Summary: PsychologicalReview Copyright 1988 by the American PsychologicalAssociation, Inc.
1988, Vol. 95, No. 1, 124-150 0033-295X/88/$00.75
Toward a Unified Theory of Similarity and Recognition
E Gregory Ashby
University of California, Santa Barbara
Nancy A. Perrin
Portland State University
A new theory of similarity, rooted in the detection and recognition literatures, is developed. The
general recognition theory assumes that the perceptual effectof a stimulus is random but that on
any singletrial it can be represented asa point in a multidimensional space. Similarity is a function
of the overlap of perceptual distributions. It is shown that the general recognition theory contains
Euclidean distance models ofsimilarity as a specialcase but that unlike them, it is not constrained
byanydistance axioms. Three experiments are reported that test the empirical validityofthe theory.
In these experiments the general recognition theory accounts for similarity data as wellas the cur-
rently popular similarity theories do, and it accounts for identification data as well as the long-
standing "champion" identification model does.
The concept of similarity is of fundamental importance in
psychology.Not only is there a vast literature concerned directly
with the interpretation of subjective similarity judgments (e.g.,
as in multidimensional scaling) but the concept also plays a cru-