Behind China’s vaunted Internet censorship are throngs of specialized police officers, fake commentators, and ever-changing technologies. But China watchers have puzzled over the system’s modus operandi. Some posts are swiftly culled, whereas others on seemingly more sensitive topics are left untouched. In the most revealing study yet of Chinese censorship, researchers describe today how they peered behind the curtain to find out what China’s censors—and presumably the government officials operating behind the scenes—fear most.

When political scientist Gary King of the Institute for Quantitative Social Science at Harvard University and Ph.D. students Jennifer Pan and Margaret Roberts began examining censorship in China in 2011, many scholars assumed that calling for policy changes, criticizing government leaders, and raising sensitive topics like the Tiananmen Square crackdown in 1989 were verboten. To test that assumption, the trio downloaded millions of social media posts from more than 1300 sites between January and July 2011, then selected roughly 127,000 of them to examine in more detail. They watched in real time as posts were taken down. Censorship in China, King says, is “like an elephant tiptoeing around. It leaves big footprints.”

In most cases, censors reacted swiftly, deleting messages within a day of posting. They also seemed to follow a surprising logic. The researchers found that posts on topics they themselves classified as highly sensitive were only slightly more likely than average to be deleted—24% of posts, versus 13% overall. That was “completely unexpected,” King says. They next looked at bursts of posts following significant events. During events with potential for collective action, the vast majority of posts were censored—regardless of whether they supported or criticized the state.

That study, published in American Political Science Review in May 2013, was blind to posts that never went online in the first place because they were snagged in an automated censorship filter. To truly understand what is censored in China, King and colleagues realized, they would need to write their own posts. And that meant creating an unprecedented participatory experiment on China’s blogs, microblogs, and forums—one that “probes much deeper than earlier studies,” says Noah Smith, a computer scientist at Carnegie Mellon University in Pittsburgh, Pennsylvania, who was not involved with the study.

Over three 1- to 2-week periods last year, the researchers oversaw assistants in China and the United States who opened 200 user accounts at 100 sites and then authored 1200 unique posts. Some commented on events involving collective action, such as volatile demonstrations over government land grabs in Fujian province. Others responded to events involving no collective action, like a corruption investigation of a provincial vice governor. For each event, the assistants authored both pro- and antigovernment posts. Posts created by the team advocating collective action were between 20% and 40% more likely to be censored than were posts not advocating it, the team reports online today in Science. Posts critical of the government, on the other hand, were not significantly more likely to be censored than supportive posts—even when they called out leaders by name. “Criticisms of the state are quite useful for the government in identifying public sentiment, whereas the spread of collective action is potentially very damaging,” Roberts explains.

Yu Xie, a sociologist at the University of Michigan, Ann Arbor, says that although the study is methodologically sound, it overemphasizes the importance of coherent central government policies. Political outcomes in China, he notes, often rest on local officials, who are evaluated on how well they maintain stability. Such officials have a “personal interest in suppressing content that could lead to social movements,” Xie says.

For the full version of this story, see this week’s issue of Science.