The so-called “paradox of voting” is a major anomaly for rational choice theories of elections. If voting is costly and citizens are rational, then in large electrorates the expected turnout would be small, for if many people voted the chance of anyone being pivotal would be too small to make the act worthwhile. Yet many people do vote, even in large national elections. To address this puzzle we construct a model of adaptive rationality: Citizens learn by simple trial and error, repeating satisfactory actions and avoiding unsatisfactory ones. (Their aspiration levels, which code current payoffs as satisfactory or unsatisfactory, are also endogenous, themselves adjusting to experience.) Our main result is that agents who adapt in this manner turn out in substantial numbers even in large electorates and even if voting is costly for everyone.Standard conceptions of rational behavior do not explain why anyone bothers to vote in a mass election…. [Turnout is] the paradox that ate rational choice theory.Fiorina (1990, 334)We would like to thank Stephen Ansolabehere, Sorin Antohi, Glenn Ellison, Dedre Gentner, Sunil Kumar, David Laitin, Tze Lai, Arthur Lupia, Elijah Millgram, Lincoln Moses, Scott Page, Tom Palfrey, John Patty, Paul Pfleiderer, Adam Simon, Joel Sobel, Carole Uhlaner, three anonymous referees, and the participants in the Political Economics seminar at the GSB, the Stanford–CalTech workshop, the UNC American Politics Research Group, the UCLA conference on Cognition, Emotion, and Rational Choice, panels at the Annual Meetings of the MPSA and the APSA, the Agent 2000 Workshop, the Seventh Annual Wallis Conference, the CMU–Pitt Colloquium Series, and the Center for Advanced Study in the Behavioral Sciences seminar series for their helpful comments. This paper was written while Ting was at the University of North Carolina at Chapel Hill, and he thanks UNC's Department of Political Science for its support. It was revised while Bendor was a Fellow at the CASBS, and he is grateful for the Center's financial and intellectual support.