This research investigated two competing explanations of the similarity effect: Byrne’s (1971) reinforcement model and the information processing perspective. A meta-analysis of 240 laboratory-based similarity studies explored moderators important to the similarity effect, including set size, proportion of similarity, centrality of attitudes, and information salience. Results indicated effects for proportion of similarity, centrality of attitudes, and information salience, and were largely consistent with predictions of the information processing perspective. We discuss the implications of these findings for the two perspectives, for other models for the similarity effect, and for the role of affect and cognition in the experience of interpersonal attraction.