Abstract: Many political failure arguments implicitly assume that voters are irrational. (Wittman 1995, 1989; Coate and Morris 1995) This paper argues that this assumption is both theoretically and empirically plausible: In politics, rationality, like information, is a collective good that individuals have little incentive to supply. In consequence, voters are frequently not only rationally ignorant but also "rationally irrational." Rational irrationality leads to both demand side and supply side political failures: Competition not only pressures politicians to act on voters' biased estimates, but selects for politicians who genuinely share those biases. The analytical framework also sheds new light on log-rolling, political shirking and advertising, and politicians' human capital.