Book excerpt: Group polarization—the tendency of groups to incline toward more extreme positions than initially held by their individual members—has been rigorously studied by social psychologists, though in a way that has overlooked important philosophical questions. This is the first book-length treatment of group polarization from a philosophical perspective. The phenomenon of group polarization raises several important metaphysical and epistemological questions. From a metaphysical point of view, can group polarization, understood as an epistemic feature of a group, be reduced to epistemic features of its individual members? Relatedly, from an epistemological point of view, is group polarization best understood as a kind of cognitive bias or rather in terms of intellectual vice? This book compares four models that combine potential answers to the metaphysical and epistemological questions. The models considered are: group polarization as (i) a collective bias; (ii) a summation of individual epistemic vices; (iii) a summation of individual biases; and (iv) a collective epistemic vice. Ultimately, the authors defend a collective vice model of group polarization over the competing alternatives. The Philosophy of Group Polarization will be of interest to students and researchers working in epistemology, particularly those working on social epistemology, collective epistemology, social ontology, virtue epistemology, and distributed cognition. It will also be of interest to those working on issues in political epistemology, applied epistemology, and on topics at the intersection of epistemology and ethics.