The reason: as portions have grown, so, too, have leftovers.
“We know that growing portion sizes increase consumption, but grossly enlarged portions also cause consumers to face more and more food leftovers,” said Aradhna Krishna of the University of Michigan Ross School of Business. “Our research reveals that unconsumed food can exert meaningful influence on people’s perceptions, affect, motivation and important health-related behavior.”
Krishna, the Dwight F. Benton Professor of Marketing, and co-author Linda Hagen of the University of Southern California, tested the idea that consumers may judge their actual consumption by looking at their leftovers.
They conducted five studies, three of which involved actual food consumption and real leftovers, with two of those further measuring behavioral outcomes, including eating behavior and exercising effort. They found that, holding the amount of food consumption equal, larger (versus smaller) food leftovers lead to reduced perceived consumption.
This difference in perceived consumption has consequences for people’s motivation to compensate for their eating. Larger (versus smaller) food leftovers cause them to eat more in a subsequent unrelated food consumption task, and also to exercise less in an explicit calorie compensation task, the researchers say.
“The psychological drivers of this phenomenon are twofold,” Hagen said. “Larger leftovers reduce perceived consumption, which leads people to feel better about themselves. And feeling better about themselves, in turn, reduces people’s motivation to compensate.”
Average portion and package sizes have increased over time, leading to increased consumption. One study by other researchers found that when portion sizes grow by 100 percent, people only eat 35 percent more—meaning they have greater portions of their food left over.
“This study demonstrates that even leftovers stemming from these enlarged portions can impact consumption subsequently, expanding the scope of portion size research and highlighting the complex ways in which enlarged portions can influence consumption behaviors,” Krishna said.