Americans are often ridiculed for lacking knowledge of world news. In August 1997, for example, a Pew Research Center survey found that 79 percent of Americans were aware that the boxer Mike Tyson had bitten the ear of an opponent, while only 40 percent knew that Britain had returned Hong Kong to China that same summer.

Despite that information deficit, champions of American public opinion have steadfastly defended its ability to fill in the holes and make rational choices. When hard decisions are required, sociologists and political scientists have long argued, a sleepy public gathers valuable tidbits from the media and comes up to speed. Scott Keeter, a Pew civic engagement expert, pointed out that in early 2003, “nearly two-thirds of the public could name North Korea as the nation admitting to a secret nuclear weapons program.”

Theory also holds that the public makes apt use of its values, which serve as a prism for interpreting the larger world. Sam Popkin, a professor of political science at the University of California, calls it “gut rationality” – a sort of homing device that allows the public to quickly combine its bedrock beliefs with a smattering of new information and make, on the whole, reasoned decisions. People learn from past experiences, daily life and the news media, and they flesh out their world view based on their default values, he concludes.

This process may be threatened, not enhanced, however, by the explosion of shortcuts available.

Sources for information are more bountiful – and arguably more partisan – than ever before: 24-hour cable news in a variety of flavors (CNN, Fox News Channel, CNBC); the Internet (Drudge, Slate, Salon and a universe of blogs); radio (Limbaugh, Franken); and, of course, newspapers (USA Today, The Wall Street Journal, this newspaper).

And this freewheeling bazaar of news choices has generated an audience that is increasingly self-segregating. Consider that a plurality of Fox News Channel’s audience is now Republican, while a plurality of CNN’s audience now consists of Democrats, according to Pew’s latest biennial survey of news habits. That poll also showed that perceptions of “media credibility” – that is, whether people think a particular news outlet can be trusted – are now more driven by ideology and partisanship than at any point in nearly 20 years of surveys.

Professor Popkin worries in particular about the area of foreign affairs.

“Attitudes will become more based on partisanship and less on the specifics of the issues,” he said, adding that “opinions about Iraq show a far larger partisan divide than for any war in the modern era.”

Social observers have fretted about information segregation for years. Cass R. Sunstein, a professor of law at the University of Chicago, argued in his 2001 book “Republic.com” that the Internet’s ability to provide personalized news – to permit users to filter out those things they don’t care about – posed a threat to democracy itself.

Democracy, he argued, depends in part on people’s being exposed to information they would not necessarily have chosen for themselves. So, too, might the concept of gut rationality be endangered in a filtered world, where people see only what they want to see, hear only what they want to hear, read only what they want to read.

Still, perhaps all is not lost.

In his new book, “The Wisdom of Crowds,” James Surowiecki argues eloquently that “under the right circumstances, groups are remarkably intelligent, often smarter than the smartest people in them.” That’s because a diversity of experience, opinion and knowledge can render the whole greater than the sum of its parts. Whether a more partisan news environment undermines or enhances the cognitive diversity of American culture – or diminishes the “gut rationality” of the public – remains to be seen.

“The public’s judgment has been pretty good over the past 75 years, when we pretended that we didn’t have a partisan media,” said Maxine Isaacs, a lecturer at Harvard’s Kennedy School of Government. “Everyone knew that we did. It’s now just more overt.”