The Delboeuf illusion: Why expanding dinner plates are expanding our waistlines

By Elaine Watson

- Last updated on GMT

Imagine these are dinner plates. Which black spot is larger?
Imagine these are dinner plates. Which black spot is larger?
An optical illusion documented by Belgian philosopher Franz Joseph Delboeuf in the 1860s could help to explain why smaller plates could help us battle the bulge, according to a new study.

While smaller plates typically encourage smaller serving sizes, this is not simply because we physically can’t pile as much food on them, but in part down to Delboeuf, argue Brian Wansink, professor of marketing at Cornell University and Koert Van Ittersum, associate professor of marketing at Georgia Institute of Technology.

Delboeuf documented a perceived difference in the size of two identical circles when one is surrounded by a much larger circle and the other one is surrounded by only a slightly larger circle, say the authors, who tested the theory on dinner plates and food portions.

Dinner plate size significantly impacts food consumption

By asking Georgia Tech students to serve the same diameter of soup (black spot in center of Delboeuf's diagram) onto dinner plates of varying sizes (the outer circles of his diagrams), Wansink and Van Ittersum demonstrated that what determines how much they serve is the relative gap between the edge of the food and the edge of the dinnerplate.

In other words, the bigger the plate, the more they served.

Thus, the students repeatedly over-served into the larger dishes and under-served into the smaller ones, and were quite unaware that they were doing it, noted the authors in a paper published in the Journal of Consumer Research.

“These results demonstrate how the size of dinnerware people use to serve themselves may significantly influence how much food they consume on a day-to-day basis.

“From a consumer welfare perspective, the effect of dinnerware size on serving behavior is significant, since the average size of a sample of dinner plates increased almost 23%, from 9.6 inches to 11.8 inches, since 1900.”

They added: “Should the size of a plate or bowl encourage a person to eat only 50 more calories a day, the resulting mathematical increase in weight would be approximately five pounds each year.”

The role of color in food serving size

But new light was also shed on the role of color in determining food portion sizes, noted the authors, who experimented with different colored foods, dinnerware and tableware.

It quickly emerged that what mattered in this regard was not the color of the plate or tablecloth per se, but the contrast​ between the color of the food and the color of the plate, or the color of the plate and the color of the table.

For example, people serving white-sauce-based pasta on a white plate are more likely to over-serve than those serving red-sauce-based food on a white plate. But somewhat counter-intuitively, those using red plates on red table cloths are less likely to over-serve.

Study designs

Study one​ (225 students at the Georgia Tech, average age 21.1)

Participants were shown a 9cm diameter petri dish of tomato soup and told to fill bowls of different sizes with soup to match the 9cm target diameter. Participants poured 8.2% less than the target serving size in the smaller bowls and 9.9% more in the larger bowls.

In a separate test, when asked to estimate the diameter of pre-served soup in different sized dishes, they perceived it to be greater in the smaller bowls than in the large ones, when in fact, it was exactly the same in all dishes.

Study two​ (47 Georgia Tech students, average age 21.7)

Participants were shown a target serving size of cereal and asked to reproduce this by serving themselves on small and large plates. Some white plates were on a white tablecloth (low color contrast); others were on a black tablecloth (high color contrast).

The lower the color contrast between the plates and the tablecloth, the more accurate the servings, although participants still over-filled the large dishes and under-filled the small ones.

Study three​ (91 Georgia Tech students, average age 21.6)

Participants were shown a target serving size of cereal and asked to draw a circle with the same diameter on a smaller and larger plate. Those alerted to the effects of the Delboeuf illusion in advance were able to complete the task more accurately, although the effects were not eliminated.

Study four​ (101 students, Georgia Tech, average age 21.6)

Participants were presented with a target serving size of cereal and asked to serve themselves the same amount of cereal on a smaller and larger plate. Again, those informed about the Delboeuf illusion in advance performed better.

Study five​ – (College reunion in upstate New York, 60 lunch goers at a buffet table)

Participants served themselves pasta with red sauce or a white sauce and were randomly given a white plate or a red plate. Those in the low color contrast conditions (eg. white on white, red on red) over-served more pasta than those in high color-contrast conditions.

Source​: Journal of Consumer Research​, vol 39 August 2012, published online ahead of print

Plate Size and Color Suggestibility: The Delboeuf Illusion’s Bias on Serving and Eating Behavior.’

Authors​: Koert van Ittersum and Brian Wansink

Related topics: The obesity problem, R&D

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1 comment

portion size illusion

Posted by Barbara Jacobs,

Even without a study, it seems that there's some factor like wanting to be sure one has "enough" and will not be deprived. Thus, in using a larger plate--and wanting to 'fill is up more" one would naturally eat more. Similarly, using a smaller plate and eve if it's "full," it is easier to eat the recommended (so I've heard) 'half of what is on the plate," which might then really amount to 1/4 of the amount of food someone would put on a larger plate, if the "perceived deprivation" theory (well, it's my own little theory anyway) is true.

Just a thought.

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