to put fruit or veggies on their trays.

After schools made the switch, the fraction of students who ate them increased from 20% to 28%. But it also caused the school to buy more fruit and vegetables, 70% of which ended up in the trash. In the end fewer than 1 in 10 additional children ate a serving.

Clearly, you can lead a kid to vegetables, but you can't make them eat them. So what does work?

A second study tested whether offering the students an incentive would encourage them to eat their fruits and veggies, essentially using bribery to change their eating habits.

At these schools, students were served a plate with a main entree and were then allowed to choose as many other items as they wanted from a selection of fruits, vegetables, and other side dishes. They were also told that if they ate at least one serving of fruit or vegetables, they would receive a reward.

Rewards varied at the different schools: a nickel immediately, a quarter immediately, a quarter two weeks from now, immediately receiving a raffle ticket that could win sporting equipment or receiving a ticket two weeks later.

Before the incentives were offered, about a third (33.6%) of the students ate a serving of fruit or veggies with lunch. After the incentive offer, this nearly doubled. And the amount that ended up in the trash decreased by about a third. Receiving the quarter immediately was the most popular incentive, raising fruit or vegetable consumption to over 70%.

This was a temporary effect. When the rewards ceased, the children returned to their earlier eating habits.

Is bribing our children a good way to get them to eat better? Some people think it would be sending our children entirely the wrong message. Others see bribery as simply an extension of the, “No dessert if you don't heat your vegetables,” discussion that occurs at many dinner tables nightly.

The two studies do clearly show that putting healthy food in front of our children doesn't necessarily mean that they're going to eat it. Other studies have found that simply changing the order in which food is laid out in the lunchroom can make a difference in what kids choose to eat.

Offering free fruit to schoolchildren in Norway did wonders for their eating habits, but for whatever reason, placing it on U.S. kids' lunch trays doesn't seem to be working nearly as well.

The first study appears in Public Health Nutrition. The second, on the use of incentives to encourage healthier eating, can be viewed here and will appear in a future edition of the Journal of Human Resources.