Does a small plate of food make you eat less?

The popular dieting trick based on the Delbouef illusion says that people will identify sizes differently when they are placed within a larger or smaller object. This illusion is understood to cause people to over-serve themselves on larger dinner plates, and under-serve on smaller ones.

Researchers analysed 56 previous studies examining the effect of smaller plates on consumption. The various studies examined whether smaller plates reduce consumption for a wide variety of conditions: food type (snackfoods, popcorn, ice-cream, breakfast cereal, rice, vegetables, fruit, etc.), plate-type (bowls vs. plates, serving platter vs plate from which the food is consumed), portion-size (fixed amount of food served, amount varied in line with the plate-size, or self-served portions), setting (consumers invited to a food laboratory vs unaware consumers in natural settings such as a buffet).
Combining all the studies showed that halving the plate size led to a 30% reduction in amount of food consumed on average. In the case of plates, reducing the diameter by 30% halves the area of the plate and reduces consumption by 30%.

What it means is that if people are provided with smaller plates, they serve themselves less, and as a result, eat less. Imagine how much of a difference switching out dinner plates could mean for weight loss targets over time.

Now, a new paper by Prof. Tzvi Ganel and Ph.D. student Noa Zitron-Emanuel debunks this trick by presenting examples of the Delboeuf illusion to two groups of experimental participants. One group was mildly food deprived, after abstaining from eating for three hours prior to the experiment. The other group was asked to eat during the hour before the experiment, and was therefore not deprived.

The results showed that both subject groups were similarly inaccurate when judging the sizes. However, the hungry participants were more likely to identify the proportions of pizza placed on larger and smaller trays correctly than people who had eaten recently.

In practical terms, the new study indicates that to “trick” ourselves into eating smaller portions to lose weight does not quite work and is less likely to fool us when we feel hungry and are dieting.