David Just and Brian Wansink, both of Cornell University's Food and Brand Lab, in Ithaca, N.Y., examined the lunch purchases of 2,314 students in grades 1-12 to see how payment methods impacted food choice.
The study, scheduled to be published in the journal Obesity, found schools that completely converted to debit systems -- as opposed to those that gave students the option to pay with cash instead -- fruit purchases were 13 percent lower and vegetable purchases were 20 percent lower. Students at debit-only schools were also more likely to purchase less healthy food options, such as candy, dessert and fried foods, the study said.
The study also found the lunches of students at debit-only schools contained 63 more calories from these less healthy foods and 32 fewer calories from healthier options, Wansink said.
Just and Wansink said the degree of parental guidance at lunchtime might be partly responsible for the study findings. If parents give children a certain amount of cash for lunch each day, they can monitor their kids' daily expenditure more closely, resulting in better lunch choices, the researchers said.
However, debit systems eliminate the restrictions of a daily cash allowance, providing kids the opportunity to spend their lunch money as they please.
A debit system which allowed parents to set daily limits or food-specific restrictions might be a compromise between convenience and guidance. Or using a "cash for cookies" rule might nudge students towards healthier foods by making them pause before impulse-purchasing less healthy options, the researchers said.