Visual Biases in Estimating Body Size

Western populations have been getting progressively heavier and the prevalence of obesity in the general population has become a major public health problem. A potential contributory factor in the rise of obesity is the inability to accurately detect weight increase using visual cues. This is a problem not only for members of the public whose weight is increasing but also for healthcare professionals who need to detect weight gain in their patients. Three main visual biases effect body judgments: contraction bias, adaptation, and Weber’s law. Contraction bias (a result of how we estimate body size) leads to people overestimating the BMI of bodies thinner than the average and significantly underestimating the BMI of bodies heavier than the average. Adaptation (the effect of seeing heavier bodies in the general population) leads to a shift in what people regard as a normal and acceptable body size toward a heavier BMI. Weber’s law (which requires a detectable size change to be a fixed proportion of the body’s total size) means that size change in obese bodies becomes increasingly difficult to detect. The cumulative effect of these visual biases is to make it harder to detect both being obese and weight increase when obese.