David Schkade, Cass R. Sunstein & Reid Hastie, When Deliberation Produces Extremism, 22 Critical Rev. 227 (2010).
Abstract: What are the effects of deliberation about political issues by likeminded people? An experimental investigation involving two deliberative exercises, one among self-identified liberals and another among self-identified conservatives, showed that participants' views became more extreme after deliberation. Deliberation also increased consensus and significantly reduced diversity of opinion within the two groups. Even anonymous statements of personal opinion became more extreme and homogeneous after deliberation.