It’s always nice to have someone to blame for a problem, so if you’re struggling to keep off weight these days, feel free to direct some spite at Joseph Delboeuf. (Don’t feel too bad–he’s long dead.) Delboeuf was a 19th-century Belgian philosopher who discovered that people perceive two identical circles differently when they’re surrounded by concentric circles of different sizes. The effect is called the Delboeuf illusion. It’s why the black circle on the left looks smaller than the one on the right:
Here’s where your waistline comes in. Cornell marketing scholar Brian Wansink, author of the forthcoming book Slim by Design, has used the Delboeuf illusion to explain how plate size has a hidden influence on how much food we eat. In a 2012 study that received wide media attention, Wansink showed that the illusory effect carries over to dishware and portion sizes. If you were to swap the black circles with Cheerios and the white ones with bowls, for instance, the bowl on the left would still look like it had less food than the bowl on the right:
The significance of this connection is striking: the design of our dishware might guide our food portions in ways we don’t even realize. Take the above image as an example. Most people would probably add some cereal to the large bowl on the left and remove some cereal from the small bowl on the right. Otherwise it doesn’t look right. But making that adjustment means the person with the small bowl will end up eating less food–for purely perceptual reasons.
So whether we know it or not, we anchor our portions to our plates. In another study, published in a recent issue of Journal of Experimental Psychology: Applied, Wansink and collaborator Koert van Ittersum of the University of Groningen report that on average people prefer a plate or bowl that’s about 70% full. Whatever the size of the dish we’re using, we fill it up until that point, which means that if you’re using smaller dishes, over time you’re eating a lot less food.
Now, one might argue that people who serve themselves smaller portions on smaller plates will simply take seconds because they’ll still be hungry. That’s bound to happen in some places. But the larger point here is that we don’t base our portions on hunger at all–we base it on plate size. So for some people 70% of a small plate won’t be enough food, but for others it just might.
Take a field experiment that Wansink and Van Ittersum conducted at several Chinese restaurants with all-you-can-eat buffets. They observed unsuspecting diners who had the option of taking a large or small plate. If people served themselves by hunger and not plate size, then all diners should have wasted the same proportion of food. In fact, those who took big plates wasted 14% while those who took small ones wasted just 8% of their good. On average, the plate, not the gut, was the guide.
What makes this insight into consumption behavior so critical is that it explains, in part, why people take larger portions even when they know they shouldn’t. In another field experiment, he and Van Ittersum observed diners who had just listened to an hour-long lecture about how environmental cues like plate size can bias food habits. Despite this fresh information, people who ate lunch at a buffet station with larger plates took 90% more food (as measured by what was left in the pans) than those who ate at one with smaller plates.
“The solution to our tendency to overeat from larger plates and bowls is not simply education,” Wansink and Van Ittersum conclude in the recent paper. “In the midst of hard-wired perceptual biases, a more straightforward action would be to simply eliminate large dinnerware–replace larger bowls and plates with smaller ones. It is easier to change your food environment than to change your mind.”
The practical implications of smaller plate design go beyond our individual belt buckles. Restaurateurs could take advantage of the plate illusion to decrease food costs without necessarily decreasing satisfaction. Those in the public health sector would welcome the change as a way to limit food waste.
Still, overeating would be the primary target of any plate design revolution. The lingering question from Wansink’s work is whether or not swapping plates will make a difference not just in portion size but in actual weight loss. His initial attempts to answer that question are encouraging. A four-month trial of more than 200 households in Syracuse found that people randomly assigned to use smaller plates lost three pounds more than those given larger dishware. So once you’re done blaming Delboeuf, be sure to thank him, too.