Abstract: How does group behavior drive extremism and challenge democratic values? The answer lies in social dynamics–the ways people influence one another. Conventional wisdom suggests that open discussion within groups will lead to compromise and moderation, yet just the opposite often occurs. In the course of exchanging opinions, like-minded people frequently develop more extreme versions of their original views on such issues as climate change, labor policy, same-sex relationships, and affirmative action. Groups ranging from citizens’ forums to judicial panels tend to squelch diversity and polarize opinion. With the Internet facilitating the formation of like-minded groups, this phenomenon may help account for the intensity and division of contemporary social and political debate. Indeed, the dangers of homogeneity and polarization within groups highlight a fundamental tension between the consequences of free speech and assembly, and the value of intellectual diversity to a civil society. In Why Groups Go to Extremes, Cass R. Sunstein argues that the key to preventing the spread of extremist views is not to suppress deliberation among the like-minded; such groups productively challenge conventional thinking and majority opinion. Instead, policymakers should develop institutions to ensure that like-minded groups encounter a diversity of opinions within civil society. The goal, Sunstein contends, must be to create opportunities for civil deliberation that expose like-minded group members to opposing views, while exposing society at large to the views of such groups.