Without heterogeneity in ideology-preferences and opinions over political actions-there would be little need for the institutions studied by political economists.1 However, the sources of ideology have received scant attention: since Marx, political economists have largely viewed ideology as driven by wealth or income-despite the fact that these variables explain little of the variation in ideology ( This paper proposes a complementary theory in which differences in ideology are also due to imperfect information processing. This theory predicts that overconfidence in one's own beliefs leads to ideological extremeness, increased voter 1 Without heterogeneity, institutions will still be useful for coordination (Weingast 1997). Traditionally, opinions have not been considered part of ideology; however, recent work has provided compelling arguments that ideology should include some beliefs (McMurray 2013a, b).