June 4 marks the 26th anniversary of the Chinese government’s deadly crackdown on pro-democracy protesters in Beijing’s Tiananmen Square. As in past years, China’s elaborate censorship apparatus has worked overtime to blunt commemorations of this bloody episode.

The effort worked to a remarkable degree, as it usually does. That is because Chinese censorship is not some bureaucratic dinosaur left over from the nation’s Leninist past. The system is omnipresent in China—dominating every form of media, from book publishing to instant messaging—and has an expansionist view of the world.

From my perch as a publisher in Hong Kong, I am especially attuned to how book censorship works on the mainland. The open secret is that the censoring kicks in before writers even start to work. They know very well that the discussion of certain subjects, events, people and attitudes can never be published in mainland China. Why would anyone want to waste time and energy undertaking such a project?

Censorship in China doesn’t work against the publishing industry. Rather, it is part of the publishing industry. The most serious round of censorship starts with those in leadership positions at publishing outlets. The prosperity of the businesses, and therefore the leaders’ loyalty, are beholden to the Communist Party. They are punishable by three-strike rules if material that displeases the party slips by censors.

These captains of the publishing industry who are dictating editorial policy are highly educated. In addition to well-known censored topics, such as minority and religious issues, that are defined by the Administrative Regulations on Publications, publishers are aware of, and sometimes participate in creating, a vast subset of censored topics, which may include feng shui, astrology or teenage romances.

Only if a written manuscript makes it past this general screening does it reach an editor, whose work on the writer’s behalf means helping to get the manuscript published by doing the first round of deletions and revisions according to the editor’s professional understanding of what should be censored. Typically for a foreign author whose work has been selected for publication and translated, this is when he or she would actually run into Chinese censorship.

The arrival of the information age has given many people in repressive nations hope of breaking censorship using new technologies. In China, the jury is still out on who is winning the war of information over the Internet and smartphones. Chinese authorities are working hard to prevent information technologies from being employed for instantaneous social mobilization, and the use of technology by the Chinese people to bypass censorship appears to remain relatively low. But the seemingly inexorable spread of social media around the world offers a possibility of a breakthrough in China.

The war being waged online by Chinese censors highlights the fundamental nature of censorship in China: It is pre-emptive rather than solely reactive, which serves Beijing’s long-run purposes better and is an effective social-engineering tool.

Censorship always works hand in hand with propaganda. Together, they don’t just limit what the Chinese can read, they help to shape people’s minds and even define what is considered being “Chinese.” The reach of this effort goes far beyond well-known banned topics such as the Great Famine of 1960-62, the Cultural Revolution of 1966-76 and Tiananmen in 1989.

In modern China, every educated person is familiar with the Marxist concept of the “superstructure.” “Humanism,” however, has no single well-defined translation, thanks to the Communist Party’s Anti-Spiritual Pollution Campaign of 1983. Without an understanding of humanism, the Chinese have been crippled in trying to comprehend the profound social changes in the West and Japan after World War II.

Not many Chinese are aware that DNA studies indicate that all modern human populations share a common African ancestry. Why would Chinese censors bother to block such information? Because the government prefers to propagate the vision of a unique and indigenousness origin for the “Chinese race,” justifying arguments for “Chinese characteristics” that contrast with universal values shared by all of humanity.

Chinese censorship is an act of willful social engineering. In disregarding or distorting social, historical and scientific facts, it encourages ignorance and prejudice. It may serve the ruling Communist Party’s current interests, but this censorship most certainly harms the people of China and, ultimately, the rest of the world.

Mr. Bao is the publisher of New Century Press in Hong Kong.