People are easily swayed by the size labels on our food, and will eat more if they believe they're consuming a "small" as opposed to a "large." And to make things worse, consumers aren't aware of their overindulgence.
This distorting effect, on both perception and eating behavior, is worse for people who aren't much concerned with their nutrition, and for those whose minds are distracted by other tasks, the study found.
The findings suggest size labels are contributing the to the country's obesity epidemic, the researchers said. Over the last several decades, food portion sizes have increased dramatically, along with people's waistlines .
"People don’t realize that they have eaten more, so they're even fooling their stomachs," said study researcher Aradhna Krishna, a professor of marketing at the University of Michigan. This can lead to unintended overconsumption, and may result in obesity, the researchers said.
The results are based on five experiments, four of which were conducted in a laboratory setting, and one in a real-life situation.
In the real-life experiment, the researchers surreptitiously manipulated the food offerings at a meeting of 76 executives. They put plates with 15 cookies each, (a weight of 2.8 ounces, or 80 grams), on tables throughout the break room. Although the plates were identical, on some tables they were labeled "medium," while on others they were labeled "large."
Executives who thought they were consuming medium-size cookies ate, on average, 0.4 ounces (12.02 grams) more of the cookies than executives who thought they were eating large cookies.
However, those who munched on the medium cookies later said they had eaten less than those who ate large cookies.
The researchers found similar results in the laboratory studies, in which participants were asked to judge the size of and then nosh on packets of pretzels, nuts and Oreo cookies.
And when subjects were given a task to do while eating, the distortions were exacerbated. For instance, participants (who made their judgments when the plate was not in front of them) said a bowl that actually held 10 mini sandwiches held only eight when it was labeled as small. But when participants had a task to do while they lunched, they said it had around seven sandwiches.
This situation resembles everyday life, where people try to do many things as once. "[They're] even less able to process more deeply what exactly the amount is, and they'll go more by the label," Krishna said.
"Small" vs. "Tall"
But what might be the effect of more ambiguous labels, for instance, Starbuck's "Venti" or Cold Stone Creamery's "Like It" sizes? While the researchers have not studied this question directly, they speculate the ambiguity may make things even worse.
"It's even more possible to indulge in this guiltless gluttony, because you can convince yourself of anything," Krishna told MyHealthNewsDaily. "If you take 'Venti,' you can convince yourself that it's really nothing big, because [the word] has no meaning for you."
To improve the situation, Krishna said it should be made clear what a normal serving size for an adult is. From there, "small," "medium" and "large" sizes should be standardized across stores and restaurant chains.
The study will be published in an upcoming issue of the Journal of Consumer Research.
This article was provided by MyHealthNewsDaily, a sister site to Live Science.