Moisés Naím / Foreign Policy

A video shows a single line of people slowly trudging up a snow-covered footpath. A shot is heard; the first person in line falls. A voice-over says, "They are shooting them like dogs." Another shot, and another body drops to the ground. A uniformed Chinese soldier fires his rifle again. Then, a group of soldiers examines the fallen bodies.

These images were captured high in the Himalayas by a member of a mountaineering expedition who claims to have stumbled upon the killing. The video first aired on Romanian television, but it only gained worldwide attention when it was posted on YouTube, the popular video-sharing Web site. Human rights groups explained that the slain were a group of Tibetan refugees that included monks, women, and children. According to the Chinese government, the soldiers had fired in self-defense after they were attacked by 70 refugees. The posted video seems to render that explanation absurd. The U.S. ambassador to China quickly lodged a complaint protesting China’s treatment of the refugees.

Welcome to the YouTube effect. It is the phenomenon whereby video clips, often produced by individuals acting on their own, are rapidly disseminated throughout the world thanks to video-sharing Web sites such as YouTube, Google Video, and others. Every month, YouTube receives 20 million visitors, who watch 100 million video clips a day. There are 65,000 new videos posted every day. Most of the videos are frivolous, produced by and for teenagers. But some are serious. YouTube includes videos posted by terrorists, human rights groups, and U.S. soldiers in Iraq. Some are clips of incidents that have political consequences or document important trends, such as global warming, illegal immigration, and corruption. Some videos reveal truths. Others spread disinformation, propaganda, and outright lies. All are part of the YouTube effect.

Fifteen years ago, the world marveled at the fabled "CNN effect." The expectation was that the unblinking eyes of TV cameras, beyond the reach of censors, would bring greater accountability and transparency to governments and the international system. These expectations were, in some sense, fulfilled. Since the early 1990s, electoral frauds that might have remained hidden were exposed, democratic uprisings energized, famines contained, and wars started or stopped, thanks to the CNN effect. But the YouTube effect will be even more intense. Although the BBC, CNN, and other international news operations employ thousands of professional journalists, they will never be as omnipresent as millions of people carrying a cell phone that can record video. Thanks to their ubiquity, the world was able to witness a shooting on a 19,000-foot mountain pass.

This phenomenon is amplified by a double echo chamber: One is produced when content first posted on the Web is re-aired by mainstream TV networks. The second occurs when television moments, even the most fleeting, gain a permanent presence thanks to bloggers or activists who redistribute them through Web sites like YouTube. Activists everywhere are recognizing the power of citizen-produced and Web-distributed videos as the ultimate testimony. The human rights group Witness arms individuals in conflict zones with video cameras so they can record and expose human rights abuses. Electoral watchdogs are taping elections. Even Islamic terrorists have adapted to this trend. Al Qaeda created a special media production unit called Al Sahab ("The Cloud"), which routinely posts its videos online, with the realistic expectation that they will be picked up by major media outlets and other Web sites.

The YouTube effect has brought other mixed blessings. It is now harder to know what to believe. How do we know that what we see in a video clip posted by a "citizen journalist" is not a montage? How do we know, for example, that the YouTube video of terrorized American soldiers crying and praying for their lives while under fire was filmed in Iraq and not staged somewhere else to manipulate public opinion? The more than 86,000 people who viewed it in the first 10 days of its posting will never know.

Governments are already feeling the heat of the YouTube effect. The U.S. military recently ordered its soldiers to stop posting videos unless they have been vetted. The Iranian government restricts connection speeds to limit its people’s access to video streaming. These measures have not stopped the proliferation of Web videos shot by U.S. soldiers in Iraq, or savvy Iranians from viewing the images they want to see. And, though Beijing has been effective in censoring the content its citizens can view, it has yet to figure out a way to prevent a growing number of videos of peasant rebellions from being posted online. In the long run, all such efforts will fail.

When it comes to having faith in what we see online, the good news is that the YouTube effect is already creating a strong demand for reliable guides — individuals, institutions, and technologies that we can trust to help us sort facts from lies. That is important, because the hope of countering the downsides of the YouTube effect will never come from government intervention. Markets and democracy do a much better job of filtering the bad from the good in the confusing tsunami of Web videos coming our way. The millions of bloggers who are constantly watching, fact-checking, and exposing mistakes are a powerful example of "the wisdom of crowds" at work. Sure, markets and democracies often fail or disappoint. But the openness these political and economic forces promote are now being assisted by a technology that is as omnipresent as we are.