In University of California v. Bakke, the landmark 1978 decision on affirmative action, the Supreme Court ruled that universities could consider race and ethnicity in their admission policies. Its rationale was simple: A diverse student body will enhance intellectual life, for everyone. “The Nation’s future depends upon leaders trained through a wide exposure to that robust exchange of ideas which discovers truth ‘out of a multitude of tongues,’ ” the court declared, quoting an earlier case striking down mandatory loyalty oaths for teachers and professors.

But on American campuses today, we’re biting our own tongues. So we’re not reaping the full benefits of diversity, which is supposed to teach us about each other.

Witness a recent survey of over 37,000 students at more than 150 colleges and universities, cosponsored by the Philadelphia-based Foundation for Individual Rights in Education, which reported that over four-fifths of students self-censor at least some of the time and one-fifth do so “fairly often” or “very often.” Over half (51%) identified racial inequality as the most difficult topic to discuss, just ahead of the George Floyd protests (43%) and transgender issues (42%).

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Students of all backgrounds are self-censoring, especially if they don’t agree with the perceived campus wisdom about race and criminal justice.

“In a class discussion after a controversial ‘racist’ incident involving a Black student and public safety officers, I wanted to express that I didn’t think the actions of the officers were entirely unjustified,” an African American student at Barnard College wrote. “I felt like I couldn’t say this because everyone around me was saying how racist it was [even though none of these students were Black but I was].”

Consider the lost opportunity for learning, in that single example. If the Black woman had spoken up, her peers would have been given the opportunity to think more deeply about their opinions about the episode. And these views would be more informed than they were when nobody was challenging them.

Now multiply that out, across our campuses, and you get a good idea of the pall of self-censorship that has descended upon all of us. At Haverford College, students said they were afraid to criticize a strike that was called after the Philadelphia police shooting of Walter Wallace Jr., an African American man with a history of mental illness. At Penn State, a student who opposed free health care for undocumented immigrants kept quiet for fear of being called “xenophobic and racist.”

At Temple University, a student said they felt “highly uncomfortable contributing my thoughts as a religious person” during a discussion in philosophy class about the afterlife. And at University of Pennsylvania, where I teach, another student said they refrained from sharing opinions in a class about Black Lives Matter.

Yet I haven’t heard many of my fellow professors — or any major university leader — decry self-censorship in higher education. Some of them are probably afraid to say what they think, just like their students are. Others imagine that diversity requires a certain kind of verbal reticence, so that all students feel comfortable on our campuses.

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That patronizes students, all in the guise of protecting them. And it flouts the spirit of affirmative action, which was premised on the idea that people of different backgrounds can teach each other. That will not always be easy or pleasant, for anybody. But if we can’t talk, we can’t learn. Period.

And that brings us back to Bakke, which also quoted a 1977 article by Princeton’s then-president William G. Bowen. According to Bowen, a diverse university would help students “to learn from their differences and to stimulate one another to reexamine even their most deeply held assumptions about themselves and their world.” I believe in that ideal, as deeply as I believe in anything. But anyone who says we’re living it on our campuses simply hasn’t been listening.

Jonathan Zimmerman teaches education and history at the University of Pennsylvania. He is the author (with cartoonist Signe Wilkinson) of “Free Speech and Why You Should Give a Damn,” which was published in April by City of Light Press.