In 2004, the press buzzed about the polarization of the political parties. Meanwhile, unnoticed, Americans had been reshaping their lives.

Over the past 30 years, the United States has been sorting itself, sifting at the most microscopic levels of society, as people have packed up and moved. Between 4 and 5 percent of the population move each year from one county to another—100 million Americans in the past decade. As they move, they cluster in communities of sameness, among people with similar ways of life, beliefs—and politics. What happened wasn’t a simple increase in political partisanship, but a more fundamental kind of self-perpetuating, self-reinforcing social division.

Americans were busy creating social resonators, and the hum that filled the air was the reverberated and amplified sound of their own voices and beliefs.

—From The Big Sort: Why the Clustering of Like-Minded America Is Tearing Us Apart Copyright © 2008 by Bill Bishop. Published by Houghton Mifflin Harcourt. All rights reserved.