doi:10.1093/pan/mpp003 The Microfoundations of Mass Polarization

Although there has been considerable attention to the question of how much polarization there is in the mass electorate, there has been much less attention paid to the mechanism that causes polarization. I provide evidence demonstrating the occurrence of individual-level conversion—individual Democrats and Republicans becoming more liberal and conserva-tive. Although over the short term most of the observed changes are quite small and cannot be distinguished from measurement error, over time and many respondents, these move-ments aggregate to generate polarization. Small individual-level preference shifts provide an important foundation for aggregate polarization. Few issues have seized the attention of political scientists—not to mention journalists—to the same extent as the debate over mass polarization. In brief, the controversy centers around whether ordinary Americans are now more ideologically divided than they were a generation ago (for the competing perspectives, contrast Fiorina, Abrams, and Pope 2005; Abramowitz and Saunders 2008). Regardless of where one stands on the level of polarization, an important question remains: what mechanism causes polarization? Most of the literature argues that conversion—individual voters becoming more extreme ove