The most controversial aspect of my new book, “The War on Cops,” is my claim that violent crime is up in many American cities because officers are backing off of proactive policing. I have dubbed this double phenomenon of de-policing and the resulting crime increase the “Ferguson effect,” picking up on a phrase first used by St. Louis’s police chief.

Violence began increasing in the second half of 2014, after two decades of decline. The Major Cities Chiefs Association convened an emergency session in August 2015 to discuss the double-digit surge in violent felonies besetting its member police departments.

The violence continued into fall 2015, prompting Attorney General Loretta Lynch to summon more than 100 police chiefs, mayors and federal prosecutors in another emergency meeting to strategize over the rising homicide rates.

Arrests, summonses and pedestrian stops were dropping in many cities, where data on such police activity were available. Arrests in St. Louis City and County, for example, fell by a third after the shooting of Michael Brown. Misdemeanor drug arrests fell by two-thirds in Baltimore through November 2015.

Chicago Mayor Rahm Emanuel told Lynch that his officers were going “fetal”: “They have pulled back from the ability to interdict,” he said. “They don’t want to be a news story themselves, they don’t want their career ended early, and it’s having an impact.”

2015 closed with a 17 percent increase in homicides in the 56 largest cities, a nearly unprecedented one-year spike. Twelve cities with large black populations saw murders rise anywhere from 54 percent in the case of the District to 90 percent in Cleveland. Baltimore’s per capita murder rate was the highest in its history in 2015.

Robberies also surged in the 81 largest cities in the 12 months after the shooting of Michael Brown in Ferguson, Mo.

In the first quarter of 2016, homicides were up 9 percent and non-fatal shootings up 21 percent in 63 large cities, according to a Major Cities Chiefs Association survey.

Chicago is a prime example of the Ferguson effect. Stops were down nearly 90 percent in the first part of this year compared with last year. Shootings citywide through July 17 were up 50 percent compared with the same period in 2015; shootings were up 87 percent compared with the same period in 2014. In Austin, on the West Side, shootings are up 220 percent compared with 2014. Through July 19, 2,234 people have been shot in the city, averaging one an hour during some weekends. Yesterday, a 6-year-old girl was seriously wounded in her abdomen while sitting on her porch, when a violent shoot-out between three cars broke out; she is one of at least 21 children younger than 13 shot so far this year, including a 3-year-old boy shot on Father’s Day who is now paralyzed for life.

(One would have assumed, pursuant to the Black Lives Matter narrative, that racist cops were responsible for a significant portion of those shootings, given that their victims have been overwhelmingly black. In fact, Chicago cops shot 11 people, all armed and dangerous, through July 19, comprising 0.5 percent of all shootings.)

This crime increase, I argue, is due to officers’ reluctance to engage in precisely the proactive policing that has come under relentless attack as racist. For the past two years, activists, academics, the press and many politicians have charged that pedestrian stops and low-level public order enforcement (also known as “broken windows” policing) are little more than biased oppression of minority citizens.

That political message is accompanied by increasing tension on the street, inflamed by the persistent allegation that racist officers are the biggest threat facing young black males today. A garden-variety Black Lives Matter march that I attended last November on Fifth Avenue in New York featured “F–––the Police,” “Murderer Cops” and “Racism Is the Disease, Revolution Is the Cure” T-shirts as well as “Stop Police Terror” signs.

Officers working in urban areas are now routinely surrounded by angry crowds when they question a suspect or make an arrest. “In my 19 years in law enforcement, I haven’t seen this kind of hatred towards the police,” a Chicago cop who works on the South Side told me in May. “People want to fight you. ‘F––– the police. We don’t have to listen,’ they say.” A police officer in Los Angeles’s Newton Division reports: “Our officers are getting surrounded, cursed and jeered at every time they put handcuffs on someone.”

Officers continue to rush to crime scenes after someone has already been victimized, sometimes getting shot at in the process. But in that large area of discretionary policing that aims to prevent crime before it occurs — getting out of a squad car at 1 a.m., for example, to question someone who appears to have a gun or may be casing a target — many officers are deciding to drive on by rather than risk a volatile, potentially career-ending confrontation that they are under no obligation to instigate. “Every cop today is thinking: ‘If this stop goes bad, I’m in the mix,’ ” says Lou Turco, president of the Lieutenants Benevolent Association in New York City.

An officer in South Central Los Angeles described the views of his fellow cops: “Guys and gals in coffee shops are saying to each other: ‘If you get out of your car, you’re crazy, unless there’s a radio call.’ ”

That officers would lessen their discretionary engagement under this barrage of criticism and hatred is both understandable and inevitable. Policing is political. If a powerful segment of society sends the message that proactive policing is bigoted, the cops will eventually do less of it. This is not unprofessional conduct; it is how the calibration of police legitimacy is supposed to work.

Cops, moreover, are human. In a speech last October at the University of Chicago law school, FBI Director James Comey said that officers in one big city precinct had recounted being surrounded and taunted from the moment they made a pedestrian stop. “’We feel like we’re under siege, and we don’t feel much like getting out of our cars,’ ” they told him.

Under such conditions, it is not surprising that proactive policing is down. Remember, such policing is discretionary. Cops don’t have to do it. And they have been told not to do it by activists and the media, who accuse them of racism for making stops in high-crime areas. The only surprise is that many of those same activists are now accusing the cops of not “doing their job,” as a result of which “people are dying,” in the words of Black Lives Matter activist Shaun King. This is the same King who launched a petition in 2014 demanding that Attorney General Eric Holder “meet with local black and brown youth across the country” who were being oppressed by “broken windows” policing and pedestrian stops.

The connection between de-policing and crime increases has been documented before. A 2005 study of de-policing after the anti-cop riots in Cincinnati in 2001 by University of Washington economist Lan Shi, for example, found a significant increase in felony crime caused by the drop-off in officer engagement.

Acknowledging the connection between de-policing and crime is unacceptable, however, to those who reject the idea that data-driven, proactive policing can lower crime.

To be sure, no one has conducted randomly controlled experiments to confirm that the current crime spike in urban areas is the result of officers reverting to a reactive style of policing. But no other explanation fits the timing of the post-Ferguson crime increase. As Comey said last October, de-policing “is the one explanation that does explain the calendar and the map and that makes the most sense to me.”

University of Missouri, St. Louis, criminologist Richard Rosenfeld reached the same conclusion in a study of the post-Ferguson crime increase for the Justice Department: “The only explanation that gets the timing right is a version of the Ferguson effect,” he told the Guardian in May.

The crime increase is real, driven by officer disengagement, and is resulting in more black lives being lost. Tomorrow: Why Ferguson effect deniers are wrong.

[Heather Mac Donald is the Thomas W. Smith fellow at the Manhattan Institute and a contributing editor to City Journal.]