HOW surprised should we be that a grand jury in Missouri failed to indict a police officer for killing an unarmed black man? In one sense, very surprised: it is very rare for grand juries to fail to indict a suspect when the state is doing its best to make a case. The standard of proof for an indictment is not that the state has made its case beyond a reasonable doubt, but that there is probable cause to prosecute. According to the Missouri courts, "probable cause" exists where "knowledge of the particular facts and circumstances is sufficient to warrant a prudent person's belief that a suspect has committed an offense". The 1996 case establishing that standard, State v Tokar, upheld a suspect's arrest for a murder committed during a burglary because the defendant and his girlfriend drove a yellow station wagon (which matched eyewitness descriptions of the perpetrators), and he had prior convictions for similar burglaries. Given this standard, and the fact that prosecutors have enormous discretion over the proceedings, one would think that indictments are fairly easy to secure whenever the state has a reasonable amount of evidence. And indeed, as Reason's Anthony Fisher writes, a grand jury refusing to hand down an indictment is "an incredibly rare thing". In federal (rather than state) courts, grand juries in 2010 failed to indict in just 11 out of 162,000 cases. If anything, conservative legal scholars in recent years have worried that grand juries are too susceptible to indicting based on flimsy cases.

At the same time, we shouldn't be at all surprised by the grand jury's verdict. While it is generally easy to get an indictment, it is extremely difficult to indict a police officer. This is especially true for homicide, as Jamelle Bouie writes. While 410 "justifiable homicides" were reported by the FBI in 2012, there were virtually no indictments of police for killing people. FiveThirtyEight's Ben Casselman wonders whether the difference is based on jurors being more inclined to trust police; prosecutors being less inclined to make cases aggressively against police; or prosecutors being forced to bring weak cases against police, due to political pressure.

We do not have the data to answer that question, and realistically, we never will. What Mr Casselman doesn't mention is the glaring issue of race. The 12-member Missouri grand jury needed nine votes to indict. Three of the jurors were black. The jurors are prohibited by law from discussing their deliberations.

I have served on one American jury, and the entirety of our discussion (which required us to be sequestered for two days) turned on whether or not we trusted the police. We ended up with a hung jury because the white jurors trusted the police, and the black jurors did not. The evidence presented in the case consisted of a bag of cocaine, and two police officers who testified that the accused had tossed it under a car while fleeing. (The only defence testimony came from a chemist who challenged the police lab's procedures for measuring the quantity of the drugs; that played little role in the jury's discussion.) For the white jurors, without any testimony contesting the police account, we had no "reasonable doubt" of the state's case. For the black jurors, and for one juror in particular, the testimony of two police officers with no corroborating witnesses or evidence was simply not enough reason to send a man to jail.

These are both reasonable ways of approaching the world. What is dangerous is how neatly the disagreement falls along racial lines. African-Americans have a radically different experience of interaction with police officers than white Americans do, and are much less likely to implicitly trust the testimony of officers accused of brutality or lawbreaking. Most white Americans do not understand this distrust, so minorities are often left with a sense that the legal system offers them no recourse. White Americans cannot understand why African-Americans do not accept the verdict of the justice system. African-Americans suspect that the system is structured in a way that denies them justice.

The decision in Missouri yesterday will widen that divide. There are a few obvious measures that could help narrow it. One would be for all police to wear body cameras while on patrol, a step Michael Brown's parents have endorsed. But technological solutions do not usually solve deep social ills. The most difficult step is less technical than ethical: white Americans need to recognise that if black Americans mistrust the police, they often have good reason to do so. Police officers sometimes employ excessive force; sometimes, they kill people for no good reason. They are almost never indicted for it. Impunity is insidious. It makes it impossible for citizens to trust that in any given failure to prosecute, justice was done.