Eyes wide apart: overestimating interpupillary distance

Our ability to perceive many aspects of the human face is so acute that some would propose that we possess specialised face-processing mechanisms. One example of this exquisite performance is our ability to discriminate differences in the lateral separation of the eyes in pairs of otherwise identical faces-see Kemp et al (1990, Perception 19 531-543). However, we have found that our ability to estimate our own interpupillary distance (IPD) shows no such accuracy. Subjects were asked to indicate, without undue rumination, their own IPD and, by way of control, the distance from the bridge to the tip of their own nose. Compared to the true distances, IPD was overestimated by more than 30% while nose length was underestimated by 11%. Although a plausible reason for the nose length underestimation is foreshortening of the image, the reason for the overestimation of eye separation remains obscure.