A review and meta-analysis of methodological and subject variables influencing the exposure-affect relationship was performed on studies of the mere exposure effect published in the 20 years following Zajonc's (1968)-seminal monograph. Stimulus type, stimulus complexity, presentation sequence, exposure duration, stimulus recognition, age of subject, delay between exposure and ratings, and maximum number of stimulus presentations all influence the magnitude of the exposure effect. Implications of these findings are discussed in the context of previous reviews of the literature on exposure effects and with respect to prevailing theoretical models of the exposure-affect relationship. Modifications of the 2-factor model of exposure effects that increase the heuristic value of the model are described. A possible evolutionary basis of the exposure effect is discussed.