In a striking empirical regularity, deliberation tends to move groups, and the individuals who compose them, toward a more extreme point in the direction indicated by their own predeliberation judgments. For example, people who are opposed to the minimum wage are likely, after talking to each other, to be still more opposed; people who tend to support gun control are likely, after discussion, to support gun control with considerable enthusiasm; people who believe that global warming is a serious problem are likely, after discussion, to insist on severe measures to prevent global warming. This general phenomenon--group polarization--has many implications for economic, political, and legal institutions. It helps to explain extremism," radicalization," cultural shifts, and the behavior of political parties and religious organizations; it is closely connected to current concerns about the consequences of the Internet; it also helps account for feuds, ethnic antagonism, and tribalism. Group polarization bears on the conduct of government institutions, including juries, legislatures, courts, and regulatory commissions. There are interesting relationships between group polarization and social cascades, both informational and reputational. Normative implications are discussed, with special attention to political and legal institutions.