Five studies examined the hypothesis that people overestimate the influence of self-interest on attitudes and behaviors. The results strongly supported the hypothesis. In Study 1, participants overestimated the impact that financial reward exerted on their peers' willingness to donate blood. In 4 subsequent studies, participants overestimated the impact that group membership had on their peers' attitudes (Studies 2, 3, and 4) and behaviors (Study 5). The tendency to overestimate the impact of self-interest on others was largely unrelated to the impact that it had on participants' own attitudes and behaviors. Implications of the lay person's belief in the power of self-interest are discussed.