The economic conditions of one’s life can profoundly and systematically influence health outcomes over the life course. Our present research demonstrates that rejecting the notion that social class categories are biologically determined—a nonessentialist belief—buffers lower-class individuals from poor self-rated health and negative affect, whereas conceiving of social class categories as rooted in biology—an essentialist belief—does not. In Study 1, lower-class individuals self-reported poorer health than upper-class individuals when they endorsed essentialist beliefs but showed no such difference when they rejected such beliefs. Exposure to essentialist theories of social class also led lower-class individuals to report greater feelings of negative self-conscious emotions (Studies 2 and 3), and perceive poorer health (Study 3) than upper-class individuals, whereas exposure to nonessentialist theories did not lead to such differences. Discussion considers how lay theories of social class potentially shape long-term trajectories of health and affect of lower-class individuals.