After the two-week period, all the intervention children showed equal increases in their liking of their target vegetable compared with the control children. However, when given the chance to eat as much of it as they wanted (knowing there was no chance of reward), the kids who had previously earned stickers chose to eat more than the kids who'd just been repeatedly exposed to the vegetable without reward.

At one- and three-month follow-up, the intervention children's increased liking of their target vegetable was sustained regardless of the specific condition they'd been in. However, in terms of increased consumption (when given the opportunity to eat their target vegetable, knowing no reward would be forthcoming), only the sticker and verbal praise children showed sustained increases.

So, how come previous studies have claimed that bribery can undermine children's intrinsic motivation, actually leading to increases in disliking of foods? Cooke and her colleagues think this may be because past lab studies have often targeted foods that children already rather liked. Consistent with this explanation, it's notable that past community studies that reported the successful use of rewards targeted unpopular vegetables just as this study did.

An important detail of the current study is that verbal praise was almost as effective as tangible reward. 'Social reward might be particularly valuable in the home,' the researchers said, 'because it may help parents avoid being accused of unfairness in offering incentives to a fussy child but not to the child's siblings.'
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Cooke, L., Chambers, L., Anez, E., Croker, H., Boniface, D., Yeomans, M., and Wardle, J. (2011). Eating for Pleasure or Profit: The Effect of Incentives on Children's Enjoyment of Vegetables. Psychological Science http://dx.doi.org/10.1177/0956797610394662