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Why you eat too much: The Delboeuf Illusion

The size of dinnerware influences how much people serve and consume. When plates and bowls are large enough that capacity is not a constraint, we consistently serve more onto relatively larger than relatively smaller dinnerware. But why?

Two researches (Van Ittersum & Wansink) set out to investigate. This is more important than you might think, plates and bowls are almost 23% larger now than they were in 1900. And that matters to your waistline. Assuming a larger plate leads to an extra 50 calories a day, that quickly extrapolates into five lbs. a year.

Researchers already know that the shape of packaging influences behavior. But it was previously unknown when and why this might happen with dinnerware and what could be done to prevent us from taking too much food.

Turns out we have to go back into history to learn about an optical illusion:

In 1865, Delboeuf documented a puzzling perceived difference in the size of two identical circles when one of the circles was surrounded by a much larger circle and the other one was surrounded by only a slightly larger circle.

“For nearly 150 years, the Delboeuf illusion has been regarded as robust, but ‘of little practical value.’ In the context of serving behavior, however, it takes on an undiscovered dimension of everyday importance.”

The authors proposed this well-established perceptual bias “may explain how and why the size of dinnerware causes serving biases that occur meal by meal” and lead to clues for compensating.

The main objective here is to determine if the well-established perceptual biases associated with the Delboeuf illusion have a corresponding effect on serving behavior. Our main contention is that the Delboeuf illusion biases consumers’ ability to accurately determine how much food they need to serve to reach the target serving size they would otherwise typically serve and consume.

Basically, when our plate is large, we sense a large gap between the edges of the serving size and the plate. We’re tricked into thinking the serving size is smaller than its actual size and we compenstate by overserving.

So what about educating people about the Delboeuf illusion?

…we expect that educating consumers about the Delboeuf illusion and its effect on serving behavior allows them to be more consciously involved and reduce or even eliminate the effects. Research suggests that expertise reduces biases in a variety of domains. Research on the effectiveness of educating people on the presence and effects of illusions also suggests that education reduces biases. However, Wansink and Van Ittersum (2005) report that while education reduces serving biases associated with pouring drinks in different shaped glasses, it did not eliminate them.

And if this is really a perceptual bias at play, manipulating the color-contrast should weaken the assimilation and contrast effects. And it does.

The authors conclude:

By introducing the Delboeuf illusion as a possible explanation and extending it from the perceptual to the behavioral domain, the studies have shown how the illusion biases serving size perceptions, serving behavior, and consumption. Whereas contrast effects explain why consumers tend to overserve when using larger bowls and plates, assimilation effects explain why they also tend to underserve when using smaller ones.

It was found that attention and education may reduce the overserving (underserving) biases associated with serving on larger (smaller) bowls and plates. Furthermore, while reducing the color-contrast between the dinnerware and its background may help reduce over- and underserving biases, increasing the color-contrast between the food and the dinnerware actually may accomplish this as well. We conclude that the Delboeuf illusion offers a key explanation as to why and when the size of dinnerware influences serving size perceptions, serving behavior, and consumption.

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Source: Van Ittersum, Koert, and Brian Wansink (forthcoming), “Plate Size and Color Suggestibility:The Delboeuf Illusion’s Bias on Serving and Eating Behavior,” Journal of Consumer Research.

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