Americans’ hostility toward political opponents has intensified to a degree not fully explained by actual ideological polarization. We propose that political animosity may be based particularly on partisans’ overestimation of the prevalence of extreme, egregious views held by only a minority of opponents but imagined to be widespread. Across five studies (N= 4993; three preregistered), we examine issue extremity as an antecedent of false polarization. Both liberals and conservatives report high agreement with their party’s moderate issues but low agreement with the extreme issues associated with their side. As expected, false polarization did not occur for all issues. Partisans were fairly accurate in estimating opponents’ moderate issues (even underestimating agreement somewhat). In contrast, partisans consistently overestimated the prevalence of their opponents’ extreme, egregious political attitudes. (Over)estimation of political opponents’ agreement with extreme issues predicted cross-partisan dislike, which…