Similarity

An enormous amount of ink has been spilled in the psychology literature on the topic of similarity. There are two reasons that this seemingly intuitive and prosaic concept has been the subject of such intense scrutiny. First, there is virtually no area of cognitive processing in which similarity does not seem to play a role. William James observed in 1890 that “this sense of Sameness is the very keel and backbone of our thinking” (p. 459). Ivan Pavlov first noted that dogs would generalize their learned salivation response to new sounds as a function of their similarity to the original tone, and this pattern of generalization appears to be ubiquitous across species and stimuli. People group things together based on their similarity, both during visual processing and categorization. Research suggests that memories are retrieved when they involve similar features or similar processing to a current situation. Problem solutions are likely to be retrieved from similar prior problems, inductive inference is largely based on the similarity between the known and unknown cases, and the list goes on and on.

Psychology clearly has an enormous stake in similarity. In fact, the depth of this stake makes it that much more unsettling that similarity can be such a slippery and temperamental construct. Perceived similarity can vary considerably with context. In fact, the act of comparison itself can change people's representations, leading them to construe things as more comparable by reinterpreting the given features or even creating new features. Similarity ratings are often asymmetric; for instance, people view North Korea to be more similar to China than the reverse. Likewise, similarity and difference are not always the inverse of one another. When given a choice between XX and OX, people choose XX as both more similar to and more different from OO. Similarity can change systematically with temporal distance and physical distance, and there is a growing body of evidence for consistent individual differences in which kinds of features drive a person's similarity judgments. Judgments of similarity can increase with simple association. For example, coffee is judged to be similar to cream because it is contextually associated with cream. In the absence of objective ways to measure psychological similarity, researchers are left to rely on participants’ subjective judgments of their own processing, which are potentially unreliable, or on data from tasks in which similarity is proposed to play a role, leading to conclusions that are potentially circular. Thus, it is possible that similarity is both the most essential and the most problematic construct in cognitive science. This entry discusses some of the important findings from the extensive literature on similarity and the ways in which psychologists have attempted to address a topic so critical to our understanding of the mind and yet so elusive.