'Demand characteristics' in body-size estimation in anorexia nervosa.

We asked 24 patients with anorexia nervosa and 30 normal controls to estimate their body-size several times, each time using different instructions. The degree of over-estimation was found to vary predictably with the wording of the instructions. Informing the subject that she had made an error without specifying the direction of the error resulted in reduced over-estimation on a subsequent trial, for both anorexics and controls. 'Internally directed' instructions were associated with a greater degree of over-estimation than 'external' instructions in both groups, but particularly in anorexic subjects. Our results indicate the necessity of controlling the 'demand characteristics' of such experiments.