Why women wear heels: a new size illusion?

It is commonly said that tall people look thinner. We asked whether an illusion exists such that the taller of two equally wide stimuli looks thinner, and conversely whether the thinner of two equally tall stimuli looks taller. Participants were randomly assigned to one of four experimental conditions: (1) to judge the horizontal extent of two identical bodies that only differed vertically; (2) to judge the vertical extent of two identical bodies that only differed horizontally; (3) to judge the vertical extent of two images, one of the entire body and the other of half a body; and (4) to judge the vertical extent of two rectangles that only differed horizontally. In all conditions we found a misestimation of the dimension that was not varied. Specifically, subjects judged (1) the taller of two bodies with the same horizontal extent as being narrower on 80.71% of trials (no illusion would result in the taller body being chosen on 50% of the trials); (2) the narrower of two bodies with the same vertical extent as being taller on 72.02% of trials; (3) half a body with the same vertical extent as the entire body as being taller on 56.25% of trials; and (4) the narrower of two rectangles with the same vertical extent as being taller on 58.57% of trials. These data confirm the folk wisdom that being thin makes you look taller and being tall makes you look thinner. The results with the rectangle stimuli suggest, however, that this illusory effect is a more general perceptual phenomenon rather than specific to bodies. We tentatively interpret this effect as a kind of "area constancy": if two identical images differ on one dimension, then they must also differ on the other in order to maintain a constant area.