Most patrons of fast-food shops are regularly asked if they'd like to "value size" or increase the portions of their meal for a few cents more.
Experts say supersized meals and a "clean plate" culture largely contribute to a national obesity rate among adults greater than 33 percent.
But for a few weeks at a time in 2010, visitors to the Panda Express franchise at Duke University in Durham, N.C., were offered the option of less -- and a surprising number of people took it.
The experiment, described in the January issue of the journal Health Affairs, "tells us that fast-food customers are receptive to interventions. They recognize that they overeat and wish that they didn't," said Janet Schwartz, an assistant professor of marketing at Tulane University and lead author of the study.
Schwartz and her colleagues had noted from other research that simply putting nutrition or calorie labels on restaurant menus doesn't seem to make much difference in how much customers order or eat.
"So we wondered whether inviting customers to take a smaller portion of a starchy side dish would activate self-control," Schwartz said. About a third to a fifth of the customers agreed to downsize, under several different conditions."
Three different tests were run at the restaurant, located at Duke's student center and near the medical center, at different times during the year, starting near the end of the lunchtime crunch.
Customers in line were first asked which of four side dishes they wanted: steamed rice, fried rice, chow mein (noodles) or mixed vegetables (only about 4 percent ever order those). Then, before moving on to pick one or more of 20 stir-fried entrees, the server asked patrons who'd picked a starchy side if they'd would like to save 200 calories by getting a half portion -- one scoop rather than two, about 5 ounces less than standard.
Thirty-three percent said yes. That share stayed the same even when servers offered customers a 25-cent discount for the smaller portion.
In the next phase, researchers asked the restaurant to make the same offer without, and then with, calorie labels posted for each menu item. That time, 21 percent took the offer before the labels were put up, but just 14 percent after the calories were posted.
And customer receipts showed that those taking smaller sides didn't make up for those portions by ordering more or heavier entrees.
"Our economists were surprised so many people were willing to take less, and wondered if these were people who take less food to begin with and throw away a lot," Schwartz said.
So in a third test, the same downsizing offer was made and 18 percent accepted. But this time, customers were approached after their meal and asked to do a "satisfaction survey" that included a check of their leftovers, including weighing them.
It turned out that the downsizers and those who took full servings all left about the same amount of food on their plates -- but the surveys showed that those who ordered the half-sides did so to deliberately cut calories and didn't eat more later in the day to "make up."
The researchers concede that the late lunch crowd at Duke may not be typical, but note Duke undergrads were only in session during the second test phase.
Calorie labels, already mandated in many cities, will soon go national for chains under federal law, and many restaurants have started offering small plates and half portions, Schwartz noted. "But of lot of them also match those special offers with promotions to eat dessert."
The researchers say offering less at the counter simply makes people stop and think what they're about to eat, rather than mindlessly chowing down. "It's just one more strategy," Schwartz said. "Some people may need someone to give them a prompt in person to eat less."
(Reach Lee Bowman at BowmanL@shns.com)
(Distributed by Scripps Howard News Service, http://www.scrippsnews.com)