TESTING THE PROTOTYPE THEORY OF CONCEPTS

Four experiments were designed to test two predictions of Prototype Th eory. The first prediction was that when the defining (necessary) feat ures of a concept are only partially matched by an instance, then char acteristic (nonnecessary) features of concepts can affect categorizati on. The test of this prediction was rendered problematic as successive experiments failed to identify clearly necessary features for a range of concepts. The second hypothesis related to the independence of fea tures in determining similarity. Most versions of Prototype Theory ass ume a linear combination of feature matches, which would predict that the effect of changing a feature on category membership should be grea test when the probability of categorization is closest to 50% (i.e. at the category border). The results showed that, contrary to this predi ction, the effect of changing a feature was greatest when other featur es were all positive, and so categorization probability was at a maxim um. The results support either a logistic combination rule for assessi ng similarity on the basis of feature match (Medin & Shaffer, 1978), o r an exponential generalization function relating similarity to protot ype to the sum of matching features (Shepard, 1987). (C) 1995 Academic Press, Inc.