Travel, it is said, broadens the mind. In my last column, I paid tribute to a writer who learned by traveling, and whose experience of Japan reshaped his attitudes toward his homeland. We live now in a century where mobility is taken for granted, at least in the developed world. If the proverb is true, our minds should be broader than ever. Yet there is a paradoxical sense in which that very freedom might narrow our outlook.

British writer G.K. Chesterton — a perceptive and intelligent conservative — saw this danger a hundred years ago. Arguing that small is beautiful, exalting the local against the cosmopolitan, he wrote:

The man who lives in a small community lives in a much larger world. ... In a large community we can choose our companions. In a small community our companions are chosen for us. ... There is nothing really narrow about the clan; the thing which is really narrow is the clique.

For Chesterton, necessity was the mother of toleration. The accidental bonds of neighbors and family members were worthy of celebration because people who had no option but to live together would be compelled to learn to accept each other's differences. The ability to form communities at will would instead encourage the formation of groupings of like-minded people; in consequence, there would be no impetus to tolerate those who thought differently.

Has this come to pass? Regarding sexual and social mores, the developed world at least seems generally a more tolerant place than it was when Chesterton was writing. Yet his formulation has its relevance. It is easier to tolerate our neighbors, after all, when we have the freedom to choose to reside among people who are like ourselves.

And there is evidence in many walks of life that people of opposing views and different backgrounds no longer meet or seek to find common ground. In cities such as Paris or Amsterdam, people whose families have lived there for generations and recent immigrants increasingly inhabit separate suburbs.

In the United States, the growing polarization of political opinion speaks for a nation in which those who hold opposing views have ever more begun to lead separate lives, ceasing to communicate with or understand each other. It is striking to compare the political map of the close 1960 presidential election, in which John F. Kennedy narrowly defeated Richard Nixon, with that of the 2004 contest, in which George W. Bush narrowly defeated John Kerry. In 1960, each candidate won some states in almost every region of the country. In 2004, the south and central regions voted solidly for Bush while Kerry took the whole western seaboard and the northeast.

Developments in the media have played their part in this. The proliferation of news sources on the Internet and satellite broadcasting is often said to have permitted the expression of a new diversity of opinions. But for opinions to be expressed and for opinions to be heard are two different matters. The traditional format of a newspaper such as The Japan Times, in which columnists of diverse political and moral positions are clustered on the same opinion page, encourages readers to evaluate a variety of positions and respond to a variety of arguments. Even if they are not swayed by those arguments, they are reminded that an uncongenial opinion is capable of civilized expression and merits serious debate.

But this depends on a diversity of opinions being expressed within one publication or news source, whereas there is an increasing trend toward many news sources each offering a single, slanted point of view. As media proliferate, consumers are ever more likely to gravitate toward outlets that express positions they already hold.

The effects of this on society are potentially corrosive. In Britain, the BBC has earned criticism for its policy of impartiality even in the reporting of acts of violence: For instance, after the London bombings of July 7, 2005, it referred to the perpetrators by neutral words such as "bombers" rather than condemnatory terms such as "terrorists." But that studied neutrality might itself be conducive to moderation. A society where most obtain their news from a single broadcaster respected for its impartiality is surely less likely to spawn extremists in the first place than one in which those who regard bombers as terrorists get their news from partisan conservative commentators while those who regard them as martyrs visit militant Web sites.

In fairness, the divisions in our societies are not absolute. The crisis that has made them more sharply visible — the war in Iraq — has also generated some vital shows of unity. Thus, the Stop the War coalition brought together socialists, Islamists and old-fashioned liberals, and people of every ethnic origin. Nevertheless, we are a long way from the global village that communications theorist Marshall McLuhan predicted the people of the world would inhabit in the 21st century. It remains true, in the words of the Dire Straits song, that "We have just one world, But we live in different ones."