This paper, forthcoming in Journal of Risk Research, examines a remedy for a defect in existing accounts of public risk perceptions. The accounts in question feature two dynamics: the affect heuristic, which emphasizes the impact of visceral feelings on information processing; and the cultural cognition thesis, which describes the tendency of individuals to form beliefs that reflect and reinforce their group commitments. The defect is the failure of these two dynamics, when combined, to explain the peculiar selectivity of public risk controversies: despite their intensity and disruptiveness, such controversies occur less frequently than the affect heuristic and the cultural cognition thesis seem to predict. To account for this aspect of public risk perceptions, the paper describes a model that adds the phenomenon of culturally antagonistic memes—argumentative tropes that fuse positions on risk with contested visions of the best life. Arising adventitiously, antagonistic memes transform affect and cultural cognition from consensus-generating, truth-convergent influences on information processing into conflictual, identity-protective ones. The paper supports this model with experimental results involving perceptions of the risk of the Zika virus: a general sample of U.S. subjects, whose members were not polarized when exposed to neutral information, formed culturally polarized affective reactions when exposed to information that was pervaded with antagonistic memes linking Zika to global warming; when exposed to comparable information linking Zika to unlawful immigration, the opposed affective stances of the subjects flipped in direction. Normative and prescriptive implications of these results are discussed.