Smaller Plate, Smaller Waist? Using Portion Size Distortion to Your Benefit

Make Optical Illusions Work in Your Favor

By Peter M. Vishton, Ph.D.William & Mary
Edited by Kate Findley, The Great Courses Daily

Overindulging in delicious desserts and sugary drinks is easy to do—especially when we don’t feel full afterwards. Peter M. Vishton, Ph.D., shows us how to enlist our subconscious to trick ourselves into consuming less.

Glasses of fresh squeezed orange juice
For dinnerware, learn how to use visual illusions of size to determine your portion control. Photo by xMarshall / Shutterstock

How Optical Illusions Work

As Americans’ portion sizes have increased over the decades, so have their waist sizes. But when you understand the nature of optical illusions and portion size distortion, you can cut back on the calories with minimal effort.

Human perception functions by comparing things. If you’re in a dimly lit room for a while, your eyes adapt to the lighting.

A moderately bright lamp might look really bright if it were switched on after you got used to that dimly lit room. Our sensory systems thus adapt to situations and respond mostly to particular things in relation to one another rather than in absolute terms.

A classic visual display called the Ebbinghaus Illusion features two central disks identical in size. One is surrounded by small disks, the other by large disks. 

Even though the projection of those two central disks onto your retina is identical in size, most people see the one surrounded by small disks as larger than the one surrounded by large disks. Our perception of an object’s size is influenced by its relation to the things around it.

Portion Size Distortion and Ice Cream

Some experimenters have found that this illusion applies to our perception of how much food is in front of us and how much we’ve eaten. Brian Wansink and his collaborators initially demonstrated this effect at an ice cream social.

Ice cream was provided in a self-serve fashion. People who attended the event would take a bowl at the beginning of the line and scoop as much ice cream as they wanted. The experimenters presented participants with different sized bowls and recorded how much ice cream people scooped out for themselves.

Presumably, carrying a small bowl doesn’t make you less hungry. Nevertheless, people with small bowls tended to take less ice cream. 

Additionally, no one was filling their bowl to maximum capacity. People would scoop ice cream into their bowls until it looked like the particular amount that they wanted to eat.

If you’re making that decision as you put your food onto a small dish, you’ll tend to stop sooner than if you’re putting your food onto a large dish. Your perception of the amount of food on your plate is based on how that amount of food compares to the size of the overall dish.

Thus, if you’re looking to cut back on your eating, one of the simplest things you can do is use smaller plates. 

Taller Glasses, Less Drink

There’s another illusion that applies to drinks. Consumption of sugary drinks has been identified as a strong contributor to our modern obesity epidemic. 

If you drink a 20-ounce soda, you’ve consumed about 16 teaspoons of sugar. That’s like eating three standard candy bars.

The calories themselves are more than most people realize, but perhaps the worst part is that they don’t satiate hunger. Most people can sip a 32-ounce soft drink and still feel like they’re ready to eat lunch or dinner. 

So how can you reduce your intake of sugary drinks? Treat your inner two-year-old with a visual illusion.

A glass of juice can come in various shapes—for example, it can be short and fat versus tall and skinny. Which one appears to have more fluid in it? 

Given a choice of glasses of various heights and widths, people will pick a mostly full, tall glass as holding the most fluid. Conversely, a short, wide glass that’s mostly empty appears to have the smallest amount. This occurs even when all of the glasses contain the same amount of fluid.

If you pour yourself a glass of juice, soda, or wine, you’re going to consume a certain number of calories. At the end, based on your consumption monitoring, you’ll have a certain sense of satisfaction.

This portion size distortion suggests that you should drink out of a tall, thin glass when you’re drinking anything but water. When humans try to perceive volume, they base that perception largely on the height of the stimulus. 

When you put the same amount of fluid into a short, wide glass, it looks like less beverage. Essentially, your inner two-year-old continues to fall for this illusion trick even if your brain has learned otherwise. 

By serving your food on smaller plates and drinks in taller glasses, you can increase your sense of how much you’ve consumed. These techniques will help you maintain a healthy eating program without having to use too much willpower.

This article was edited by Kate Findley, Writer for The Great Courses Daily.
Image of Professor Peter Vishton

Dr. Peter M. Vishton is Associate Professor of Psychology at William & Mary. He earned his Ph.D. in Psychology and Cognitive Science from Cornell University. Before joining the faculty of William & Mary, he taught at Northwestern University and served as the program director for developmental and learning sciences at the National Science Foundation.

About Kate Findley 104 Articles
Kate is a writer, novelist, and blogger living in Los Angeles. She has been writing for The Great Courses since 2017. It incorporates her two favorite things: writing and learning.