In this research we examine estimates of American social class mobility—the ability to move up or down in education and income status. Across studies, overestimates of class mobility were large and particularly likely among younger participants and those higher in subjective social class—both measured (Studies 1–3) and manipulated (Study 4). Class mobility overestimates were independent of general estimation errors (Study 3) and persisted after accounting for knowledge of class mobility assessed in terms of educational attainment and self-ratings. Experiments revealed that mobility overestimates were shaped by exposure to information about the genetic determinants of social class—a faux science article suggesting genetic constraints to economic advancement increased accuracy in class mobility estimates (Study 2)—and motivated by needs to protect the self—heightening the self-relevance of class mobility increased overestimates (Study 3). Discussion focused on both the costs and benefits of overestimates of class mobility for individuals and society.