Trays started disappearing from university cafeterias several years ago. Not for the usual reason, to be used as sleds in winter, or for the best reason, for college-age secret agents to slip under their jackets as impromptu torso protection against enemy shivs or poisoned blow darts.

Trays have been on their way out just because they hold too much food. If you're walking through a cafeteria line, all that real estate is a cue to take more food than you might want to eat.

This came up on World Food Day last week, at an event I moderated at Bloomberg headquarters with Craig Hanson, director of the People & Ecosystems Program at the World Resources Institute, and Marcus Samuelsson , the celebrated chef whose current restaurants include the Red Rooster Harlem. Hanson is overseeing a series of WRI reports about the projected world food shortage. His team's research suggests that the agricultural sector will have to produce 60 percent more kilocalories in 2050 than in 2006, to feed more than 9 billion people.

Better university dining won't make a dent in what WRI calls the "food gap." But it's a start in addressing the astonishingly large problem of food waste — about a quarter of food produced globally is lost or thrown away. In North America, 61 percent of the waste occurs at the consumer level. On average, we each throw out enough food to feed a growing kid every day.

Traylessness is a fun reminder that behavioral cues are a vital tool in resource conservation — for cafeteria food, gasoline, water or any of the other things feared to be "peaking ." In 2008, students at 25 schools collectively threw out about 11,500 pounds less food on days when trays disappeared from cafeterias, as much as a 30 percent drop in waste per student, according to a 2008 study by Aramark Higher Education.

That's translating to tens of thousands of dollars in savings for schools every year. Early adopters included University of Maine at Farmington, Grand Valley State University, in Allendale, Michigan, University of North Carolina at Chapel Hill and the University of Florida.

The trick, universities have said, is good communication. The schools that dumped their trays most readily claim successful communication to students, explaining the environmental and financial savings, and health benefits, of taking less, according to Aramark.

Good communication helps to explain trayless cafeterias, just as it does grocery-store bag taxes in liberal Montgomery County, Maryland, or carbon cap-and-trade policy in Europe. But successful changes like this also depend primarily on something deeper than that: a willingness to change, something many of us lack in abundance. Marylanders and Europeans, like young, idealistic, sustainability-minded college students, were already politically receptive to action before the communication campaign began. If that were all it took, we might have had a binding global climate change treaty 15 years ago.

On a personal level, a greater willingness to change might lead me to carry chopsticks all the time, as A.J. Jacobs suggested in his 2012 book Drop Dead Healthy , to encourage less food-shoveling.

They'd also double as self-defense, I suppose, if I were ever set upon by a hoard of college-aged secret agents. After all, I know they're not wearing trays.

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