Your body also contains little packets of genetic material that sit on the ends of your chromosomes. Theyre called telomeres. Each time your cells divide, their telomeres get a little shorter, and when they become too short, you die. This is normal aging. But guess what else shrinks your telomeres? Chronic stress.

If words can cause stress, and if prolonged stress can cause physical harm, then it seems that speech at least certain types of speech can be a form of violence. But which types?

This question has taken on some urgency in the past few years, as professed defenders of social justice have clashed with professed defenders of free speech on college campuses. Student advocates have protested vigorously, even violently, against invited speakers whose views they consider not just offensive but harmful hence the desire to silence, not debate, the speaker. Trigger warnings are based on a similar principle: that discussions of certain topics will trigger, or reproduce, past trauma as opposed to merely challenging or discomfiting the student. The same goes for microaggressions.

This idea that there is often no difference between speech and violence has stuck many as a coddling or infantilizing of students, as well as a corrosive influence on the freedom of expression necessary for intellectual progress. Its a safe bet that the Pew survey data released on Monday, which showed that Republicans views of colleges and universities have taken a sharp negative turn since 2015, results in part from exasperation with the speech equals violence equation.

The scientific findings I described above provide empirical guidance for which kinds of controversial speech should and shouldnt be acceptable on campus and in civil society. In short, the answer depends on whether the speech is abusive or merely offensive.

Offensiveness is not bad for your body and brain. Your nervous system evolved to withstand periodic bouts of stress, such as fleeing from a tiger, taking a punch or encountering an odious idea in a university lecture.

Entertaining someone elses distasteful perspective can be educational. Early in my career, I taught a course that covered the eugenics movement, which advocated the selective breeding of humans. Eugenics, in its time, became a scientific justification for racism. To help my students understand this ugly part of scientific history, I assigned them to debate its pros and cons. The students refused. No one was willing to argue, even as part of a classroom exercise, that certain races were genetically superior to others.

So I enlisted an African-American faculty member in my department to argue in favor of eugenics while I argued against; halfway through the debate, we switched sides. We were modeling for the students a fundamental principle of a university education, as well as civil society: When youre forced to engage a position you strongly disagree with, you learn something about the other perspective as well as your own. The process feels unpleasant, but its a good kind of stress temporary and not harmful to your body and you reap the longer-term benefits of learning.

Whats bad for your nervous system, in contrast, are long stretches of simmering stress. If you spend a lot of time in a harsh environment worrying about your safety, thats the kind of stress that brings on illness and remodels your brain. Thats also true of a political climate in which groups of people endlessly hurl hateful words at one another, and of rampant bullying in school or on social media. A culture of constant, casual brutality is toxic to the body, and we suffer for it.

Thats why its reasonable, scientifically speaking, not to allow a provocateur and hatemonger like Milo Yiannopoulos to speak at your school. He is part of something noxious, a campaign of abuse. There is nothing to be gained from debating him, for debate is not what he is offering.

On the other hand, when the political scientist Charles Murray argues that genetic factors help account for racial disparities in I.Q. scores, you might find his view to be repugnant and misguided, but its only offensive. It is offered as a scholarly hypothesis to be debated, not thrown like a grenade. There is a difference between permitting a culture of casual brutality and entertaining an opinion you strongly oppose. The former is a danger to a civil society (and to our health); the latter is the lifeblood of democracy.

By all means, we should have open conversations and vigorous debate about controversial or offensive topics. But we must also halt speech that bullies and torments. From the perspective of our brain cells, the latter is literally a form of violence.

Lisa Feldman Barrett, a professor of psychology at Northeastern University, is the author of How Emotions Are Made: The Secret Life of the Brain.

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A version of this op-ed appears in print on July 16, 2017, on Page SR9 of the New York edition with the headline: When Is Speech Violence?.

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When Is Speech Violence? - New York Times