“Conservatives may be self-selecting out of graduate school, but they’re doing it on a rational basis,” he told me. “It’s become clear to them that they’re unlikely to succeed at the same level as someone going into these fields with more socially approved political convictions and attitudes.” They’re discouraged not by a letter from the director of graduate studies but rather by more subtle obstacles blocking the way to tenure, in Dr. Wood’s view. Writing in The Chronicle of Higher Education in March, he criticized liberal social scientists for failing to heed their own extensive research into bias.

“The most effective way to keep out a whole class of people who are unwelcome isn’t to bar entry, but to make sure that very few in that class will want to enter,” Dr. Wood wrote. “If it comes down to it, entry can still be impeded through other techniques, the feminist and the multiculturalist vetoes on the faculty search committee being the deadliest as far as conservatives go, although there are others.”