Abstract

Similarity is a compelling part of everyday experience . In the visual world, objects that are similar in shape or color may seem to leap to our attention . In conceptual processing, we have an immediate sense of whether a pair of concepts is similar . The prominence of similarity in conscious experience has made it an important explanatory construct in psychological theories . New problems are assumed to be solved on the basis of their similarity to known problems (e .g ., Reed, Ernst, & Banerji, 1974; Ross, 1987) . Objects are assumed to be classified on the basis of their similarity to some stored category representation (e .g ., Medin & Schaffer, 1978 ; Reed, 1972). Predictions of new features of an item may be based on what other similar items have those features (e .g ., Blok & Gentner, 2000 ; Heit & Rubinstein, 1994 ; Osherson, Smith. Wilkie, Lopez, & Shafir, 1990 ; Sloman, 1993) . Yet despite extensive work on mechanisms of similarity, there has been very little discussion of why and how similarity is important in cognitive processing beyond the general recognition that similarity often provides a good basis for generalization (Shepard, 1987) . In this chapter, we consider the role of similarity in the cognitive architecture and the relationship of similarity to automatic processing . We suggest that some types of similarity are determined automatically . When the cognitive system recognizes similarities . they influence cognitive processing, even when the person does not intend processing to be affected by similarities . To support this claim, we first outline three approaches to similarity . Then, we examine how similarity can influence both low-level processes like attention and memory retrieval and higher cognitive processes like analogical reasoning and decision making . Next, we explore a number of examples in which cognitive processing is in-