A concern about national character runs through the Supreme Court’s free-expression cases. In 1927, Justice Louis Brandeis wrote that free speech could rarely be a threat to “courageous, self-reliant men, with confidence in the power of free and fearless reasoning.” In a 1989 decision striking down laws against flag-burning, Justice William Brennan quoted that comment and explained that the court was striking a blow for the country’s “resilience.”

An important reason that we celebrate free speech, then, is that — to put the point negatively — we don’t want to be fearful and brittle people, or a fearful and brittle people.

That’s exactly what Mary Katharine Ham and Guy Benson think we’re in danger of becoming. Their new book, “End of Discussion,” is about the culture rather than the law of free speech. They take aim at what they call the “outrage industry,” which uses exaggerated claims of offense to shut down debate.

Many of the examples Ham and Benson cite are well known: the defenestration of Brendan Eich as chief executive officer of Mozilla for refusing to profess support for same-sex marriage; the disinvitation of commencement speakers for violating arcane campus orthodoxies; the firing of liberal pundit Juan Williams from NPR for admitting while on Fox News that he “gets nervous” when he sees people “in Muslim garb.” Other stories they recount didn’t make national news. They mention a disc jockey at a North Carolina bar who was told not to return to work after playing the song “Blurred Lines,” which a patron said might be a “trigger” for victims of sexual assault.

What typically gets forgotten in these incidents is a sense of proportion. Ham and Benson acknowledge that there’s a reasonable debate to be had about that song; what’s unreasonable is making someone lose his paycheck over playing it. They don’t oppose boycotts in principle, and I doubt they’d deny that a CEO could say something so offensive that it morally obligates his company to oust him. But they think these tactics should be used sparingly — and not against people who merely oppose same-sex marriage (even though they themselves support it).

The authors are conservative commentators, and they concede that receiving hyperbolic responses to their opinions is part of their jobs. What’s especially objectionable about the outrage industry, they write, is that it targets people who are not in public life, like that DJ. It makes more and more of daily life resemble a political campaign in its potential for career-ending gaffes.

They concede as well that conservatives are sometimes guilty of attempting to squelch debate: “genuine opposition to war has too often been cast as opposing ‘the troops,’ ” and criticism of Israeli policy as anti-Semitic. A more systematic treatment of such examples would have improved the book. But the authors shouldn’t be faulted for a lack of even-handedness, because the phenomenon they’re analyzing is mostly found, at least these days, among liberals. “Because of their indisputable dominance in academia, media, and entertainment, liberal outrages have a natural advantage of amplification.”

The authors don’t speculate about why outrage is a growth industry, but others have. Some point to the rise of social media: Mobilizing vitriol has never been cheaper. Others point to the decline in the social force of religion, on the theory that a new set of taboos is replacing an older one. Perhaps part of the explanation is that our campuses are full of people brought up to treasure the sanctity of their feelings. And the left has been emboldened by recent political victories and demographic trends.

Whatever the causes, Ham and Benson make a strong case that intolerance is making the U.S. less free, and less fun. They conclude with some practical advice for people of all political persuasions: Listen to your ideological opponents; don’t give gratuitous offense, but don’t be cowed by bluster either; that sort of thing. None of it is earth-shattering, but the authors have amply established that all of it needs to be said.

The most effective response to the outrage industry, in fact, may be the one they model: Hold it up to gentle mockery. For that service, all those who want our country to remain the home of courageous, self-reliant men and women should thank them.

Ramesh Ponnuru, a Bloomberg View columnist, is a senior editor for National Review, where he has covered national politics for 18 years, and a visiting fellow at the American Enterprise Institute.