The number that generates the most discussion around the Tiananmen massacre, which began 25 years ago today, is typically the death toll. China's official statistic is 241 dead, which virtually no one believes. Two eyewitness accounts, by a Swiss ambassador and the Red Cross, estimated that 2,600 or 2,700 were killed in Beijing alone, just one of the cities where Chinese troops mowed down peaceful protesters. Both estimates were withdrawn under Chinese government pressure.

But there's another number that, while even less scientific, may be a better metric for understanding the June 1989 crackdown and what it means for China today. It's the number of Beijing university students who, in an informal survey, recognized the "tank man" photo that for the rest of the world is an immediately recognizable symbol of the 1989 protests and massacre: just 15 out of 100.

even students who can recognize tank man often insist he was an escaped mental patient

China's campaign to enforce a national amnesia of one of the most traumatic events in its recent history has been so staggeringly successful that even most university students are unaware of its best-known symbol. But this matters more than just for denying the victims' families, and the nation, its right to mourn and reflect. It has depoliticized a generation, an ongoing muting of Chinese voices that extends well beyond just the memory of June 4.

The number comes from NPR's Louisa Lim, who for her excellent book on China's national amnesia after Tiananmen interviewed 100 students at the prestigious Beijing universities that, in 1989, were absolutely central to the student movement and protests. These students today have a historical connection to the 1989 movement, and are some of the smartest and Internet-savviest young people in China. They also have unusually easy access to foreign media and are disproportionately likely to have traveled abroad. If any young people in China would know about Tiananmen, surely it would be them.

But they didn't. Here's what Lim found when she showed them the photo that would be so famous in any country but the one where it was taken:

The students I spoke to are the crème de la crème, the best-educated students in China, yet the vast majority of them looked at the photo with the slightest flicker of recognition. "Is it in Kosovo?" one astronomy major asked. A student pursuing a Ph.D in marketing hazarded a guess, "Is it from South Korea?" "I feel that it looks a bit like Tiananmen Square. But it isn't, is it?" asked one student doing postgraduate work in education at Beijing Normal University. Out of 100 students, 15 correctly identified the picture; two whom had never seen it before but had guessed correctly. In fact, the number of students who mistakenly believed it to be a photo of a military parade was higher, at 19, than those who recognized it.

Immediately after the June 4 massacre, the Chinese government began an aggressive campaign to censor the incident so heavily as to erase it from the memories of its billion-plus citizens, to immediately declare it a non-event, a necessary police action against a small band of counter-revolutionary rebels, and then to insist that it had never happened at all. They succeeded far beyond what most observers at the time, even cynically-minded close observers of China, imagined possible.

Keep this number in mind the next time you hear about Chinese students' vaunted performance on international standardized tests. Only 15 out of 100 seem to know about the most important event in the last quarter-century of their country's history. It would be sort of as if only 15 out of 100 Harvard university students were able to recognize an iconic photo of the September 11 attacks, which killed about as many people in New York as the Tiananmen massacre killed in Beijing.

CHINESE STUDENTS HAVE BEEN SO politically neutered THAT THEY'RE NOT EVEN AWARE OF THEIR OWN POWERFUL HISTORY

This number is necessarily non-scientific. National polling on Tiananmen is of course impossible in China; Lim has acknowledged that even the remembrances in her book may get her banned from the country permanently.

A survey of 100 students can only tell you so much about national opinion in the world's most populous country. But I will say that it tracks awfully closely with my own anecdotal experience, and that of others in China whom I've spoken to. Chinese journalist Helen Gao, a member of China's post-1989 generation, writes in today's New York Times that she did not have a single conversation with a peer about Tiananmen until she was a junior in college.

The difference between reality and popular impression is again telling on the tank man photo. No one knows for sure who he is, and although he quickly became one of the most famous Chinese people in the world, his identity has never been confirmed and what happened to him after that photo is a mystery. China has succeeded in making him, like the Tiananmen massacre he has come to symbolize, in many ways simply not exist.

Even Chinese citizens who move to the United States as students and spend years exposed to American media and education will often tell me that the Tiananmen massacre never happened or was a minor disturbance necessarily and near-bloodlessly quelled by civic-minded troops. The line on the "tank man" photo is typically that he was an escaped mental patient who wandered too close to some troops and was harmlessly carried back to his hospital bed. Even otherwise liberal-minded Chinese who have access to all the information will, of their own accord, actively participate in holding up the government's version of events.

And that is what makes this number so important, perhaps even as or more important than the death toll. While the Chinese government killed hundreds of its own citizens on June 3 and 4, 1989, it has since then done something that may quietly be nearly as cruel and consequential: enforced a public amnesia of one of the most traumatic events in its modern history.

But the national amnesia has been more damaging than even to deny China a chance to reflect and mourn on the tragedy. In the 1980s, Chinese students were so politically involved that they led one of the most important popular movements of the late 20th century. Today, due in large part to a government campaign that has included a brutally state-enforced national amnesia, Chinese students are so apolitical, so focused on jobs and wealth, that they're not even aware of their own powerful history.