Similarity Involving Attributes and Relations : Judgments of Similarity and Difference Are Not Inverses

Conventional wisdom and previous research suggest that similarity judgments and difference judgments are inverses of one another. An exception to this rule arises when both relational similarity and attributional similarity are considered . When presented with choices that are relationally or attributionally similar to a standard, human subjects tend to pick the relationally similar choice as more similar to the standard and as more different from the standard. These results not only reinforce the general distinction between attributes and relations but also show that attributes and relations are dynamically distinct in the processes that give rise to similarity and difference judgments . The question of what makes things seem alike or seem different is fundamental to cognition. Models of learning imply that the learning of a task is facilitated if it is similar to another task that is already part of a learner's repertoire (Thorndike, 1966) . Stimulus generalization occurs as a function of how similar the new stimulus is to the conditioned stimulus (Pavlov, 1927 ; Shepard, 1987) . In memory models it has been assumed that X reminds people of Y if it is similar to Y (Kolodner, 1984 ; Schank, 1982), and in theories of categorization it has been assumed that classification of new examples is based on their similarity to known examples or to a category prototype (Medin & Smith, 1984 ; Oden, 1987) . In short, similarity is a central concept in cognitive science . Correspondence and reprint requests to : Douglas L. Medin, Dept . of Psychology, University of Michigan, 330 Packard Rd ., Ann Arbor, MI 48103 . 64 Copyright © 1990 American Psychological Society PSYCHOLOGICAL SCIENCE