A natural hypothesis is that a media outlet’s perspective reflects the ideology of its owner. Indeed, much regulatory policy is premised on precisely this view. Policy makers sometimes take a jaundiced view of media consolidation on the grounds that high levels of cross-ownership reduce the range of political perspectives available to consumers.

From their study of newspapers, however, Mr. Gentzkow and Mr. Shapiro, find little evidence to support this hypothesis. After accounting for confounding factors like geographic proximity, they find that two newspapers with the same owner are no more likely to be ideologically similar than two random papers. Moreover, they find no correlation between the political slant of a paper and the owner’s ideology, as judged by political donations.

So, if not the owner’s politics, what determines whether a newspaper leans left or right? To answer this question, Mr. Gentzkow and Mr. Shapiro focus on regional papers, ignoring the few with national scope, like The Times. They find that potential customers are crucial.

If a paper serves a liberal community, it is likely to lean left, and if it serves a conservative community, it is likely to lean right. In addition, once its political slant is set, a paper is more likely to be read by households who share its perspective.

Religiosity also plays a role in the story, and it helps Mr. Gentzkow and Mr. Shapiro sort out cause and effect. They find that in regions where a high percentage of the population attends church regularly, there are more conservatives, and newspapers have a conservative slant. They argue that because newspapers probably don’t influence how religious a community is, the best explanation is that causation runs from the community’s politics to the newspaper’s slant, rather than the other way around.

The bottom line is simple: Media owners generally do not try to mold the population to their own brand of politics. Instead, like other business owners, they maximize profit by giving customers what they want.

These findings speak well of the marketplace. In the market for news, as in most other markets, Adam Smith’s invisible hand leads producers to cater to consumers. But the findings also raise a more troubling question about the media’s role as a democratic institution. How likely is it that we as citizens will change our minds, or reach compromise with those who have differing views, if all of us are getting our news from sources that reinforce the opinions we start with?