China is increasingly denying entry to foreign scholars to punish those who work on issues that Beijing deems to be politically troublesome. If the government hopes to drive foreign scholars into self-censorship, this policy is self-defeating. Earlier this month, Elliot Sperling, an American professor of Tibetan history, was denied entry at the airport in Beijing, though he held a one-year visa. While Mr. Sperling believes that he was being punished for his support of Ilham Tohti, a politically moderate Uighur economics professor under arrest on charges of inciting separatism, the Chinese authorities did not inform Mr. Sperling why his visa was being canceled.

China takes a seemingly random approach to denying visas to foreign scholars and never gives an explanation. This leaves scholars guessing about what would offend China and who might be banned. A scholar critical on sensitive issues could freely come and go for years, until suddenly entry is denied, which is what happened to the China scholar Jonathan Mirsky, for instance. China forces scholars to avoid sensitive topics and to know what is acceptable without showing where the red line is.

China is especially sensitive about minority issues in Tibet and Xinjiang, and about democracy. Perry Link of the University of California and Andrew Nathan of Columbia University, two of America’s leading scholars on China, have been denied entry into the country since the ’90s for publishing “The Tiananmen Papers,” which looked into the 1989 crackdown on the democracy movement.