Science That Matters

Last year, a group of psychology researchers at the University of Pennsylvania published a paper on what they call “unit bias.”

In their study, the researchers would top up a bowl of an M&Ms in the hallway of an apartment building each day. Next to the bowl was a scoop and a sign that said, “Eat Your Fill: please use the spoon to serve yourself.” Some days, they left it with a tablespoon sized spoon. Other days, they provided a quarter-cup sized spoon (quadruple the size). The result? An average of 1.67 times more M&Ms were taken on the days that the big scoop was there than on the days when it was the spoon.

The researchers did similar experiments with large and small tootsie rolls and with halved and unbroken pretzels. The results were similar in each case: when the “unit” of food was larger, people took more. While the amount taken did not vary in proportion to size of the unit (i.e., people did not take 4 times as many M&Ms), the effect was clear. The researchers concluded that, “consumption norms promote both the tendency to complete eating a unit and the idea that a single unit is the proper portion.”

Dan Lockton of the Architectures of Control blog has ruminated on the implications of this research and described the way that it played into more expensive foods with larger portion sizes (versus two-for-one deals) at fast food changes like McDonald’s.

• Review/summary on the British Pyschological Society research digest blog
• Read the article in Psychological Science (sub. req.)

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