Like freedom of speech, the presumption of innocence before proof of guilt is something that almost everyone agrees is important in principle, but are occasionally reluctant to apply in practice. In recent weeks we have witnessed some examples of this reluctance that, to me, seem chilling. Eric Garner was an obese African-American who was killed by police officers holding him in a chokehold while they arrested him for illegally selling individual cigarettes in New York City. His last words are here.

Virtually everyone who has seen the video agrees that they acted with an extreme amount of force against a man who was not fighting back although he was resisting arrest (passively – that is, in a way that would not harm the officers).

A Grand Jury found that the police officers who killed Eric Garner did not act unlawfully. I defer to the Grand Jury on this, but assuming they are correct this suggests that the scope for lawful killing by police officers is extremely broad. As law professor Glenn Reynolds (and others) has noted, killings by police are treated much more sympathetically by juries than killings by civilians.

Michael Brown was an African-American teenager who was shot and killed by a police officer during an arrest after he (seemingly) robbed a convenience store in Ferguson, Missouri. There is still some disagreement about what happened here. The initial reports suggested that the officer executed Brown as he fled or begged for his life, but the subsequent Grand Jury investigation seems quite conclusive that Brown assaulted the police officer. The Grand Jury’s conclusions prompted looting by people in Ferguson.

If Brown’s shooting was unjust, the Garner lesson applies. But if the narrative found by the Grand Jury is correct then the protests, lootings and slandering of the police officer involved are wrong. In that case, it is the media’s presumption of guilt on the part of the police officer involved (even after the Grand Jury verdict) that has led to significant destruction and violence. People suspended the innocence principle to advance a political point, and the results have been bleak.

Jackie is a student at the University of Virginia by a Rolling Stone article which alleged that she had been gang-raped by a group of fraternity men. Last week Rolling Stone retracted the story after a number of facts given by Jackie in her story proved to be false.

The aftermath of the Rolling Stone story has been extremely disturbing, with very prominent people proudly dispensing with the innocence principle. The Washington Post ran a piece titled “No matter what Jackie said, we should automatically believe rape claims” (this was later changed to “generally” believe them). The Guardian’s Jessica Valenti wrote that “I choose to believe Jackie. I lose nothing by doing so, even if I’m later proven wrong”, and that “the current frenzy to prove Jackie’s story false – whether because the horror of a violent gang rape is too much to face or because disbelief is the misogynist status quo – will do incredible damage to all rape victims.” [my emphasis]

Has Valenti considered that someone else may lose something if we chooses to believe an accusation that is untrue? Or that we may have other reasons than misogyny or incredulity to want to know if a criminal accusation is false?

Sexual assault is very common, but this does not mean that false accusations do not occur. An estimated 1.5% to 7.5% of accusations may be false. Staggeringly, a 2012 study that used DNA testing of old physical evidence and exonerated between 8% and 15% of convicted rapists.

I know why Valenti is eager to believe Jackie: because not believing a genuine story is horrendous for the victim and makes other rape victims less likely to come forward, and hence makes rape an easier crime to commit. But the inverse is also true: believing a false story is horrendous for the wrongly-accused and makes other false accusations more likely. (The Rolling Stone story did not name individuals, but guilt-by-implication can still be enormously harmful.)

In all of these cases, people who would normally say that the presumption of innocence before proof of guilt is a good thing have assumed the opposite. The rule might work in general, they may say, but this case is an exception. Police need to be able to subdue people resisting arrest. The death of an 18-year old must be unjust. Rape is too serious an allegation to question.

Like the principle of free speech, the innocence principle only produces good results if we apply it rigidly and in cases where doing so may feel deeply unsettling.

The innocence principle matters because people who seem guilty may in fact be innocent. This is why mechanisms like jury trials exist – like the ‘thick’ version of free speech that I argued for recently, they are a mechanism for sorting the truth from lies.

Hayek speculated that liberal institutions like these evolved over time, because the societies that lacked them eventually fell behind the ones that upheld them. Politically and culturally, we may be witnessing an erosion of these institutions now. That would be a catastrophe. But it is not too late to change course.