The researchers’ plan was to ask subjects after the movie how many calories they thought they had eaten and to weigh their remaining popcorn. The thought was that if people only overeat because food tastes good and because they are hungry, the people given the stale popcorn shouldn’t have eaten any of it, both because it didn’t taste good and because the study was done right after dinner, so they shouldn’t have been very hungry.
Here’s what happened: People given fresh popcorn ate 45% more from the extra-large containers than from the large ones. That’s about 190 calories more. But again, if you told them, hey you ate about 45% more than a person with a medium-size container, what would they say? “No I didn’t.”
Even when the popcorn was stale, people ate 34% more from the bigger bucket. When asked what they thought about the popcorn, the typical reply was, “Oh God, I can’t get the taste out of my mouth. Why did you do that to me?” But although they hated the popcorn, they still ate it, and those with bigger buckets ate more.
This experiment demonstrates that when people start eating, they really don’t have an idea of how much they should eat. No one thinks, “I could really use about 112 grams of popcorn today.” Nor do most people notice that they are less hungry than they want to be or more full than they desire. Without those internal bodily cues, people end up using external cues, such as the size of the serving container, to determine how much to eat. And on some level, it seems more normal to take a large serving from a large container than to take a small serving from a large container.
Shape matters, too
The developmental psychologist Jean Piaget described the concept called “conservation of volume.” Essentially, it says that up until a certain developmental stage, young children think that a tall, erect, narrow dish contains more liquid than a flat dish with equal capacity. It is only when children develop cognitively that they can understand that the same amount of water can appear differently in different-shaped vessels.
But understanding that concept and being able to “eyeball” serving sizes in different-size glasses are two different things. And both children and adults can be taken in by optical illusions. When looking at an upside down “T,” for example, most people estimate that the vertical line is longer than the horizontal line by about 20% even when in reality, the two lines are the same length. For whatever reason, people tend to focus on vertical dimensions more than horizontal dimensions and to underestimate horizontal dimensions. This tendency affects people’s perceptions of how much liquid drinking glasses of different shapes hold. Typically, people think that long skinny glasses hold more than short wide glasses.
A study conducted with 133 teens at a nutrition and fitness camp in New Hampshire confirmed the difficulty most people have in correctly estimating how much a short wide glass holds. During breakfast, as campers came through the cafeteria line, each was given either a really tall 22-ounce glass or a very short and wide 22-ounce glass. The campers then got their breakfast and their juice or soda. As they exited the line, the researchers asked them how many ounces of juice or soda they thought they had poured, and their answers were recorded. The researchers then weighed how much they had actually poured.
Because people typically pay more attention to height than width, the researchers expected that campers would pour a lot more liquid into the short wide glasses than the tall skinny glasses, and that is exactly what happened. They poured 88% more into short wide glasses than tall skinny glasses, or about 10 ounces versus about 5.5 ounces. However, they all believed they poured less into the shorter glasses. A typical comment was “I am following my diet well. A little bit more, but not that high.” And these were teens who had been educated about portion sizes, portion control, and calorie estimation.