This paper studies, theoretically and empirically, the role of overcondence in political behavior. Our model of overcondence in beliefs predicts that overcondence leads to ideological extremeness, increased voter turnout, and increased strength of partisan identification. Moreover, the model makes many nuanced predictions about the patterns of ideology in society, and over a person's lifetime. These predictions are tested using unique data that measure the overcondence, and standard political characteristics, of a nationwide sample of over 3,000 adults. Our numerous predictions find strong support in these data. In particular, we document that overcondence is a substantively and statistically important predictor of ideological extremeness and voter turnout.