Ties That Bind: Reconciling Discrepancies Between Categorization and Naminf

Ties That Bind: Reconciling Discrepancies Between Categorization and Naming Kenneth R. Livingston (livingst@vassar.edu) Department of Psychology and Program in Cognitive Science Vassar College, 124 Raymond Avenue, Box 479 Poughkeepsie, New York 12604 Janet K. Andrews (andrewsj@vassar.edu) Department of Psychology and Program in Cognitive Science Vassar College, 124 Raymond Avenue, Box 146 Poughkeepsie, New York 12604 Patrick Dwyer (padwyer@vassar.edu) Department of Psychology Vassar College, 124 Raymond Avenue, Box 3532 Poughkeepsie, New York 12604 Abstract We present the results of a study designed to show that dissociations between lexical and similarity- based boundary partitions for a set of items can be produced in the laboratory. This is achieved by an incremental process of learning to assign a category label to items increasingly far removed (in similarity space) from the center of that category and approaching a different category. This process occurs in parallel with a compression effect in psychological similarity space such that increasingly distant items labeled as members of category A nonetheless come to be viewed as more similar to category B (the category to which they are in fact closer in pre- category learning similarity space) than they are by people who have not learned the category distinction. Introduction Although patterns of similarity are related in a more complex way to categorization and concept learning than was once thought, the evidence is mounting that such relationships are crucial to the partitioning of a set of items into categories (Medin, 1989; Medin, Goldstone, and Gentner, 1993; Goldstone, 1994a; 1994b). Among the most interesting recent findings is the discovery that the psychological similarity space usually assumed in the effort to measure relationships among category members is not static, but may actually undergo a change in its metric properties during the process of category learning. Thus, for example, Goldstone and his colleagues have found repeatedly that people who have learned to categorize a set of items make more reliable discriminations between pairs of items that cross the category boundary than people who have not learned to categorize them (Goldstone, 1994a; 1994b) This expansion or acquired distinctiveness effect seems to involve a stretching of the psychological similarity space in the region of the category boundary, with the result that smaller changes along the category relevant dimensions are sufficient to produce a just noticeable difference (JND). Category clusters might also be formed by a process of compression in the region of similarity space that contains a set of items, with the result that a larger change is required to produce a JND. A number of researchers have found evidence for this process as well (Kurtz, 1996; Livingston, Andrews, and Harnad, 1998). Either of these processes, expansion or compression of the similarity space, is sufficient to produce a categorical distinction between sets of items, and thus looks promising as a general mechanism for concept learning. On such an account, a concept is formed when regions of psychological similarity space are warped so as to create a relatively more compact representational structure that can be more readily manipulated as a unit. This description of a general mechanism for category learning is called into question, however, by recent findings concerning the relationship of language labels to category similarity structures. Malt, Sloman, Gennari, Shi, and Wang (1999) asked native speakers of English, Spanish, and Chinese to name each member of a set of sixty containers. They then asked these same people to sort these sixty items into groups based on their overall similarity. Perhaps not surprisingly, language communities differ in their lexical partitioning of the set of items. Chinese speakers partition the set using five words, English speakers use 7, and Spanish speakers use 15. The variation in grain is not the result of simply forming a greater number of subcategories in,