How do you get people to eat less? Turns out that, for a significant number of people, the answer may be as simple as asking them. That’s the conclusion of a study in the February issue of Health Affairs, by Janet Schwartz of Tulane’s Freeman School of Business and coauthors, that has been receiving a lot of recent attention.

The researchers had servers at a Chinese fast-food restaurant ask customers whether they wanted to reduce the portion size of three starchy side dishes, which would save approximately 200 calories per dish. They consistently found that 14-33 percent of diners accepted the offer, even when they were not offered a discount for doing so. What’s more, customers who accepted the offer did not compensate by leaving less food uneaten or by ordering entrees with more calories.  The invitation to take less food may have been particularly effective because the side dishes targeted “are not focal to the meal and may be particularly subject to mindless eating,” write Schwartz and colleagues Jason Riis of Harvard, Brian Elbel of New York University, and Dan Ariely of Duke.

That downsizing attracted so many takers is particularly important given the disappointing results from other strategies aimed at getting Americans to cut calories, such as posting calorie counts on menus. Schwartz and her colleagues say the downsizing offers steer a middle course between the passive strategy of posting calorie counts and the more aggressive strategies of offering healthier default options or simply banning unhealthy food options altogether. “Although sometimes effective, such tactics are not always practically or politically feasible,” the authors note.

The lesson, Schwartz and her colleagues say, is that many people are willing to cut calories by taking smaller portions, but only if affirmatively prompted. The option to spontaneously ask for less food was always available to customers during the study, but virtually no one did so, despite the fact that many participants believed that restaurant portion sizes were too big and found smaller sizes appealing. When servers offered customers the option to downsize, it introduced a possibility that diners do not normally consider during the “automatic behavior” of fast-food ordering, and it gave customers an opportunity to exercise self-control before being faced with the temptation of a full plate.

“Considering the past success of “supersizing” meals as a means of changing people’s immediate and long-term eating behavior, our findings offer hope that ‘downsizing’ could be effective in driving a return to more moderate ordering and consumption in food-service settings,” the authors write. They see numerous possibilities for expanding this type of intervention to other types of food: “For example, a Big Mac “value meal” with a large order of french fries and a large cola drink provides 1,350 calories, of which only 540 come from the Big Mac. Consumers might be willing to downsize the french fries or the drink, or both, to reduce the calories from approximately 800 to 600, realizing substantial calorie savings without having to change or sacrifice the focal Big Mac.”

Schwartz and coauthors suggest that voluntary downsizing offers may appeal to both restaurateurs and policymakers. “Knowing that there is consumer demand for smaller portions may encourage restaurateurs to take action themselves, especially if they want to avoid policies that force downsizing (for example, policies that dictate default serving sizes more in accordance with the 2010 dietary guidelines),” they write. For the policy community, encouraging voluntary adoption of downsizing “may prove to be an appealing alternative to imposing costly and controversial policies intended to drive behavior in the same direction.”