Why Everyone You Know Thinks the Same as You

washingtonpost.com:

"Studies show that most people interested in politics associate nearly exclusively with others who have similar political beliefs. In fact, research by sociologist David Knoke at the University of Minnesota shows that if you know whether a person's friends are Republicans, Democrats or independents, you can predict with near certainty that person's political views.

Homophily may help explain some of the bitter partisanship of our times -- when your friends are drawn exclusively from one half of the electorate, it is not surprising that you will find the views of the other half inexplicable."

Living in an echo-chamber -- I actually wrote an engineering paper on this phenomenon last year at a cooperative-control conference. Basically, when you associate exclusively with like-minded people, you become irrationally confident in your shared views.