Blog | Thursday, December 1, 2011

QD: News Every Day--It's true: Smaller plates encourage smaller portions

A simple optical illusion might encourage better eating habits, researchers found.

The Delboeuf illusion makes equal size circles appear to be different sizes by surrounding them with larger or smaller concentric rings. Applied to eating, smaller plates make the food servings appear larger.

One problem is that the size of commercially available dinnerware has increased from 9.6 inches to 11.8 inches in the past century. Eating only 50 calories a day more as a result equals enough calories to add five pounds of weight annually.

Practical implications of the research include encouraging people to replace larger plates and bowls with smaller ones, choose plates that contrast starkly with food, and even choose tablecloths that match their dinnerware, the researchers noted. Those with eating disorders or elderly people who need to eat more could follow the opposite advice to improve their intake.

Researchers conducted five studies on different variables to test how the Delboeuf illusion affected how people dish out their dinner. Results, which were published online Nov. 11 in the Journal of Consumer Research, are also published in full at the researcher's university website.

The first study isolated plate size. College students were shown a 9 cm bowl filled with tomato soup. They then tried to match the serving at another table that had one of seven differently sized bowls with diameters larger and smaller than 9 cm.

Participants poured 8.2% less soup into the three smaller bowls and 9.9% more into the three larger bowls. Those who poured into a similarly sized control bowl served an insignificant .9% less.

Next, the students looked at a target serving of soup in a randomly sized serving bowl. The bowls were shaped so that no matter the diameters of the bowl, the soup itself had a consistent diameter in the bowl. The students were asked to determine how much larger or smaller the soup was in the bowl compared to the target diameters.

Participants perceived the diameter of the smaller bowls to be 8.9% larger than the diameter of the target serving and perceived the diameter of the larger bowls as 8.6% smaller. The similarly sized control bowl was seen as an insignificant .8% larger.

Paydirt!! courtesy of Erik B. HansenThe second of the five studies looked at color contrast between dinnerware and a tablecloth. Students were asked to look at a target serving of cereal and then serve themselves a similar diameter at four stations (large and small white bowls sitting on black and white tablecloths).

Students in the high-contrast condition served 9.8% more using large plates and 13.5% less on the smaller plate. The low-contrast station significantly reduced overserving on large plates (9.8% vs. .3%) and reduced undeserving on small plates (-13.5% vs. -4.7%).

The third of the five studies assessed attentive servers to inattentive ones. Large and small plates were shown with a target size of cereal, but half the subjects were shown the servings for less than two seconds while the others were allowed one minute to study the portions. Students then had to draw a circle around the serving with the same diameter as the target serving size on the smaller and larger plates.

Inattentive students drew circles 8.3% larger on the larger plates and 11.2% smaller on the smaller plates. Attentive students drew circles 1% larger on larger plates and 7.2% smaller on larger plates, showing that mindful servers could adjust somewhat for the Delboeuf illusion.

The fourth study directly told randomized participants about the Delboeuf illusion and its impact on serving sizes. Control and experimental subjects were then offered small and large plates. Plate size influenced serving sizes, but less so among informed subjects. Large plate servings were reduced from 10.6% more to 4.4% more and small plate servings were reduced from 7.1% to 4.3% less.

The fifth study recruited adult lunch goers randomized to two buffet tables serving pasta in either red or white sauces, one sauce per each table. Once in line, the adults were randomly offered red or white plates that were 27.3 cm. The adults served themselves and a hidden scale recorded the serving amounts. A recommended serving of pasta according to the Food Guide Pyramid and the Diabetic Exchange System is 114.3 g.

Those in low-contrast servings overserved significantly more pasta than high-contrast scenarios than low-contrast ones (182.7 g vs. 140.6 g). Low-contrast servings (red sauce on red plate, or white sauce on white plate) didn't differ much (184 g vs. 181.5 g), and neither did the high-contrast scenarios (141.5 g vs. 139.8 g).

"The solution to our tendency to overeat from larger plates and bowls is not simply education. In the midst of hardwired perceptual biases, a more straightforward action would be to simply eliminate larger dinnerware--replace our larger bowls and plates with smaller ones," the researchers concluded. "It may be easier to change our personal environments than to change our minds."