Cass Sunstein thinks that hanging out with those you agree with can cause extremism:
[M]uch of the time groups of people end up thinking and doing things that group members would never think or do on their own. This is true for groups of teenagers, who are willing to run risks that individuals would avoid. It is certainly true for those prone to violence, including terrorists and those who commit genocide. It is true for investors and corporate executives. It is true for government officials, neighbourhood groups, social reformers, political protestors, police officers, student organisations, labour unions and juries. Some of the best and worst developments in social life are a product of group dynamics, in which members of organisations, both small and large, move one another in new directions.
(Hat tip: 3QD)