Faces reversed in contrast cannot be readily recognized, an effect absent in object recognition. Why? Four factors: expertise, reflectance (pigmentation), high similarity, and the need to discriminate metrically varying smooth surfaces have been offered as explanations. Observers achieved expertise on discriminating smoothly shaped, pigmented, non-face blobs with positive contrast, where distractor similarity matched that of a set of faces in shape and reflectance. On a match-to-sample task, reversal of contrast between sample and matching images had no effect when matching such blobs, but markedly degraded performance when matching faces suggesting that this effect is unique to faces.