Aristotle proposed that to achieve happiness and success, people should cultivate virtues at mean or intermediate levels between deficiencies and excesses. In stark contrast to this assertion that virtues have costs at high levels, a wealth of psychological research has focused on demonstrating the well-being and performance benefits of positive traits, states, and experiences. This focus has obscured the prevalence and importance of nonmonotonic inverted-U-shaped effects, whereby positive phenomena reach inflection points at which their effects turn negative. We trace the evidence for nonmonotonic effects in psychology and provide recommendations for conceptual and empirical progress. We conclude that for psychology in general and positive psychology in particular, Aristotle's idea of the mean may serve as a useful guide for developing both a descriptive and a prescriptive account of happiness and success.