People prefer to be with people like themselves. For all the celebration of "diversity," it's sameness that dominates. Most people favor friendships with those who have similar backgrounds, interests and values. It makes for more shared experiences, easier conversations and more comfortable silences. Despite many exceptions, the urge is nearly universal. It's human nature.

Perhaps America's greatest glory is to rise above this self-absorption. People with many different heritages and beliefs have blended into a cohesive society. At some point, most people subordinate their own firmly held convictions and loyalties to the larger nation. This is more than patriotism; it's the identity of "being an American." But it is in constant tension with the differences that divide Americans.

The latest manifestation of this is what Bill Bishop calls "the Big Sort." By that, he means that Americans have increasingly "clustered in communities of sameness, among people with similar ways of life, beliefs, and, in the end, politics." Republican fundamentalists congregate with other Republican fundamentalists. Liberal Democrats herd with other liberal Democrats. Environmentalists decamp to Portland, Ore. Child-centered Republican families move to the exurbs of Dallas and Minneapolis.

The increasing segregation of America by social and cultural values -- not just by income -- helps explain America's growing political polarization, Bishop argues in his new book (naturally: "The Big Sort"). Because prosperity enables more Americans to live where they please, they gravitate to lifestyle ghettos -- and that has significant political implications. Citing studies of social psychology, Bishop says that group consciousness actually amplifies likes and dislikes. Views become more extreme. People become more self-righteous and more suspicious of outsiders.

It's not red and blue states so much as red and blue counties. Bishop -- a recovering newspaper columnist -- collaborated with Robert Cushing, a retired professor of sociology from the University of Texas, to examine voting patterns in presidential elections. They classified counties as politically lopsided if one candidate won by 20 percentage points or more. Their findings are stunning. In the 1960 Kennedy-Nixon election, a virtual dead heat, 33 percent of counties qualified. By 2000, also a dead heat, the share was 45 percent. In 2004, it was 48 percent. In 1976, it had been as low as 27 percent.

Not surprisingly, many neighborhoods today have mostly Obama or McCain yard signs, not a competitive mixture. Though he dislikes this sorting, Bishop, for good reason, is not contemptuous of it: He discovered it through personal experience. When he and his wife moved to Austin, they instinctively selected a neighborhood called Travis Heights. It had people like them, who turned out to be highly liberal. How liberal? Well, in 2000, almost 60 percent of Texans voted for George W. Bush. In Travis Heights, Bush finished third, after Al Gore and Ralph Nader. (Despite his views, Bishop's analysis is evenhanded.)