“It probably is the largest effort ever to selectively censor human expression,” says Harvard University social scientist . “They don’t censor everything. There are millions of Chinese [who] talk about millions of things. But the effort to prune the Internet of certain kinds of information is unprecedented.”

King has just completed two studies that peer into the Chinese censorship machine — including a field experiment within China that was conducted with extraordinary secrecy. Together, the studies refute popular intuitions about what Chinese censors are after.

The censors actually permit “vitriolic criticism” of China’s leaders and governmental policies, King and his colleagues — Jennifer Pan and Margaret Roberts — found. But the censors crack down heavily on any move to get people physically mobilized to act on such criticism.

“What they’re after is any attempt to move people,” King says. “Any attempt to [motivate] collective action.”