structure. Perceptual classification learning experiments were conducted in which presentation frequencies of individual exemplars were manipulated. The exemplars had varying degrees of similarity to members of the target and contrast categories. Classification accuracy and typicality ratings increased for exemplars presented with high frequency and for members of the target category that were similar to the high-frequency exemplars. Typicality decreased for members of the contrast category that were similar to the high-frequency exemplars. A frequency-sensitive similarity-to-exemplars model provided a good quantitative account of the classification learning and typicality data. The interactive relations among similarity, frequency, and categorization are considered in the General Discussion. Among the most well-established findings in the categorization literature is that categories have "graded structures"

this paper and the writing of the paper were supported, in part, by grants to Dominic W. Massaro from the Public Health Service (PHS R01 NS 20314), the National Science Foundation (BNS 8812728), a James McKeen Cattell Fellowship, and the graduate division of the University of California, Santa Cruz. Cohen & Massaro On the Similarity of Categorization Models 2