Paying to eat vegetables doesn’t pay, or why you can’t believe The Wall Street Journal

When I decided to quit bitching and start blogging, the last thing I imagined was that the first thing I’d write about was spinach. Or broccoli. Or extrinsic motivation—the fancy psychology term for carrots and sticks.  And in this case, not even carrots, since The Wall Street Journal and the Journal of Health Economics are now telling parents that to motivate kids to eat vegetables you should add cash to the deal.

I thought this myth was busted nearly 20 years ago, when a team at the University of Rochester reviewed 128 separate studies, finding across the board that tangible rewards “significantly undermined free-choice intrinsic motivation.” Meaning, if you pay kids candy or cash to eat carrots, you reduce the odds a carrot will ever be its own crunchy reward. (References come at the end. This is #1.)  I always forget that scene where the monster, supposedly dead, pops his ugly head up and growls.

So: Three behavioral economists from Carnegie Mellon, University of Pennsylvania and Brigham Young University give 8000 kids at 40 schools in Utah each a token worth a quarter every day for either three weeks or five if they’ll eat at least one serving of fruits or vegetables at lunch (#2). Behavioral economists love this game. Similar school studies were run twice in 2013, then again in 2015. None produced a conclusive result.

So here come these guys, pockets full of tokens festooned with pictures of apples and carrots, lumbering into school lunchrooms. They plant research assistants by the trash cans to interrogate the kids, make sure no sneaky Pete dumps zucchini on the floor, and take notes on 403,922 dirty trays. They check the number of servings each child takes, the number she actually eats. The schools, meanwhile, set to receive their own cash rewards in proportion to the produce their pupils consume, warn kids that cheaters will cause the whole school to lose the whole payout.  None of this, the researchers assure us, should have any effect on what kids eat once the tokens have stopped flowing.  And they see no need to bother with a control group—say, a school with all the lunchtime surveillance and threats, but no payment.

Here are the numbers that seduced The Wall Street Journal.  When the research began, fewer than 40% of the kids ate at least one serving a day. By week three, that percentage nearly doubled.  Even better, two months after the study, more kids were still downing more produce. In schools where tokens were distributed for three weeks, nearly half the kids still ate at least a serving a day two months on.  In schools where tokens were distributed for five weeks, the rate was 54%.  Amazing!  A teeny tiny extrinsic reward had changed eating habits long term.

Or not.  In fact, though the last-recorded rates of consumption were higher than rates at the start, they’d declined as soon as the token stream went dry, declined again after one month, declined yet again after two.  Those research assistants still stood sentry by the trash cans counting orange peels, but kids, in the absence of payment, slid back.

Which was not even the worst of it.

The worst, unreported in the Journal, was the amount of extra produce these students nibbled down, even at best.  At best, at peak token distribution, kids averaged one-third of one extra serving per day. Two months later, at 5-week schools, the average dropped to barely an extra tenth of a serving . At 3-week schools, two months on, kids were eating exactly seven hundredths of one extra serving of fruit or vegetables.

Maybe these numbers prompt high-fives among behavioral economists.  Maybe they thrill writers at The Wall Street Journal, or maybe said writers don’t know how to read published research.  But if you’re a mommy, a daddy, or a school nutritionist, they’re a joke.

“Science,” says the Journal, “says it’s OK to pay your children to eat their fruits and vegetables.”

No. It doesn’t.  And that’s why I’m launching this blog.



1. Deci, E.L., Koestner, R. and Ryan, R.M., 1999. “A Meta-Analytic Review of Experiments Examining the Effects of Extrinsic Rewards on Intrinsic Motivation,” Psychological Bulletin, 125( 6,) 627-6

2. Loewenstein, G., Price, J. and Volpp, K., 2016. “Habit formation in children: Evidence from incentives for healthy eating.” Journal of Health Economics, 45, 47-54.


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