BEIJING

Censorship in China conjures the image of rigid, unsmiling authority, but that disapproving scowl can give way to a different expression — and not always a consistent one. A film, for example, might be banned for 20 years, while the novel on which it is based sells briskly throughout that same period.

This might seem puzzling, but the reason is simple. China has more than 500 publishing houses, each with its own editor in chief (and de facto censor); if a book is rejected by one publisher there’s still a chance another will take it. In contrast, films are not released until officials in the state cinema bureau in Beijing are satisfied, and once a film is banned it has no hope of being screened.

When it comes to censorship in China, the primary factors are often economic, not political. Publishing houses that were once government financed have operated as commercial enterprises for years now. Editors are under pressure to make the biggest profit they can. Even if a book carries some political risks, a daring editor will take the gamble if there’s a chance it will be a best seller.

To be sure, there are some limits in book publishing — the Tiananmen Square protests of 1989 are taboo, for example — but fewer than in film. That’s because film censors, unlike book publishers, don’t have to worry about making profits. Each script is scrutinized, and only after it’s approved can filming commence. Review of the finished product is even more exacting. Even if they were to reject every project that comes their way, it wouldn’t affect their salaries, so they are not prepared to take on the least political risk. That’s why the Cultural Revolution and other sensitive topics are regularly discussed in print but remain off-limits on film. If you go to a cinema, all you’ll see, basically, are martial-arts films, palace dramas, love stories and comedies — and a few American movies.