Quiz question: What is the most important free speech ruling in the history of the U.S. Supreme Court?

Quiz question: What is the most important free speech ruling in the history of the U.S. Supreme Court?

The best answer may well be New York Times v. Sullivan, decided 50 years ago this month. In that case, the court ruled, for the first time, that the First Amendment shields speakers and writers from libel suits.

The court’s ruling has fundamentally affected not only our law but also our culture. Whenever you open a newspaper, visit a website or even do a Google search, the information that you see might well be a testimony to the enduring importance of New York Times v. Sullivan.

Not long after the case was decided, on March 9, 1964, the great free speech scholar Alexander Meiklejohn proclaimed that it was an occasion for "dancing in the streets." On the occasion of its 50th anniversary, the dancing has continued.

But amid the justified celebration, we should pay close attention to the dark side of New York Times v. Sullivan. While it is has granted indispensable breathing space for speakers, it has also created a continuing problem for public civility and for democratic self-government.

The case grew out of a libel action brought by L.B. Sullivan, a public official in Montgomery, Ala., whose responsibilities included supervision of the local police department. Sullivan complained about an advertisement, published by the New York Times, which objected to "an unprecedented wave of terror" against civil-rights demonstrators in Alabama.

To support that objection, the advertisement identified a serious of actions allegedly taken by Montgomery police and other officials against those demonstrators. Some of the allegations were false.

For example, the advertisement inaccurately stated that the police department had "ringed" the campus of the Alabama State College. It alleged, also inaccurately, that the police had bombed Martin Luther King’s home. As the court acknowledged, "the police were not only not implicated in the bombings, but had made every effort to apprehend those who were."

In ruling that the free speech principle safeguards even defamatory statements, the court seized the high moral ground. It affirmed the "profound national commitment to the principle that debate on public issues should be uninhibited, robust, and wide-open." It added that public debate will inevitably "include vehement, caustic, and sometimes unpleasantly sharp attacks on government and public officials."

But what about falsehoods? The court insisted that "erroneous statement is inevitable in free debate." It said that constitutional protection of falsehoods was necessary to provide "breathing space," even if those falsehoods were defamatory. If those criticizing public officials were compelled to guarantee the truth of their claims, the court argued, they would engage in self-censorship.

To prevent that self-censorship, the court produced its famous requirement of "actual malice" — which forces public officials to prove that anyone saying or publishing a falsehood did so knowingly or with "reckless disregard" of the truth. That standard sets an exceedingly high bar. Those who spread falsehoods often do so unknowingly, and terrible sloppiness need not count as recklessness.

In light of the court’s ruling, critics usually do not have to worry about getting ensnared in an expensive libel suit if their target is a public official (a category that the court eventually broadened to "public figures," including entertainers and athletes). Even if you’re negligent — that is, you should have known what you’re saying was untrue and defamatory — you are likely to be protected.

When it comes to public figures, all sorts of false allegations are permissible, whether they involve birth certificates, drug abuse, sexual misconduct or income tax fraud. One result is that those who seek public office put their reputation at immediate risk.

One of the goals of the court’s ruling was to protect self- government, but the effects on self-government are not all good. Talk show hosts, bloggers and users of social media can spread ugly falsehoods in an instant — exposing citizens to lies that may well cause them to look on their leaders with unjustified suspicion.

False accusations are hardly new. But New York Times v. Sullivan can claim at least some responsibility for adding to a climate of distrust and political polarization in the U.S.

True, the marketplace of ideas is open to those who seek to correct the record, but sometimes the truth arrives too late. As Jonathan Swift noted hundreds of years ago, "Falsehood flies, and the truth comes limping after it."

Nonetheless, the court got the balance right in New York Times v. Sullivan: A free society cannot have "uninhibited, robust, and wide-open" debate without breathing space for falsehoods. But as we celebrate the court’s ruling, we should be willing to excuse those who choose to sit out the dance.

Cass R. Sunstein, the Robert Walmsley university professor at Harvard Law School, is a Bloomberg View columnist. He is the author of two new books, "Why Nudge?" and "Conspiracy Theories and Other Dangerous Ideas," and is a former administrator of the White House Office of Information and Regulatory Affairs. Follow him on Twitter at @CassSunstein.