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

Van Ittersum, Koert, and Brian Wansink (2012). Plate Size and Color Suggestibility: The Delboeuf Illusion’s Bias on Serving and Eating BehaviorJournal of Consumer Research, 39(2), 215-228. doi: 10.1086/662615.

There is growing evidence that the size of dinnerware influences how much people serve and consume during a meal. However, it remains unclear why this might happen which prevents the identification of moderating conditions or possible solutions.

A visual perception bias called the Delboeuf Illusion which is caused by contrast and assimilation may explain the change in eating behavior. The main objective of in this study was to determine whether the well-established perceptual biases associated with this illusion have a corresponding effect on serving behavior.

To conduct this research we set out to test four hypotheses. First, that people would under serve themselves when a given smaller dinnerware and over serve themselves when given larger dinnerware. Second, that reducing the color contrast between dinnerware and the background (tablecloth, place mat or other) would reduce under serving on small plates and over serving on large plates. Third, that making people more attentive and aware of their serving size would reduce under serving on small plates and over serving on large plates. Finally, that educating people about the Delboeuf Illusion would reduce under serving on small plates and over serving on large plates, although not eliminate the effect.

To test these hypotheses, we conducted five experiments. In the first experiment, 225 college students were shown a target serving size of soup in a petri dish and then asked to serve soup into a different sized bowl, but maintain the diameter of soup they had observed in the target. In the second experiment, 47 college students were asked to observe a target serving of cereal and asked to reproduce the same circle on plates that were either larger or smaller than the target. The color contrast between the plate and background was also manipulated. In the third experiment, 91 college students were asked to observe a target serving of cereal and asked to draw a circle on either a small or large plate with the same diameter. Participants were allowed to examine the target bowl for either 2 seconds or 1 minute. In the fourth study, 101 college students were asked to observe a target serving of cereal and serve themselves cereal of the same diameter on a smaller or larger plate. Before proceeding to serve the cereal, half of the participants were educated about the Delboeuf Illusion. In the fifth experiment, 60 adults were led to two buffet tables. One of the tables served only pasta premixed with red sauce while the other served only pasta premixed with white sauce. Participants were randomly given either a large red plate or large white plate.

Results from these experiments demonstrate that the Delboeuf Illusion may be the explanation for why and how dinnerware size influences serving behavior. Overall we found that people over serve themselves when using larger dinnerware and under serve themselves when using smaller ones. Increasing the attention and education of people reduced the over and under serving biases. Furthermore, reducing the color contrast between dinnerware and the background, or increasing the color contrast between the plate and food also reduces over and under serving biases. Thus, these results point to practical, real-world applications of the Delboeuf Illusion.