The influence of personal BMI on body size estimations and sensitivity to body size change in anorexia spectrum disorders

• Anorexia spectrum disorders (ANSD) and control participants estimated body size using novel 3D stimuli.
• Controls’ estimates depend on BMI, show contraction bias and follow Weber's law.
• Lowest BMI ANSD women's estimates are very accurate and extremely sensitive.
• Thereafter, ANSD women over-estimate body size in direct proportion to their BMI.
• These marked differences cannot be explained away by psychological concerns alone.

In this cross-sectional study, we investigated the influence of personal BMI on body size estimation in 42 women who have symptoms of anorexia (referred to henceforth as anorexia spectrum disorders, ANSD), and 100 healthy controls. Low BMI control participants over-estimate their size and high BMI controls under-estimate, a pattern which is predicted by a perceptual phenomenon called contraction bias. In addition, control participants’ sensitivity to size change declines as their BMI increases as predicted by Weber's law. The responses of women with ANSD are very different. Low BMI participants who have ANSD are extremely accurate at estimating body size and are very sensitive to changes in body size in this BMI range. However, as BMI rises in the ANSD participant group, there is a rapid increase in over-estimation concurrent with a rapid decline in sensitivity to size change. We discuss the results in the context of signal detection theory.