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SOME folks in Texas recently decided to start a new community “containing 100% Ron Paul supporters”. Mr Paul is a staunch libertarian and, until recently, a Republican presidential candidate. His most ardent fans are invited to build homesteads in “Paulville”, an empty patch of west Texas. Here, they will be free. Free not to pay “for other people's lifestyles [they] may not agree with”. And free from the irksome society of those who do not share their love of liberty.

Cynics chuckle, and even Mr Paul sounds unenthusiastic about the Paulville project, in which he had no hand. But his followers' desire to segregate themselves is not unusual. Americans are increasingly forming like-minded clusters. Conservatives are choosing to live near other conservatives, and liberals near liberals.

A good way to measure this is to look at the country's changing electoral geography. In 1976 Jimmy Carter won the presidency with 50.1% of the popular vote. Though the race was close, some 26.8% of Americans were in “landslide counties” that year, where Mr Carter either won or lost by 20 percentage points or more.

The proportion of Americans who live in such landslide counties has nearly doubled since then. In the dead-heat election of 2000, it was 45.3%. When George Bush narrowly won re-election in 2004, it was a whopping 48.3%. As the playwright Arthur Miller put it that year: “How can the polls be neck and neck when I don't know one Bush supporter?” Clustering is how.

County-level data understate the degree of ideological segregation, reckons Bill Bishop, the author of a gripping new book called “The Big Sort: Why the Clustering of Like-Minded America is Tearing Us Apart”. Counties can be big. Cook County, Illinois, (which includes Chicago), has over 5m inhabitants. Beaverhead County, Montana, covers 5,600 square miles (14,400 square kilometres). The neighbourhoods people care about are much smaller.

Americans move house often, usually for practical reasons. Before choosing a new neighbourhood, they drive around it. They notice whether it has gun shops, evangelical churches and “W” bumper stickers, or yoga classes and organic fruit shops. Perhaps unconsciously, they are drawn to places where they expect to fit in.

Where you live is partly determined by where you can afford to live, of course. But the “Big Sort” does not seem to be driven by economic factors. Income is a poor predictor of party preference in America; cultural factors matter more. For Americans who move to a new city, the choice is often not between a posh neighbourhood and a run-down one, but between several different neighbourhoods that are economically similar but culturally distinct.

For example, someone who works in Washington, DC, but wants to live in a suburb can commute either from Maryland or northern Virginia. Both states have equally leafy streets and good schools. But Virginia has plenty of conservative neighbourhoods with megachurches and Bushites you've heard of living on your block. In the posh suburbs of Maryland, by contrast, Republicans are as rare as unkempt lawns and yard signs proclaim that war is not the answer but Barack Obama might be.

At a bookshop in Bethesda (one of those posh Maryland suburbs), Steven Balis, a retired lawyer with wild grey hair and a scruffy T-shirt, looks up from his New York Times. He says he is a Democrat because of “the absence of alternatives”. He comes from a family of secular Jews who supported the New Deal. He holds “positive notions of what government actions can accomplish”. Asked why he moved to Maryland rather than Virginia, he jokes that the far side of the river is “Confederate territory”. Asked if he has hard-core social-conservative acquaintances, he answers simply: “No.”

Groupthink

Because Americans are so mobile, even a mild preference for living with like-minded neighbours leads over time to severe segregation. An accountant in Texas, for example, can live anywhere she wants, so the liberal ones move to the funky bits of Austin while the more conservative ones prefer the exurbs of Dallas. Conservative Californians can find refuge in Orange County or the Central Valley.

Over time, this means Americans are ever less exposed to contrary views. In a book called “Hearing the Other Side”, Diana Mutz of the University of Pennsylvania crunched survey data from 12 countries and found that Americans were the least likely of all to talk about politics with those who disagreed with them.

Intriguingly, the more educated Americans become, the more insular they are. (Hence Mr Miller's confusion.) Better-educated people tend to be richer, so they have more choice about where they live. And they are more mobile. One study that covered most of the 1980s and 1990s found that 45% of young Americans with a college degree moved state within five years of graduating, whereas only 19% of those with only a high-school education did.

There is a danger in this. Studies suggest that when a group is ideologically homogeneous, its members tend to grow more extreme. Even clever, fair-minded people are not immune. Cass Sunstein and David Schkade, two academics, found that Republican-appointed judges vote more conservatively when sitting on a panel with other Republicans than when sitting with Democrats. Democratic judges become more liberal when on the bench with fellow Democrats.

Residential segregation is not the only force Balkanising American politics, frets Mr Bishop. Multiple cable channels allow viewers to watch only news that reinforces their prejudices. The internet offers an even finer filter. Websites such as conservativedates.com or democraticsingles.net help Americans find ideologically predictable mates.

And the home-schooling movement, which has grown rapidly in recent decades, shields more than 1m American children from almost any ideas their parents dislike. Melynda Wortendyke, a devout Christian who teaches all six of her children at her home in Virginia, says she took her eldest out of public kindergarten because she thought the standards there were low, but also because the kids were exposed to a book about lesbian mothers.

“We now live in a giant feedback loop,” says Mr Bishop, “hearing our own thoughts about what's right and wrong bounced back to us by the television shows we watch, the newspapers and books we read, the blogs we visit online, the sermons we hear and the neighbourhoods we live in.”

Shouting at each other

One might ask: so what? If people are happier living with like-minded neighbours, why shouldn't they? No one is obviously harmed. Mr Bishop does not, of course, suggest curbing Americans' right to freedom of association. But he worries about some of its consequences.

Voters in landslide districts tend to elect more extreme members of Congress. Moderates who might otherwise run for office decide not to. Debates turn into shouting matches. Bitterly partisan lawmakers cannot reach the necessary consensus to fix long-term problems such as the tottering pensions and health-care systems.

America, says Mr Bishop, is splitting into “balkanised communities whose inhabitants find other Americans to be culturally incomprehensible.” He has a point. Republicans who never meet Democrats tend to assume that Democrats believe more extreme things than they really do, and vice versa. This contributes to the nasty tone of many political campaigns.

Mr Bishop goes too far, however, when he says the “big sort” is “tearing [America] apart”. American politics may be polarised, but at least no one is coming to blows over it. “We respect each other's views,” says Mrs Wortendyke of the few liberals in the home-schooling movement. “We hate each other cordially,” says the liberal Mr Balis.