Une petit yogurt, s’il vous plait

Americans typically eat yogurt out of 8-ounce containers. By contrast, the typical yogurt in a French market weighs just more than half that, about 125 grams. This seemingly pointless fact may hide a fundamental psychological truth about how humans make all sorts of choices in life.

That at least is the theory of University of Pennsylvania psychologist Andrew Geier and his colleagues, who are studying what they label “unit bias.” The number one, they argue, is a “natural unit,” and in the realm of food and diet that means one serving. The French don’t double up on their tiny yogurts to get the same volume of food or caloric intake as Americans. Instead, they simply stop eating after one serving, and therefore eat less overall, and therefore are more slender and healthier than overweight Americans.

That’s a heavy social burden for a simple yogurt container to carry. So Geier and colleagues decided to test the “power of one” in everyday settings. They did three similar experiments. In one, they put out a bowl of Tootsie Rolls for public consumption; some days the Tootsie Rolls were large, and other days they were small. They did the same test with Philadelphia-style soft pretzels; some days they put out whole pretzels, while other days they cut them in half. Finally, they put out a large bowl of M&Ms, alternating a tablespoon-sized serving spoon with a spoon four times that size.

The results, as reported in the June issue of the APS journal Psychological Science, were unambiguous. Whatever their junk food of choice, people helped themselves to substantially more when they were offered supersized portions. Put another way, offering small portions effectively controlled how much people ate.

Why are snackers so mesmerized by the number one? The scientists don’t know for sure, but they speculate that unit bias may result from a combination of learning and culture. Many American children are indoctrinated from early on to “clean their plates,” reinforcing the notion that a plateful is the appropriate amount to eat at a meal. And people may also limit themselves to one serving so as not to appear greedy or gluttonous.

Whatever the dynamic at work, the researchers believe that unit bias affects many choices we make every day. We typically go to one movie, not two, whether the movie is 90 minutes long or three hours. We only ride the roller coaster once at the amusement park, regardless of the ride’s length (there are deeply disturbed exceptions to every rule, of course).

But the most obvious implications of these snack-food experiments are in the public health arena, where overeating and obesity increasingly threaten Americans’ well-being. Interestingly, food marketers appear to have picked up on this bit of psychological insight. A couple years ago the Nabisco company started selling junk food in 100-calorie “snack” units. Weight-conscious Americans loved them to the tune of $100 million a year, and many other food companies soon followed suit. Now we can get everything from Pringles to Sprite to Chips Ahoy! in prepackaged 100-calorie snacks. Chips are still chips, but it’s better than pigging out on a family- size unit.

These findings are reported in “Unit Bias: A New Heuristic That Helps Explain the Effect of Portion Size on Food Intake” in the July issue of Psychological Science.