My new report for the Center for the Study of Partisanship and Ideology gives the lie to these claims. Based on eight comprehensive surveys of academic and graduate-student opinion across the U.S., Canada and Britain, it buttresses the findings of numerous studies to provide hard data on the absence of viewpoint diversity and presence of discrimination against conservative and gender-critical scholars. High-profile activist excesses are mere symptoms of a much wider problem of progressive authoritarianism. Roughly 1 in 3 conservative academics and graduate students has been disciplined or threatened with disciplinary action. A progressive monoculture empowers radical activist staff and students to violate the freedom of political minorities like conservatives or “gender-critical” feminists, who believe in the biological basis of womanhood—all in the name of emotional safety or social justice.

Political discrimination is pervasive: 4 in 10 American academics indicated in a survey this summer that they would not hire a known Trump supporter for a job. In Canada, the share is 45%, while in Britain, 1 in 3 academics wouldn’t hire a Brexit supporter. Between one-fifth and half of academics and graduate students are willing to discriminate against right-leaning grant applications, journal submissions and promotion cases. On a four-person panel, this virtually guarantees that a conservative will face discrimination.

Meanwhile, only 28% of American academics say they would be comfortable sitting with a gender-critical scholar over lunch, less even than the 41% who would sit with a Trump-voting colleague. Somehow this has become acceptable in a way it never would be for a person from a religious, as opposed to political, minority.

Some 75% of American and British conservative academics in social sciences and humanities say their departments offer a hostile climate for their beliefs. Nearly 4 in 10 American centrist faculty concur. This produces a chilling effect that results in self-censorship: I found that merely 9% of Trump-supporting academics say they would feel comfortable expressing their political beliefs to a colleague. Their progressive counterparts admit as much, with only 14% of all U.S. academics saying a Trump supporter would feel comfortable expressing his beliefs. In Britain, only 18% of Brexit-supporting academics would feel comfortable sharing their views, even though 52% of the British electorate supported Brexit.

Fully 7 in 10 conservative American academics say they self-censor in their teaching, research or academic discussions. Conservative scholars shy away from asking questions that go against the progressive consensus out of fear for their careers. This chilly climate is picked up by conservative and centrist students, which in turn influences the pipeline of who continues on to graduate school and the professoriate.

I find that conservative master’s students in social sciences and humanities fields are significantly more likely than other master’s students to believe their beliefs don’t fit in academia. Conservatives who think their politics wouldn’t fit are significantly less likely than others to be interested in pursuing an academic career. This isn’t about money. Conservative master’s students are no more likely than progressive students to say that pay is a factor discouraging them from going into academia. In effect, there is a feedback loop: Low viewpoint diversity reproduces the hostile climate that sustains the progressive monoculture that has developed in many faculties over the past four decades.

The result of this hostile environment is conformity to a culture that is out of alignment with the nation’s. As in previous studies, I find a low level of political diversity, with only 5% of American scholars in the social sciences and humanities identifying as conservative. In the U.S. and Canada, academics on the left outnumber those on the right by a ratio of 14 to 1.

Many academics, most of whom are not radicals, are aware of the problem. A positive sign is that only 1 in 10 academics support “canceling” controversial right-wing professors by firing them from their jobs. But younger academics are twice as likely to endorse a dismissal campaign as older faculty. Doctoral candidates are around three times as likely, suggesting the problem of political intolerance is likely to get worse. More conservatives on the faculty would increase resistance to censorship, making it more difficult for progressive intolerance to succeed.

While some counsel a hands-off approach to the problem, the laissez-faire strategy has failed for nearly four decades. My new report shows why: Few academics want to cancel their controversial colleagues, but a growing minority discriminates against them and isn’t actively opposed to their cancellation. This helps explain why there isn’t enough internal pressure to protect academic freedom. The problem is now spilling off campus.

At this point, only a proactive approach can work, such as the policies recently announced in Britain, in which public universities are to be audited and potentially fined for academic freedom violations each year by the government. In the U.S., state or federal authorities must regulate public universities to ensure they protect the First Amendment rights of staff and students and don’t discriminate against political minorities. Unless reforms come from outside the academy, universities will continue to be monocultures in which conservative ideas aren’t given a fair hearing.

Mr. Kaufmann is a professor of politics at Birkbeck College, University of London and a board member at Center for the Study of Partisanship and Ideology.