AUDIE CORNISH, HOST:

China is also known for having one of the most sophisticated systems of censorship in the world. Its force of so-called Internet Police is believed to be between 20 to 50,000 strong. But just what they're looking for is more complicated than some once thought. Researchers at Harvard University have been studying hundreds of Chinese social media sites, and you might be surprised by what they found does and does not get censored.

Gary King is director of Harvard's Institute of Quantitative Social Science. He's here to talk more about it.

Hi there, Gary.

GARY KING: Hi there.

CORNISH: So what key words or ideas end up getting censored and why? What did you find in your research?

KING: Well, the really interesting thing is that what everybody really thought is that they were censoring criticisms of the government. But it turns out they're not censoring criticisms of the government. There are many vitriolic posts that are about the government and the policies and even the leaders. But in fact, they're not censoring that at all. What they censor instead are efforts at collective action. Anytime people get together for any reason - either to criticize government, to support the government or things totally unrelated to the government - that's what they censor.

CORNISH: So give me an example.

KING: So if there is a dissident that says we should all march on some place in China, they'll censor that. If everybody wants to get together for something totally unrelated to the government, unrelated to policies, they'll censor that as well. You want to have a barbecue with - or the equivalent of a barbecue in China with 200 people, they might censor that.

We have an example where they censored an interesting collective action. So, after the earthquake in Japan, there was a rumor that spread throughout one region of China, that if you ate salt it would protect you from the radiation. It was completely false, but people decided that was true and they started rioting, trying to get more salt. And there was a wave of censorship about that. It wasn't a criticism of the government. It had nothing to do with the government at all.

CORNISH: So has China effectively found a way to control the Internet?

KING: Yes. So it's very interesting. Everybody thought that what they were doing was censoring criticism of themselves so that they would look good in what remained. They certainly are censoring. They're censoring about 13 percent of all posts - all of the millions of posts in China.

The government, of course, is perfectly fine with collective action that the government controls. You want to have 50,000 people at a planned sporting event, no problem. But if you want to organize it and you're outside of the government, they're going to come get you.

There were, by the way, to other areas that were very heavily censored. One was pornography. They really try to censor that. Most other the governments censor that to some degree. And the second one, which is even more interesting, is - although I said that you can criticize the Chinese government, there's one area in which you may not criticize the Chinese government in Chinese social media, and that's if you try to criticize the censors. So you can criticize pretty much anybody in the government except the people that do the censoring.

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