Typicality effects in logically defined categories

Subjects learned to categorize geometric designs by some rule combining two features, x and y. When xy stimuli (stimuli with both features) are always positive, the rule (concept) is inclusive disjunctive; when xy stimuli are always negative, the rule is exclusive disjunctive. In three other experimental conditions, the probability of xy stimuli being positive during acquisition was .25, .50, and .75. In a variety of postacquisition tests, xy stimuli were chosen as prototypical despite their less than consistent occurrence in the positive category. Stimuli with one relevant feature (x or y), although consistently assigned to the positive category, were often evaluated as poorer examples of the concept. These results are interpreted in terms of a schema model in which information acquired during learning is organized according to probability density functions over feature dimensions. This theory appears to be general enough to accommodate the evidence from laboratory studies of both logical and natural concepts.