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Calorie underestimation is often alleged to contribute to obesity, but is obesity really caused by the underestimation of the number of calories contained in large fast–food meals? We propose a psychophysical model of meal size estimation, hypothesizing that estimation biases are caused by the size of a meal, regardless of the body mass of the person doing the estimation. We asked 55 students to estimate the size of 8 different fast–food meals in calories and found that both low–BMI and high–BMI people underestimated calories consistently.
In a second study, 156 students chose the size of a sandwich, portion of fries, and soft drink they preferred and were asked to estimate the number of calories contained in the meal they had created. The students were divided into 3 groups: one informed of the biasing effect of meal size but asked to estimate the total caloric content of the meal, one that was identical to Study 1, and one in which participants were asked to estimate the caloric content of each component of their preferred meal. We found that students with a high BMI choose larger meals than those with low BMI's and that as a result, high–BMI students are more prone to underestimation. However, after controlling for the size of the meal, the estimations of both groups were identical.
In a follow–up study at 3 different fast–food restaurants, 147 customers were asked to estimate the number of calories in their meals. All participants underestimated the caloric content of their meal, but once again were more accurate in estimating the calories in smaller meals.
A fourth study was conducted on 405 dietetic professionals, who were asked to estimate the number of calories in 3 fast–food meals containing the same ingredients but of different sizes or to estimate the number of calories a low– or high–BMI person would guess. We found that dietitians are prone to psychophysical underestimation, but less so than consumers. Additionally, dietitians wrongly predict that high–BMI people will underestimate more than low–BMI people or themselves.
Based on these data, we conclude that biases in calorie estimation are caused by meal size, not body size, and that piecemeal estimations are more accurate than total meal estimations.
Chandon, Pierre and Brian Wansink (2007). Is Obesity Caused by Calorie Underestimation? A Psychophysical Model of Fast–Food Meal Size Estimation. Journal of Marketing Research, 44:1, 84–99. Reprinted with permission from Journal of Marketing Research, published by the American Marketing Association. Chandon and Wansink, 44:1, 84–89.