It was just before midnight when the first police officer hit Mark Covell, swiping his truncheon down on his left shoulder. Covell did his best to yell out in Italian that he was a journalist but, within seconds, he was surrounded by riot-squad officers thrashing him with their sticks. For a while, he managed to stay on his feet but then a baton blow to the knee sent him crashing to the pavement.

Lying on his face in the dark, bruised and scared, he was aware of police all around him, massing to attack the Diaz Pertini school building where 93 young demonstrators were bedding down on the floor for the night. Covell's best hope was that they would break through the chain around the front gates without paying him any more attention. If that happened, he could get up and limp across the street to the safety of the Indymedia centre, where he had spent the past three days filing reports on the G8 summit and on its violent policing.

It was at that moment that a police officer sauntered over to him and kicked him in the chest with such force that the entire lefthand side of his rib cage caved in, breaking half-a-dozen ribs whose splintered ends then shredded the membrane of his left lung. Covell, who is 5ft 8in and weighs less than eight stone, was lifted off the pavement and sent flying into the street. He heard the policeman laugh. The thought formed in Covell's mind: "I'm not going to make it."

The riot squad were still struggling with the gate, so a group of officers occupied the time by strolling over to use Covell as a football. This bout of kicking broke his left hand and damaged his spine. From somewhere behind him, Covell heard an officer shout that this was enough - "Basta! Basta!" - and he felt his body being dragged back on to the pavement.

Now, an armoured police van broke through the school gates and 150 police officers, most wearing crash helmets and carrying truncheons and shields, poured into the defenceless building. Two officers stopped to deal with Covell: one cracked him round the head with his baton; the other kicked him several times in the mouth, knocking out a dozen teeth. Covell passed out.

There are several good reasons why we should not forget what happened to Covell, then aged 33, that night in Genoa. The first is that he was only the beginning. By midnight on July 21 2001, those police officers were swarming through all four floors of the Diaz Pertini building, dispensing their special kind of discipline to its occupants, reducing the makeshift dormitories to what one officer later described as "a Mexican butcher's shop". They and their colleagues then illegally incarcerated their victims in a detention centre, which became a place of dark terror.

The second is that, seven years later, Covell and his fellow victims are still waiting for justice. On Monday, 15 police, prison guards and prison medics finally were convicted for their part in the violence - although it emerged yesterday that none of them would actually serve prison terms. In Italy, defendants don't go to jail until they have exhausted the appeals process; and in this case, the convictions and sentences will be wiped out by a statute of limitations next year. Meanwhile, the politicians who were responsible for the police, prison guards and prison medics have never had to explain themselves. Fundamental questions about why this happened remain unanswered - and they hint at the third and most important reason for remembering Genoa. This is not simply the story of law officers running riot, but of something uglier and more worrying beneath the surface.

The fact that this story can be told at all is testament to seven years of hard work, led by a dedicated and courageous public prosecutor, Enrico Zucca. Helped by Covell as well as his own staff, Zucca has gathered hundreds of witness statements and analysed 5,000 hours of video as well as thousands of photographs. Pieced together, they tell an irrefutable tale, which began to unfold as Covell lay bleeding on the ground.

The police poured into the Diaz Pertini school. Some of them were shouting "Black Bloc! We're going to kill you", but if they genuinely believed they were confronting the notorious Black Bloc of anarchists who had caused violent mayhem in parts of the city during demonstrations earlier in the day, they were mistaken. The school had been provided by the Genoa city council as a base for demonstrators who had nothing to do with the anarchists: they had even posted guards to make sure that none of them came in.

One of the first to see the riot squad bursting in was Michael Gieser, a 35-year-old Belgian economist, who subsequently described how he had just changed into his pyjamas and was queuing for the bathroom with his toothbrush in his hand when the raid began. Gieser believes in the power of dialogue and, at first, he walked towards them saying, "We need to talk." He saw the padded jackets, the riot clubs, the helmets and the bandanas concealing the policemen's faces, changed his mind and ran up the stairs to escape.

Others were slower. They were still in their sleeping bags. A group of 10 Spanish friends in the middle of the hall woke up to find themselves being battered with truncheons. They raised their hands in surrender. More officers piled in to beattheir heads, cutting and bruising and breaking limbs, including the arm of a 65-year-old woman. At the side of the room, several young people were sitting at computers, sending emails home. One of them was Melanie Jonasch, a 28-year-old archaeology student from Berlin, who had volunteered to help out in the building and had not even been on a demonstration.

She still cannot remember what happened. But numerous other witnesses have described how officers set upon her, beating her head so hard with their sticks that she rapidly lost consciousness. When she fell to the ground, officers circled her, beating and kicking her limp body, banging her head against a near-by cupboard, leaving her finally in a pool of blood. Katherina Ottoway, who saw this happen, recalled: "She was trembling all over. Her eyes were open but upturned. I thought she was dying, that she could not survive this."

None of those who stayed on the ground floor escaped injury. As Zucca later put it in his prosecution report: "In the space of a few minutes, all the occupants on the ground floor had been reduced to complete helplessness, the groans of the wounded mingling with the sound of calls for an ambulance." In their fear, some victims lost control of their bowels. Then the officers of the law moved up the stairs. In the first-floor corridor they found a small group, including Gieser, still clutching his toothbrush: "Someone suggested lying down, to show there was no resistance. So I did. The police arrived and began beating us, one by one. I protected my head with my hands. I thought, 'I must survive.' People were shouting, 'Please stop.' I said the same thing ... It made me think of a pork butchery. We were being treated like animals, like pigs."

Officers broke down doors to the rooms leading off the corridors. In one, they found Dan McQuillan and Norman Blair, who had flown in from Stansted to show their support for, as McQuillan put it, "a free and equal society with people living in harmony with each other". The two Englishmen and their friend from New Zealand, Sam Buchanan, had heard the police attack on the ground floor and had tried to hide their bags and themselves under some tables in the corner of the dark room. A dozen officers broke in, caught them in a spotlight and, even as McQuillan stood up with his hands raised saying, "Take it easy, take it easy," they battered them into submission, inflicting numerous cuts and bruises and breaking McQuillan's wrist. Norman Blair recalled: "I could feel the venom and hatred from them."

Gieser was out in the corridor: "The scene around me was covered in blood, everywhere. A policeman shouted 'Basta!'. This word was like a window of hope. I understood it meant 'enough'. And yet they didn't stop. They continued with pleasure. In the end, they did stop, but it was like taking a toy away from a child, against their will."

By now, there were police officers on all four floors of the building, kicking and battering. Several victims describe a sort of system to the violence, with each officer beating each person he came across, then moving on to the next victim while his colleague moved up to continue beating the first. It seemed important that everybody must be hurt. Nicola Doherty, 26, a care worker from London, later described how her partner, Richard Moth, lay across her to protect her: "I could just hear blow after blow on his body. The police were also leaning over Rich so they could hit the parts of my body which were exposed." She tried to cover her head with her arm: they broke her wrist.

In one corridor, they ordered a group of young men and women to kneel, the easier to batter them around the head and shoulders. This was where Daniel Albrecht, a 21-year-old cello student from Berlin, had his head beaten so badly that he needed surgery to stop bleeding in his brain. Around the building, officers flipped their batons around, gripping the far end and using the right-angled handle as a hammer.

And in among this relentless violence, there were moments when the police preferred humiliation: the officer who stood spread-legged in front of a kneeling and injured woman, grabbed his groin and thrust it into her face before turning to do the same to Daniel Albrecht kneeling beside her; the officer who paused amid the beatings and took a knife to cut off hair from his victims, including Nicola Doherty; the constant shouting of insults; the officer who asked a group if they were OK and who reacted to the one who said "No" by handing out an extra beating.

A few escaped, at least for a while. Karl Boro made it up on to the roof but then made the mistake of coming back into the building, where he was treated to heavy bruising to his arms and legs, a fractured skull, and bleeding in his chest cavity. Jaraslaw Engel, from Poland, managed to use builders' scaffolding to get out of the school, but he was caught in the street by some police drivers who smashed him over the head, laid him on the ground and stood over him smoking while his blood ran out across the Tarmac.

Two of the last to be caught were a pair of German students, Lena Zuhlke, 24, and her partner Niels Martensen. They had hidden in a cleaners' cupboard on the top floor. They heard the police approaching, drumming their batons against the walls of the stairs. The cupboard door came open, Martensen was dragged out and beaten by a dozen officers standing in a semicircle around him. Zuhlke ran across the corridor and hid in the loo. Police officers saw her and followed her and dragged her out by her dreadlocks.

In the corridor, they set about her like dogs on a rabbit. She was beaten around the head then kicked from all sides on the floor, where she felt her rib cage collapsing. She was hauled up against the wall where one officer kneed her in the groin while others carried on lashing her with their batons. She slid down the wall and they hit her more on the ground: "They seemed to be enjoying themselves and, when I cried out in pain, it seemed to give them even more pleasure."

Police officers found a fire extinguisher and squirted its foam into Martensen's wounds. His partner was dragged by her hair and tossed down the stairs head-first. Eventually, they dragged Zuhlke into the ground-floor hall, where they had gathered dozens of prisoners from all over the building in a mess of blood and excrement. They threw her on top of two other people. They were not moving, and Zuhlke drowsily asked them if they were alive. They did not reply, and she lay there on her back, unable to move her right arm, unable to stop her left arm and her legs twitching, blood seeping out of her head wounds. A group of police officers walked by, and each one lifted the bandana which concealed his identity, leaned down and spat on her face.

Why would law officers behave with such contempt for the law? The simple answer may be the one which was soon being chanted outside the school building by sympathetic demonstrators who chose a word which they knew the police would understand: "Bastardi! Bastardi!" But something else was happening here - something that emerged more clearly over the next few days.

Covell and dozens of other victims of the raid were taken to the San Martino hospital, where police officers walked up and down the corridors, slapping their clubs into the palms of their hands, ordering the injured not to move around or look out of the window, keeping handcuffs on many of them and then, often with injuries still untended, shipping them across the city to join scores of others, from the Diaz school and from the street demonstrations, detained at the detention centre in the city's Bolzaneto district.

The signs of something uglier here were apparent first in superficial ways. Some officers had traditional fascist songs as ringtones on their mobile phones and talked enthusiastically about Mussolini and Pinochet. Repeatedly, they ordered prisoners to say "Viva il duce". Sometimes, they used threats to force them to sing fascist songs: "Un, due, tre. Viva Pinochet!"

The 222 people who were held at Bolzaneto were treated to a regime later described by public prosecutors as torture. On arrival, they were marked with felt-tip crosses on each cheek, and many were forced to walk between two parallel lines of officers who kicked and beat them. Most were herded into large cells, holding up to 30 people. Here, they were forced to stand for long periods, facing the wall with their hands up high and their legs spread. Those who failed to hold the position were shouted at, slapped and beaten. Mohammed Tabach has an artificial leg and, unable to hold the stress position, collapsed and was rewarded with two bursts of pepper spray in his face and, later, a particularly savage beating. Norman Blair later recalled standing like this and a guard asking him "Who is your government?" "The person before me had answered 'Polizei', so I said the same. I was afraid of being beaten."

Stefan Bauer dared to answer back: when a German-speaking guard asked where he was from, he said he was from the European Union and he had the right to go where he wanted. He was hauled out, beaten, given a face full of pepper spray, stripped naked and put under a cold shower. His clothes were taken away and he was returned to the freezing cell wearing only a flimsy hospital gown.

Shivering on the cold marble floors of the cells, the detainees were given few or no blankets, kept awake by guards, given little or no food and denied their statutory right to make phone calls and see a lawyer. They could hear crying and screaming from other cells.

Men and women with dreadlocks had their hair roughly cut off to the scalp. Marco Bistacchia was taken to an office, stripped naked, made to get down on all fours and told to bark like a dog and to shout "Viva la polizia Italiana!" He was sobbing too much to obey. An unnamed officer told the Italian newspaper La Repubblica that he had seen brother officers urinating on prisoners and beating them for refusing to sing Faccetta Nera, a Mussolini-era fascist song.

Ester Percivati, a young Turkish woman, recalled guards calling her a whore as she was marched to the toilet, where a woman officer forced her head down into the bowl and a male jeered "Nice arse! Would you like a truncheon up it?" Several women reported threats of rape, anal and vaginal.

Even the infirmary was dangerous. Richard Moth, covered in cuts and bruises after lying on top of his partner, was given stitches in his head and legs without anaesthetic - "an extremely painful and disturbing experience. I had to be held down." Prison medical staff were among those convicted of abuse on Monday.

All agree that this was not an attempt to get the detainees to talk, simply an exercise in creating fear. And it worked. In statements, prisoners later described their feeling of helplessness, of being cut off from the rest of the world in a place where there was no law and no rules. Indeed, the police forced their captives to sign statements, waiving all their legal rights. One man, David Larroquelle, testified that he refused and had three of his ribs broken. Percivati also refused and her face was slammed into the office wall, breaking her glasses and making her nose bleed.

The outside world was treated to some severely distorted accounts of all this. Lying in San Martino hospital the day after his beating, Covell came round to find his shoulder being shaken by a woman who, he understood, was from the British embassy. It was only when the man with her started taking photographs that he realised she was a reporter, from the Daily Mail. Its front page the next day ran an entirely false report describing him as having helped mastermind the riots. (Four long years later, the Mail eventually apologised and paid Covell damages for invasion of privacy.)

While his citizens were being beaten and tormented in illegal detention, spokesmen for the then prime minister, Tony Blair, declared: "The Italian police had a difficult job to do. The prime minister believes that they did that job."

The Italian police themselves fed the media with a rich diet of falsehood. Even as the bloody bodies were being carried out of the Diaz Pertini building on stretchers, police were telling reporters that the ambulances lined up in the street were nothing to do with the raid, and/or that the very obviously fresh injuries were old, and that the building had been full of violent extremists who had attacked officers.

The next day, senior officers held a press conference at which they announced that everybody in the building would be charged with aggressive resistance to arrest and conspiracy to cause destruction. In the event, the Italian courts dismissed every single attempted charge against every single person. That included Covell. Police attempts to charge him with a string of very serious offences were described by the public prosecutor, Enrico Zucca, as "grotesque".

At the same press conference, police displayed an array of what they described as weaponry. This included crowbars, hammers and nails which they themselves had taken from a builder's store next to the school; aluminium rucksack frames, which they presented as offensive weapons; 17 cameras; 13 pairs of swimming goggles; 10 pen-knives; and a bottle of sun-tan lotion. They also displayed two Molotov cocktails which, Zucca later concluded, had been found by police earlier in the day in another part of the city and planted in the Diaz Pertini building as the raid ended.

This public dishonesty was part of a wider effort to cover up what had happened. On the night of the raid, a force of 59 police entered the building opposite the Diaz Pertini, where Covell and others had been running their Indymedia centre and where, crucially, a group of lawyers had been based, gathering evidence about police attacks on the earlier demonstrations. Officers went into the lawyers' room, threatened the occupants, smashed their computers and seized hard drives. They also removed anything containing photographs or video tape.

With the courts refusing to charge the detainees, the police secured an order to deport all of them from the country, banning them from returning for five years. Thus, the witnesses were removed from the scene. Like the attempted charges, all the deportation orders were subsequently dismissed as illegal by the courts.

Zucca then fought his way through years of denial and obfuscation. In his formal report, he recorded that all the senior officers involved were denying playing any part: "Not a single official has confessed to holding a substantial command role in any aspects of the operation." One senior officer who was videoed at the scene explained that he was off duty and had just turned up to make sure his men were not being injured. Police statements were themselves changeable and contradictory, and were overwhelmingly contradicted by the evidence of victims and numerous videos: "Not a single one of the 150 officers reportedly present has provided precise information regarding an individual episode."

Without Zucca, without the robust stance of the Italian courts, without Covell's intensive work assembling video records of the Diaz raid, the police might well have evaded responsibility and secured false charges and prison sentences against scores of their victims. Apart from the Bolzaneto trial which finished on Monday, 28 other officers, some very senior, are on trial for their part in the Diaz raid. And yet, justice has been compromised.

No Italian politician has been brought to book, in spite of the strong suggestion that the police acted as though somebody had promised them impunity. One minister visited Bolzaneto while the detainees were being mistreated and apparently saw nothing or, at least, saw nothing he thought he should stop. Another, Gianfranco Fini, former national secretary of the neo-fascist MSI party and the then deputy prime minister, was - according to media reports at the time - in police headquarters. He has never been required to explain what orders he gave.

Most of the several hundred law officers involved in Diaz and Bolzaneto have escaped without any discipline or criminal charge. None has been suspended; some have been promoted. None of the officers who were tried over Bolzaneto has been charged with torture - Italian law does not recognise the offence. Some senior officers who were originally going to be charged over the Diaz raid escaped trial because Zucca was simply unable to prove that a chain of command existed. Even now, the trial of the 28 officers who have been charged is in jeopardy because the prime minister, Silvio Berlusconi, is pushing through legislation to delay all trials dealing with events that occurred before June 2002. Nobody has been charged with the violence inflicted on Covell. And as one of the victims' lawyers, Massimo Pastore, put it:

Fifty-two days after the attack on the Diaz school, 19 men used planes full of passengers as flying bombs and shifted the bedrock of assumptions on which western democracies had based their business. Since then, politicians who would never describe themselves as fascists have allowed the mass tapping of telephones and monitoring of emails, detention without trial, systematic torture, the calibrated drowning of detainees, unlimited house arrest and the targeted killing of suspects, while the procedure of extradition has been replaced by "extraordinary rendition". This isn't fascism with jack-booted dictators with foam on their lips. It's the pragmatism of nicely turned-out politicians. But the result looks very similar. Genoa tells us that when the state feels threatened, the rule of law can be suspended. Anywhere.

For 9 minutes and 29 seconds, Derek Chauvin pressed his knee into the neck of George Floyd, an unarmed Black man. This deadly use of force by the now-former Minneapolis police officer has reinvigorated a very public debate about police brutality and racism.

As protests have spread around the globe, the pressure is on police departments and politicians, particularly in the United States, to do something — from reforming law-enforcement tactics to defunding or even abolishing police departments.

And although researchers are encouraged by the momentum for change, some are also concerned that, without ample evidence to support new policies, leaders might miss the mark. Many have been arguing for years about the need for better data on the use of force by the police in the United States, and for rigorous studies that test interventions such as training on how to de-escalate tense interactions or mandating the use of body-worn cameras by officers. Those data and studies have begun to materialize, spurred by protests in 2014 after the deadly shooting of Michael Brown in Ferguson, Missouri, and the death by chokehold of Eric Garner in New York City.

Government officials, academic researchers and media outlets launched data-collection projects around that time to better understand the frequency of police violence and the risk factors that contribute to it. From these growing data sets come some disturbing findings. About 1,000 civilians are killed each year by law-enforcement officers in the United States. By one estimate, Black man are 2.5 times more likely than white men to be killed by police during their lifetime.¹ And in another study, Black people who were fatally shot by police seemed to be twice as likely as white people to be unarmed.²

“We have enough evidence that tells us that action needs to be taken,” says Justin Nix, a criminologist at the University of Nebraska Omaha. “One thousand deaths a year does not have to be normal.” Since Nature reported last September on what the data say about racial bias and police killings, new evidence has continued to support a link. Data from California show that police stopped and used force against Black people disproportionately, compared with other racial groups, in 2018 (see go.nature.com/2bgfrah). A December 2019December 2019 paper reported that bias in police administrative records results in many studies underestimating levels of racial bias in policing, or even masking discrimination entirely.³

The data are still limited, which makes crafting policy difficult. A national data set established by the FBI in 2019, for example, contains data from only about 40% of US law-enforcement officers. Data submission by officers and agencies is voluntary, which many researchers see as part of the problem.

“Most agencies do not collect that data in a systematic way,” says Tracey Meares, founding director of the Justice Collaboratory at Yale Law School in New Haven, Connecticut. “I hope when people think about the science of this that they understand what we know, what we don’t know and why we don’t know it,” she says. “Policing, in large part for historical reasons, has proceeded in kind of a science-free zone.”

Bad apples

Scientists must often think creatively to work around the limitations in the data. Mark Hoekstra, an economist at Texas A&M University in College Station, has attempted to decipher the role of race in police officers’ use of force, by comparing responses to emergency calls.

Based on information from more than two million 911 calls in two US cities, he concluded that white officers dispatched to Black neighbourhoods fired their guns five times as often as Black officers dispatched for similar calls to the same neighbourhoods⁴ (see ‘Answering the call’). Hoekstra wonders whether the factors that contribute to an officer using excessive force might be predicted in a similar way to how US Major League Baseball teams use sophisticated statistical models to predict whether a player has the potential to be a future all-star.

Scientists have tried to identify some predictive factors, such as racial bias, a bad temper, insecure masculinity and other individual characteristics, many of which can be identified through simulations already used in officer training⁵. Nix suggests that such screening could help with vetting officers before they are recruited. But raising the bar for hiring might be impractical, he cautions, because many police departments are already struggling to attract and retain highly qualified candidates.

Similar forecasting models could recognize patterns of bad behaviour among officers. Data from the New York City Police Department suggests that officers who had repeated negative marks in their files were more than three times as likely to fire their gun as were other officers⁶.

Such wrongdoing might even be contagious. Another study, published in February, looked at complaints filed against police officers in Chicago, Illinois. It found that although only a small percentage of officers shoot at civilians, those who have done so often serve as “brokers” in the social networks within policing⁷. Other officers connected to them were also found to be at greater risk of shooting.

But carrying out disciplinary action, let alone firing a police officer, is notoriously difficult in the United States. Union contracts give officers protections that have been tied to increases in misconduct⁸. In many states, a bill of rights for law-enforcement officers shields personnel from investigations into misconduct. “One thing we need to take a hard look at are those state laws and union contracts that provide either flawed or overly protective procedures that insulate officers from appropriate accountability,” says Seth Stoughton, a former police officer who is a law professor at the University of South Carolina in Columbia.

Lawrence Sherman, director of the Cambridge Centre for Evidence-Based Policing in Cambridge, UK, suggests that states have the constitutional power to license, or revoke, the power of any individual to serve as a police officer. “If a state agency was keeping track of everyone’s disciplinary history, they might have taken Derek Chauvin out of the policing business ten years ago,” says Sherman. Chauvin had received 18 complaints against him even before he put his knee on Floyd’s neck. “We monitor performance of doctors,” Sherman adds. “Why don’t we monitor the performance of police officers?”

Even officers who are fired for misconduct are frequently rehired. The police officer in Cleveland, Ohio, who fatally shot 12-year-old Tamir Rice in 2014 had previously resigned from another police department after it had deemed him unfit to serve. The Cleveland police did not review the officer’s personnel file before hiring him, The New York Times reported in 2015. An investigation of public records from Florida showed that about 3% of that state’s police force had previously been fired or had resigned in lieu of being dismissed. The study, published in May, found that these officers tended to move to smaller agencies which served a slightly larger proportion of Black residents residents, but with no significant difference in crime rates⁹. They also appeared to be more likely to commit misconduct in the future compared to officers who had never been fired.

“If an officer is fired for misconduct, or resigned to avoid an investigation, they shouldn’t be able to get hired by another agency,” says Stoughton. “This is a low-hanging fruit.”

Federal legislation introduced last month targets barriers to good and fair policing. One bill would effectively end the doctrine of qualified immunity, by which courts have largely prevented officers from being successfully sued for abuse of power or misconduct since the mid-1960s (ref. ¹⁰). A similar bill proposes a number of measures intended to increase police accountability, training and data collection, including a national police misconduct registry to keep record of when an officer is fired or quits. Although Democrats in Washington DC broadly support the bills, Republicans unveiled a competing, weaker proposal that does not address the issue of qualified immunity. This came on 17 June — a day after President Donald Trump signed an executive order that incentivizes police reform. The order drew swift criticism over its relatively narrow breadth and lack of teeth.

Robin Engel, director of the Center for Police Research and Policy in Cincinnati,Ohio, suggests that the real capacity for change is at the state and local levels. “There’s a collective citizen call to action now to hold political leaders responsible for ensuring that the police are collecting data, releasing data and operating with best practices,” says Engel.

Evidence-based policing

It remains unclear which law-enforcement practices are actually best, largely because of a lack of data and science. “We’re operating in the dark about what are the most effective strategies, tactics and policies to move forward with,” Engel says.

Political leaders and activists pushing for change in the United States have widely endorsed body-worn cameras, de-escalation training, implicit-bias training, early intervention systems, the banning of chokeholds, and civilian oversight since the tragedies of 2014. A survey of 47 of the largest US law-enforcement agencies between 2015 and 2017 found that 39% changed their use-of-force policies in 2015-2016 and revised their training to incorporate tactics such as de-escalation. Among the agencies surveyed, officer-involved shootings dropped by 21% during the study period¹¹.

“But as we have seen in the last several weeks — from Minneapolis and from the police response to the protests — there’s a great deal that still has to change in policing,” says Laurie Robinson, a criminologist at George Mason University in Fairfax, Virginia.

Researchers are advocating collection of better data, such as tracking situations in which force was avoided by de-escalation strategies or, when force was used, recording whether it was at a lower level than it might previously have been.

The Oklahoma City Police Department is among agencies working to fill that void. It now collects details on the applicability of each specific de-escalation tactic and technique any time force is used. “Since the implementation of our de-escalation policy, our use-of-force numbers have decreased,” states Megan Morgan, a police sergeant and spokesperson for the department.

The collection of data might itself hold police officers more accountable. In one study, a requirement that officers file a report when they point their guns at people but do not fire was associated with significantly reduced rates of gun death¹².

The use of body-worn cameras could be among the easiest interventions to enhance accountability. The technology gained traction after a randomized experiment published in 2014 compared shifts in which all officers wore cameras all the time with shifts in which they never did¹³. The likelihood of force being used by officers with cameras was roughly half that of officers without cameras. Furthermore, camera-wearing officers received about one-tenth the number of complaints as did officers without cameras.

Results of more-recent studies have been mixed. When the Las Vegas Metropolitan Police Department in Nevada implemented body cameras, it experienced significant drops in both the rate of complaints and the use of force¹⁴. But when the

The Metropolitan Police Department of the District of Columbia did the same, it found no benefits (see go.nature.com/3heuxac). The differences might have more to do with policies that allow officers to choose when to turn on their cameras, as well as a lack of controls for situations in which one officer shows up wearing a camera while another does not, notes Sherman. The latter could dilute true differences in the rates of complaints or uses of force.

“It would be a travesty if we got rid of body cams,” says Sherman. “They very often help to clarify what happened.”

Evidence suggests that encouraging officers to listen to citizens’ views before making decisions and to generally demonstrate an interest in working with members of a community can be another effective intervention. A one-day training programme based on these principles of procedural justice — a model of policing that focuses on respect, neutrality and transparency — was shown to reduce both citizen complaints and use of force by officers in the Chicago Police Department¹⁵.

“If police are to be of service to communities, they need to build trust with communities that are likely to distrust them,” says Thomas O’Brien, a researcher at the Social Action Lab at the University of Illinois in Urbana–Champaign. His work suggests that such trust-building requires the police to both acknowledge its role in creating the distrust, as well as apologize for it¹⁶. Any half-hearted attempts at reconciliation could backfire, he says. Special training can be difficult, however, particularly in smaller jurisdictions, which have been shown to have a higher rate of police shooting civilians¹⁷ (see ‘Small-town problems’).

In the wake of Floyd’s death, many calls for change have gone beyond police reform to defunding police departments — reducing their public funding and reallocating resources to other programmes — or dismantling them altogether. Some researchers caution against fully abolishing police departments. That could have “disastrous consequences”, says Engel. “It’s better to work within and demand significant and meaningful change, and then hold them accountable for that change.”

However, Engel does support proposals that would begin “carving off pieces” of law-enforcement agencies’ current responsibilities that might fall outside their expertise — or might not require an armed response — such as issues of homelessness, drug abuse and mental illness. In New York City, the police purview goes as far as to include enforcement of street-vendor licences. Across the United States, an arrest is made every 3 seconds; less than 5% of these are for serious violent crimes, according to the Vera Institute of Justice in Brooklyn, New York (see go.nature.com/3fbwmcn) .

Curtailing police encounters could also result in fewer crimes. Research published last year found that Black and Latino boys who are stopped more often by police are more likely to commit crimes months later¹⁸.

Stoughton also emphasizes the role of racial bias in society, as evidenced in the months leading up to Floyd’s murder by the fatal shooting of a 25-year-old Black man, Ahmaud Arbery, by two white men while he was jogging in Georgia, and by a white woman’s 911 call to falsely report being threatened by a Black birdwatcher in New York City’s Central Park. “I have become convinced that we do not have a race problem in policing,” says Stoughton. “Rather, we have a race problem in society that is reflected in policing.”

Nature 583, 22-24 (2020)

doi: https://doi.org/10.1038/d41586-020-01846-z

Updates & corrections

Update 26 May 2021: On 20 April 2021, Derek Chauvin was convicted of causing the death of George Floyd. The text has been modified to include updated information on how long Chauvin knelt on Floyd’s neck.

References

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Abstract

Objective:
There is widespread belief that people with mental disorders are overrepresented in police encounters. The prevalence of such interactions is used as evidence of extensive problems in our health care and social support systems. The goal of this study was to estimate the rates of police arrests among people with mental disorders, police involvement in pathways to mental health care, and police calls for service involving persons with mental disorders.

Methods:
A systematic review was performed with seven multidisciplinary databases. Additional studies were identified by reviewing the reference lists of all included records and by using the “related articles” and “cited articles” tools in the Web of Science database. Studies were included if they were published in peer-reviewed journals, reported primary research findings, and were written in English.

Results:
Eighty-five unique studies covering 329,461 cases met inclusion criteria. Data reported in 21 studies indicated that one in four people with mental disorders have histories of police arrest. Data from 48 studies indicated that about one in ten individuals have police involved in their pathway to mental health care. Data reported in 13 studies indicated that one in 100 police dispatches and encounters involve people with mental disorders.

Conslusions:
These estimates illuminate the magnitude of the issue and supply an empirically based reference point to scholars and practitioners in this area. The findings are useful for understanding how local trends regarding police involvement in the lives of people with mental disorders compare with rates in the broader research literature.

Contemporary health care systems, social programs, and policing models have been designed in such a way that contact between people with mental disorders and the police is inevitable. Attending to mental health crises, working with witnesses and victims of crime, searching for those who have absconded from inpatient and residential care, and identifying people who have mental health needs and connecting them to services are the foreseeable duties for today’s police officer (1,2). In addition, officers are called on to intervene in criminal acts—from public disturbances to more serious incidents involving threatened or actual violence—perpetrated by people exhibiting a variety of mental healt problems, such as dementia, intoxication, intellectual disability, or serious mental illness (3).

To many police, interacting on a routine basis with people who have mental disorders is problematic (2). Police chiefs have asserted that officers should not be the mental health response agency of first resort and that mental health situations consume too much time of frontline officers, diverting precious resources from core law enforcement activities (4). For example, police officers routinely express concern about the amount of time spent waiting in hospital emergency rooms in combination with the prospect that individuals with mental disorders may not be admitted to inpatient care (1). Research has shown that contacts involving people with mental disorders place great demands on police resources (5). Police officers express frustration with deficiencies in the health and social service systems that severely constrain their ability to resolve situations involving people with mental disorders in a timely and appropriate manner. Serious concerns have also been raised by people with mental disorders about police interventions, particularly those that involve the use of force (6,7).

Media and academic discourses suggest that these encounters are a common and growing phenomenon, or “crisis” (8), but how truly common are these interactions? Although published reviews have examined the prevalence of mental disorders in various criminal justice populations and settings, such as correctional institutions (9,10), there has been no synthesis of rates within the context of policing. This study took stock of general trends within the extant knowledge by examining the rates of interaction between police and people with mental disorders. Published data on the following three rates were synthesized: police arrests among people with mental disorders, police involvement in pathways to mental health care, and police calls for service involving persons with mental disorders.

Methods

A systematic search was conducted in PubMed, PsycINFO, Web of Science, JSTOR, Criminal Justice Abstracts, Sociological Abstracts, and the National Criminal Justice Reference Service. The search was first performed in September 2009 to scope the literature as part of a larger study focused on gathering self-reported data from people with mental illnesses about their encounters with the police. After the primary data were analyzed and published, the search was updated in June 2015 and more carefully analyzed for patterns. Combinations of the following terms were searched: (police* OR law enforce*) AND ( bipolar* OR mania* OR mental ill* OR mental disorders OR schizophreni* OR psycho*). Searches with large numbers of records (>250) were narrowed by adding the interact* term. Additional studies were located by reviewing the reference lists of all included records and by using the related articles and cited articles tools in the Web of Science database.

Studies were included if they were published in peer-reviewed journals, reported primary research findings, and were written in English. No restrictions were placed on publication year or methodological design. Studies were excluded if they reported data in such a way that an exact numerator and denominator, used for calculating rates, could not be ascertained. In addition to studies that met general eligibility criteria, studies on arrest rates were included if they provided data on histories of arrest over the lifetime or over extended periods among persons with mental disorders and were excluded if they used only police-involved samples or examined arrest rates over a brief period (fewer than five years). Studies of rates of police involvement in care pathways were included if they reported data on the proportion of people who interacted with police on their way to inpatient or outpatient mental health services, including emergency or compulsory psychiatric care, and were excluded if they used only police-involved samples. Studies of police calls for service were included if they provided data on the proportion of police dispatches or encounters that involved people with mental disorders and were excluded if they focused only on use-of-force incidents, used only samples detained in police custody, or focused only on calls to police–mental health crisis teams. [The excluded studies are listed in a table in an online supplement to this article.]

For every included study, data about selected variables were extracted and entered into Excel and SPSS. Data were extracted by a research assistant and then verified by the author. Student’s t tests, Spearman rank correlation, and analyses of variance were performed to assess differences in average rates across study characteristics. Because of the possibility that studies with exceptionally large samples—exceeding the combined samples of the remaining studies—would distort the findings, those studies were removed from calculations of the overall rates described in the text.

Results

Data from smaller studies—those with fewer than 500 individual cases—were combined for summary in Tables 1–3. Rates that include all studies are reported in the tables. In total, 85 unique studies with 329,461 cases involving contact between the police and people with mental disorders were included and synthesized. Cases covered individuals and events.

Police Arrests of People With Mental Disorders

Arrest rates can provide an approximation of formal interactions between police and the community. In relation to police arrest histories, the search uncovered 22 studies that included 126,852 individuals with mental disorders (3,11–31) (Table 1). After removal of one study with a relatively large sample (N=73,579) (19), the overall results indicated that 25% of persons with mental disorders (13,304 of 53,273 individuals) have been arrested by police at some point in their lifetime.

Discussion

This review synthesized rates reported in 85 unique studies of contact between police and people with mental disorders. The estimates varied considerably depending on the type of rate. Overall, the findings suggest that typically one in four people with mental disorders have histories of police arrest, which is substantially higher than estimated rates among general adult populations in Canada (95) and the United Kingdom (96) but in line with rates in the United States (97,98). This review also found that about one in ten individuals encountered police in their pathway to mental health care, and one in 100 police dispatches and encounters involve people with mental disorders. Such results shed empirical light on claims made by journalists, politicians, police personnel, and scholars about the frequency with which the police and people with mental disorders interact.

As is the case with most reviews of this nature, the included studies reported a wide range of rates, varying across the populations studied and the methods employed. This body of research predominantly consists of samples drawn from the United States, with some indication that U.S. police officers, compared with those in other countries, may play a greater role in connecting people to mental health services. When viewed optimistically, this might suggest that U.S. police officers are more actively involved, compared with their counterparts in other countries, in facilitating precharge diversion processes aimed at preventing people with mental disorders from further involvement with the criminal justice system. Conversely, this finding may also signal serious problems associated with the criminalization of people with mental disorders in the United States, whereby criminal justice processes, agents, and institutions are used to manage mental health issues. Another source of variation in rates was the diverse methods used for establishing the presence of an arrest history and a mental disorders, such as self-report versus official records, although these findings were mixed across the three types of rates examined in this review. Methodological quality of the studies was not formally appraised and controlled in this review, but the rates varied across different methodological characteristics, such as the size of the samples and the various ways in which mental disorder was operationalized.

One problem that has plagued research in this area is how to establish valid and reliable indicators of mental disorder in the context of police contacts. Scholars have developed rigorous field observation methods to reduce biases inherent in police-generated data (90,94). A limitation of this approach is that it is resource-intensive and difficult to perform on a large scale. Sophisticated data algorithms also have been produced to identify persons with mental disorders within administrative police databases (80,81,83,84). Although this method can be carried out on large data sets and is relatively easy to replicate, it is dependent on the reporting practices of dispatchers and police officers and, as such, likely underestimates the number of encounters that actually involve people with mental disorders. Triangulating data and methods, as was done in this review, enhances the validity of such rates, but the underreporting of mental disorders remains an issue. A number of factors influence whether mental disorders will be discovered by, or reported to, the police, including relevance to the encounter, concealability, the skills of police personnel, and the fact that people who more often interact with the police, such as men and persons from racial-ethnic minority groups, may be the least motivated to disclose mental health issues ( 99). It would be prudent for researchers and decision makers to be mindful of these methodological issues when comparing the rates of different jurisdictions and when drawing conclusions about the nature and magnitude of the phenomenon.

Rates of contact between the police and people with mental illness tell us more about the patterned social arrangements in society than the criminality of individuals with mental disorders. Numerous studies expose the powerful social forces and diverse situations that bring these two groups together. Police interactions involving people with mental illnesses are often casual in nature, unrelated to offending behavior, and resolved through informal means (80,93,100). The criminalization of illicit substances, the coercive underpinnings of mental health policies and services, and the intersecting functions of our mental health and criminal justice institutions are all implicated in these trends (101,102). Consistent with other reviews (103), this study revealed that rates of contact between police and people with mental disorders were correlated with a number of individual, organizational, and contextual factors, including homelessness, substance use problems, gender and race, and social disorganization. In addition to methodological differences, these characteristics are likely major sources of variation in rates between studies and jurisdictions. Interestingly, one study indicated that building the capacity for police services to respond appropriately to people with mental disorders, such as through training or specialized response teams, may increase the rate of mental health–related calls for services; however, the drivers of such changes (reporting practices versus greater reliance on police services) are not well understood and warrant greater research attention. What is clear is that reducing the rates revealed in this study will require that improvements be made to the undesirable social context in which people with mental disorders find themselves: terrible situations requiring routine and recurrent intervention by the police.

A number of methodological limitations must be noted. First, it is likely that a sizeable number of studies reporting on arrest histories among people with mental disorders were not captured by the literature search because they focused on other substantive issues and their titles, Abstracts, or keywords did not contain necessary terms (that is, police* OR law enforce*). Second, the literature search was first performed in 2009 and then replicated in 2015; consequently, it was not possible to determine and report on the total number of studies screened and assessed for eligibility. Third, methodological quality of the included studies was not formally appraised, and variability in quality was not controlled. Fourth, an initial search of the gray literature revealed few unpublished documents reporting primary data and satisfying other inclusion criteria. As such, the review was restricted to published research, which may pose a threat to the validity of the findings on account of publication bias.

The gross estimates of rates presented in this study will be of interest to researchers and police personnel who focus their efforts on understanding and improving such interactions; however, the study did not engage with a number of central questions. For instance, are these interactions increasing in frequency? Is the nature of these encounters changing? Are repeated contacts becoming more common? In what ways do interactions with police differ for people with or without mental disorders? How are rates affected by the implementation of specific strategies, including police training and specialized response models, aimed at improving encounters between police officers and people with mental disorders? What role does the presence of a mental disorders, as opposed to other factors, actually play in these interactions? Such questions have important implications for policy makers, police leadership, and health care planners and are worthy of additional research.

Conclusions

Numerous coroner inquests and public inquiries after tragic events have created a heightened awareness of issues surrounding police encounters with people who have mental disorders. A prevailing sentiment is that many of these interactions are undesirable, unnecessary, and avoidable. By providing estimates regarding the rates of contact between police and people with mental disorders, this study illuminates the magnitude of the issue and supplies an empirically supported reference point to scholars and practitioners in this area.

Author and article information

Dr. Livingston is with the Department of Sociology and Criminology, Saint Mary’s University, Halifax, Nova Scotia, Canada (e-mail: jamie.livingston@smu.ca).

Dr. Livingston reports no financial relationships with commercial interests.

The author thanks Zachary MacMillan, Karen Chu, and Michelle Pritchard for their contributions to the project.

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