Experiment 1: Participants overestimated the diameter of food portions by 5% and the visual area of food portions by 10% on plates with wider rims compared to plates with very thin rims (P<0.0001). The effect of rim width was greater with larger food portion sizes. Experiment 2: Participants overestimated the diameter of food portions by 1.5% and the visual area of food portions by 3% on plates with rim coloring compared to plates with no coloring (P=0.01). The effect of rim coloring was greater with smaller food portion sizes.

Within-subjects experimental design. Experiment 1 tested the effect of rim width on perceived food portion size. Experiment 2 tested the effect of rim coloring on perceived food portion size. In both experiments, participants observed a series of photographic images of paired, side-by-side plates varying in designs and amounts of food. From each pair, participants were asked to select the plate that contained more food. Multi-level logistic regression examined the effects of rim width and coloring on perceived food portion size.

INTRODUCTION

Small changes in the food environment can alter perceptions of food size, influence food choices, and reduce how much food people serve themselves and consume (1). These environmental changes include, but are not limited to alterations in portion size (2-6), container size (7-11), food variety (12, 13), and visual cues about the amount consumed (8). As a result, it may be possible to develop environmental interventions that bypass conscious, cognitive self-control and, instead, “passively” or “mindlessly” produce consistent reductions in energy intake and weight loss over time.

Plate size is one environmental factor that researchers have focused on because it may be an easy factor to manipulate. In the last three decades, the size of the average dinner plate increased more than a third (36%) (14). Observational data suggest that the increase in the average size of plates used at meals is concurrent with the rise in obesity rates in the United States (15). Research using mathematical modeling indicates that a small increase in dishware size can lead to a substantial increase in energy intake, particularly if food is energy dense (16). Experimental studies show that larger plates make a given portion size look smaller (17). However, smaller plates did not reduce food intake at meals in laboratory studies (18). It is possible that reducing total plate size may be insufficient to reduce intake because substantially smaller plates may make people more aware of smaller portion sizes, which might subsequently result in caloric compensation. However, not all same-sized plates are equal. It may be necessary to pay more attention to plate design characteristics, such as plate color and rim design.

Plate designs that make use of visual illusions, such as the Delboeuf illusion, may independently affect perceived portion size and subsequent food intake. The Delboeuf illusion demonstrates that a circle appears larger when surrounded by a slightly larger circle, but smaller when surrounded by a much larger circle (19) ( ). The proximity and the color contrast of the concentric circles affect the illusion (20-23). Research suggests that when the space between concentric circles is small (right image of ), the circles appear to be one whole object (24). Therefore, people are more likely to emphasize the similarities between the two circles and perceive the inner circle as looking larger than it actually is (25). Conversely, when the space between concentric circles is large (left image of ), the circles are perceived as two different objects. Therefore, people will emphasize the differences between the two circles (23), leading the inner circle to be perceived as smaller than it actually is (26). In the case of food on a plate, a portion of food may be perceived as larger on a small plate, but smaller on a large plate due to the space between the food and the edge of the plate. Increasing color contrast between the circles has also been shown to enhance the differences between concentric circles (22). Manipulating rim coloring may highlight the edge of the outer circle and color contrast leads people to perceive the size of the inner circle as larger than it actually is (22, 27, 28) and perceive a larger food portion size when the color of the plate is different from the color of the food.

Recent research by Van Ittersum and Wansink illustrates that the Delboeuf illusion influences serving behavior as the diameter of dishware increases and when there is a color contrast between food and dishware (17). Big bowls (larger diameter) lead people to serve more food and small bowls (smaller diameter) lead people to serve less food. People also serve themselves more food when food and plates had similar coloring (low-contrast). Conversely, people serve themselves less food when food and plates were different colors (high-contrast).

The current study extends previous research on how the Delboeuf illusion applies to food on a plate. Because the rim of a round plate can be conceptualized as an outer circle surrounding food, we hypothesized that the Delboeuf illusion would apply to plate rims surrounding food on a plate. We describe two experiments in which we manipulated images of plate rim designs and food portions on plates. Our goal was to determine the effect of rim width and color on the perception of portion size. To do this, we quantified perceived food portion size for each rim design used in the study. Experiment 1 examines the effect of rim width on perceived food portion size. Experiment 2 examines the effect of rim coloring on perceived food portion size. Prior work suggests that the Delboeuf illusion is strongest when the outer circle is only slightly larger and/or the color is more greatly contrasted with the inner circle (20-23). Therefore, in Experiment 1 we hypothesized wider rims (and thus the closer the inner edge of the rim surrounds the food) would cause food to appear larger on the same sized plate (Hypothesis 1). In Experiment 2 we hypothesized colored rims (highlighting the rim to create contrast) would cause food to appear larger than uncolored rims (Hypothesis 2). Results could help inform a superior plate design to influence perceived portion size.