The Psychology of Dieting Part 1: Plate Size and Portion Size

Over time, the average size of the American plate has been slowly increasing. In the early 1900s, the average hovered around 9.5 inches in diameter. Today, that average has increased to 11.5 inches in diameter. Are bigger plates causing us to eat more? It is quite possible.

One of the most well-known perceptual biases in cognitive psychology is known as the Delbouef Illusion. Basically, this effect demonstrates that we tend to estimate the sizes of objectives relative to the sizes of objects around them. When a circle is drawn around another circle, it will make the inner circle appear larger.

When we eat, we tend to judge how much we’ve eaten based on the space that is filled on our plates. If the food portion is lower relative to the size of the plate, we think we’re getting less; so, we heap a few more spoonfuls onto the plate to fill it out.

Dinnerware Size and the Delbouef Illusion

In this image, which helping of Cheerios do you think is larger? Obviously, this is a trick question. They are both the same size. However, the serving of Cheerios in the larger bowl appears smaller, because it is dwarfed by the size of the bowl. So, if you happened to be the one eating these Cheerios, what are you going to do? Chances are, you are going pour more cereal into the bowl, because your brain is telling you that it isn’t enough food.

One of the most famous experiments of contemporary food psychology centers around the consumption of popcorn in a movie theater. The researchers tested two different groups. One group was given fresh popcorn in two different sizes–120g and 240g. The other group was given stale popcorn in the same two sizes.

At the end of the movie, the researchers collected the leftover popcorn and counted the consumption from each group. In group that was given fresh popcorn, those who had the larger buckets ate 45.3% more popcorn. But the fact that the other group was given stale popcorn did not prevent them from eating more when the container was larger. In the second group, participants ate 33.6% more stale popcorn when it was served in larger buckets than when it was served in smaller buckets.

The implications of such experiments are huge. We don’t like things being empty. When we use a shopping cart at the store, we want to fill it. When we move into a bigger house, we want to fill it with furniture. And when we eat from bigger plates, we want to fill them with food.

So, here’s my recommendation: take a look in your cabinets. How big are the plates that you use? How big are the cups that you drink from? Even look at your silverware. How big are your spoons? Is there opportunity for you to make smaller bites? If you’re serious about losing weight, the easiest way to do it may just be to replace your existing dinnerware with dinnerware that trains you to eat less. Smaller plates; smaller waist.


Ittersum, K., & Wansink, B. (n.d.). Plate Size and Color Suggestibility: The Delboeuf Illusion’s Bias on Serving and Eating Behavior. Journal of Consumer Research, 215-228.

Wansink, B., & Kim, J. (n.d.). Bad Popcorn In Big Buckets: Portion Size Can Influence Intake As Much As Taste. Journal of Nutrition Education and Behavior, 242-245.

Photo Credit (Plated Spaghetti): Hector Alejandro licensed via Creative Commons


About Douglas E Rice

Douglas E Rice is just a guy who likes to learn stuff.
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