Visual biases in judging body weight.

OBJECTIVES: There has been a steady rise in obesity levels in Western countries, and a contributory factor is people's failure to recognize weight gain. Two important visual perceptual biases, contraction bias and Weber's law, that have hitherto been ignored in the obesity literature could contribute to this problem. Contraction bias predicts that the weight of obese bodies will be underestimated and the degree of underestimation will increase as body mass index (BMI) increases. Weber's law predicts that change in the body size will become progressively harder to detect as their BMI increases.

METHODS: In Experiment 1, 29 women participants estimated the weight of 120 women varying in their body mass. In Experiment 2, 28 women participants judged which body was the heavier in a 2-alternative forced choice paradigm.

RESULTS: In Experiment 1, as predicted the participants showed a progressive underestimation of overweight and obese bodies, β1  = 0.71, t = 26.96, p < .0001. For Experiment 2, there was a significant effect of the BMI of the bodies being judged on the just noticeable difference needed to discriminate between them: F(1, 196) = 89.39, p < .0001 for 3D bodies and F(1, 86.5) = 44.57, p < .0001 for digital photographs.

CONCLUSIONS: Normal visual perceptual biases influence our ability to determine body size: contraction bias and Weber's law mean that as bodies become overweight and obese, it is harder to judge their weight and detect any increase in size. These effects may therefore compromise people's ability to recognize weight gain and undertake compensatory weight control behaviours. Statement of contribution What is already known on this subject? It is common knowledge that obesity levels in the West are rapidly rising and that people fail to recognize weight gain. What has not been widely recognized before is that there are sound perceptual reasons for this failure. Here, we identify two such perceptual biases. What does this study add? Weber's law and contraction bias compromise people's ability to recognize weight gain. It becomes progressively harder to discriminate the size of bodies as their body mass index increases. This compromises the ability to recognize weight gain and undertake compensatory behaviours.