Similarity- versus rule-based categorization

An influential study by Rips (1989) provides the strongest evidence available that categorization cannot be reduced to similarity. In Rips's study, subjects were presented a sparse description of an object that mentioned only a value on a single dimension (e.g., "an object 3 inches in diameter"), followed by two categories (e.g., pizza and quarter), where one allowed more variability on the relevant dimension than did the other (the diameter of pizzas is more variable than that of quarters). Subjects judged the described object to be more likely to be a member of the variable category (pizza), but more similar to the nonvariable category (quarter). This dissociation between categorization and similarity strongly implies that categorization was not based on similarity. In our first experiment, we used sparse descriptions like Rips's, as well as rich descriptions that contained features characteristic of the nonvariable category. We found that categorization tracked similarity with both kinds of descriptions. In a second experiment, we modified our procedure to be more like that of Rips's by requiring subjects to think aloud while making their decisions. Now, like Rips, we found a dissociation between similarity and categorization with sparse items; with rich descriptions, categorization again tracked similarity. These findings are consistent with the hypothesis that categorization can be done in two ways, by similarity and by rule. An exclusive reliance on rule-based categorization seems to occur only when the description of the to-be-categorized object does not contain any features characteristic of any relevant category.