China’s WeChat originated as a WhatsApp clone, but later evolved into the single-most important tool for connecting people in China. Yet it’s never been clear exactly how China’s internet censors have attempted to control information that spreads in the app. That’s partly because you likely wouldn’t know if you got censored in the first place.

A new study from The Citizen Lab, a research group at the University of Toronto, reveals that censorship on WeChat occurs primarily in group chats rather than one-on-one chats between two people, and often in such a way where the sender of a text isn’t even aware a piece of text has been scrubbed. The discoveries illuminates how China’s government attempts to keep its citizens blind to the scope of its censorship regime.

The researchers set out find the extent to which certain keywords got scrubbed from conversations between two or more users in WeChat. To do this, in June 2016 the team posed as a Chinese WeChat user and sent out 26,821 keywords containing terms that had been censored on other apps, including Tom-Skype (a made-for-China version of Skype) and YY (a live broadcast app). A corresponding Canadian user in the two-way chat would then report back to say whether or not the message had been received.

The report states that out of the entire sample, only one term—Falun Gong (法轮功)—had been scrubbed. When they ran an identical test in August, even that text mysteriously passed without censorship.

Yet when they tested group chats, they found multiple cases in which certain keywords triggered a removal. Specifically, while sensitive terms used in isolation were unlikely to trigger censorship (say “June 4th,” a reference to the Tiananmen Square protests, brutally put down on June 4, 1989), it took effect when they were used in a full sentence or with other keywords.

The researchers also discovered that when WeChat censored a message, the sender received no notice informing him that his text had not reached the intended recipient.

The Citizen Lab Sample screenshots show that Chinese users could not successfully send out texts reading “June 4th Student Democracy Movement” and “My birthday is June 4th. I am a student. I am reading a book about democracy movements.”

Jason Q. Ng, research fellow at The Citizen Lab, speculates that either the government, or WeChat, or both parties have singled out group chats rather than one-to-one chats because they are more likely to help users mobilize politically. There have been several cases in which people used WeChat group chats to plan gatherings. Last June, for example, four people were arrested for using the service to protest the building of a waste incineration plant in Ningxia, Hunan.

“The size of the audience matters for a lot because as the audience increases you have the potential for greater spillover to actual moblization on the streets,” says Ng. “The fact that you can reach out to more people than just a single person makes it more risky that it could go viral or provoke street protests.”

The researchers also discovered that WeChat only censors content for users who bind their account to a mainland Chinese phone number when they first register to use the app. As a result, it primarily affects mainland Chinese residents. However, if they move outside China or change their phone numbers—for instance, Chinese students studying abroad—they are still subjected to the same censorship.