The city of Chicago is conducting a long, bloody experiment in what happens to a gang-ridden municipality in the absence of effective policing.

It is keeping the morgue depressingly busy. Seven hundred sixty-two people were murdered in the city in 2016, a nearly unheard of 50 percent increase over the year before. This is more than New York and Los Angeles — both larger cities — combined, and the worst figure in 20 years.

While everyone on the left pays obeisance to the slogan Black Lives Matter, Chicago Mayor Rahm Emanuel runs a jurisdiction where black lives have been getting cheaper almost by the day, especially in its poorest areas.

The lion’s share of Chicago’s spike of violence has occurred in five of Chicago’s 22 police districts. Eighty percent of the victims were rated by the police department as likely to be involved in gun violence, which means that the city is adept at identifying people as potential victims — just not at keeping them from getting shot.

Holiday weekends in Chicago reliably provide fodder for cable TV in the astonishing tallies of shootings and murders (a dozen people killed and 27 shootings over the Christmas weekend). Overall, more than 4,300 people were shot in the city last year. A woman told CNN she told her kids from a very young age what to do when they hear gunshots, a grim maternal duty in a city where gunplay is so routine.

The equation that accounts for the rising body count is simple: As the Chicago police have become less aggressive, the gangs have become more aggressive and more people have been killed. Chicago demonstrates that in swathes of inner-city America, you can have a chastened, passive police department, or a modicum of public order, but not both.

Chicago’s authorities courted the anti-police agitation of the past few years with their desperate mishandling of the controversial shooting of 17-year-old Laquan McDonald, killed by an officer in 2014. The city avoided releasing the dashcam video of the incident for a year, until after Rahm Emanuel’s re-election. When it finally did, the police representations about the threat represented by McDonald looked to be false (the cop who shot him is now awaiting trial on murder charges).

With the police on their back heels, the city further hamstrung them. It discouraged minor drug arrests. It required the police to fill out two-page contact cards (with 70 different fields) whenever stopping anyone. These forms are then forwarded on to the American Civil Liberties Union.

The city would have been much better served by forthrightness in the McDonald case from the beginning, coupled with vigorous support of tough-minded, forward-leaning policing. Instead, it got the worst of both worlds.

Fearful of becoming the next “viral video,” harassed and mocked when out doing their job in tough neighborhoods, beleaguered by paperwork, the police have suffered a crisis in morale and clearly pulled back. Documents obtained by “60 Minutes” show an 80 percent drop in stops from almost 50,000 in August 2015 to under 9,000 a year later, and arrests dropping from roughly 10,000 to 7,000.

This reduced police presence on the streets has been a boon only to anti-police ideologues (the ACLU welcomes it) and the city’s myriad gangs. Chicago Police Superintendent Eddie Johnson says that the surge in violence has been driven by “emboldened offenders who acted without a fear of penalty from the criminal-justice system.”

There is much about Chicago that can’t be readily fixed, but it is fully within the city’s power to make its criminal offenders feel less emboldened. Chicago simply needs to stop, arrest and jail more dangerous people. The only alternative is the continuation of the city’s current experiment in chaos that is making the city unlivable for the people unfortunate enough to inhabit its most violent precincts.

It goes without saying that their lives matter. Chicago should begin to act like it.

Rich Lowry can be reached via e-mail: comments.lowry@nationalreview.com.