Peaceful marches, standoffs between protesters and law enforcement, and, in some cases, riots — over the past year and a half, there has been a rising movement critical of excessive use of force by police. At the same time, there has been a troubling spike in murders in some big US cities.

Some criminal justice experts believe the two trends may be connected.

The idea — dubbed the "Ferguson effect" after the Ferguson, Missouri, protests that elevated the nationwide Black Lives Matter movement — is that the protests have demoralized police officers, leading them to back off from proactive policing, while emboldening criminals, who now see the police as more vulnerable. The result: more crime.

This would be a particularly concerning development, since crime nationwide, including homicides and violent crime, has plummeted since the 1990s.

Early data from the Brennan Center for Justice shows there was, indeed, a very big increase in murders in 25 of the 30 most populous US cities from 2014 to 2015 — 14.6 percent. But overall crime has actually dropped in the 19 cities with available data — by 5.5 percent. The Brennan Center report argues that it's too early to say whether the decades-long crime drop is really reversing.

Some criminologists, meanwhile, argue that if crime or homicide rates are increasing, the Ferguson effect may not explain all, most, or even any of the rise. After all, we still don't fully know why crime declined in the past couple of decades — there are literally dozens of theories for why, and none is wholly accepted by experts as a satisfactory explanation. So it's possible other factors are contributing to the crime rise, some of which we may not even know exist yet.

Still, the Ferguson effect has gained a lot of traction, especially in conservative circles that are more supportive of police and believe that tougher policing explains most or all of the crime drop in the past couple of decades.

I reached out to one of the Ferguson effect theory's lead proponents, Heather Mac Donald, to hear the best case for this idea. Mac Donald was one of the first people to put forward the idea of a Ferguson effect through her columns in the Wall Street Journal. What follows is a transcript of our conversation, edited for length and clarity.

German Lopez: Why do you believe there's a Ferguson effect?

Heather Mac Donald: Because the rise in homicides and shootings in many cities across the country for the last year and a half or so has been extremely dramatic. It has reversed the ongoing trends.

Officers say they are reluctant to engage in proactive policing. That testimony is backed up by data on officer activity where such data exists and is publicly available — such as in Los Angeles and Baltimore, where arrests just plummeted after the Freddie Gray case.

Police chiefs and law enforcement officials testify to the significance of the crime increase and to the likely or possible reason for it, which is the reluctance of officers to engage in discretionary enforcement.

GL: If I'm understanding you correctly, there's three points of evidence in your view: One, what police are saying. Two, the arrest data. And three, the rise in murders. You put all those together to say there is a Ferguson effect, correct?

HM: Right.

GL: In your original column, you cited the St. Louis police chief, who I think came up with the name "Ferguson effect." But murders were actually going up in St. Louis before Michael Brown's death and the Ferguson protests. Doesn't that suggest that something else might be going on, as well?

HM: Well, I think murders were bouncing around month to month after the protests and riots. I think they accelerated and, you know, that's what [St. Louis Police Chief Sam] Dotson was seeing. But that's just St. Louis. I would defer to Chief Dotson as to what he's seeing and what he's hearing from his officers.

But I don't see how that explains the rest of the country. We had the first six months of 2014, violent crime was down almost 5 percent. That drop was completely erased by the next six months of 2014, and violent crime has continued to go up. So I just don't think that what the critics are saying about St. Louis explains what's happening elsewhere.

GL: You mentioned that this is a nationwide trend. The Brennan Center has pushed back on this. It just released an update to its analysis projecting that crime is down in 2015 by 5.5 percent, although murders will increase overall by 14.6 percent, in big US cities. What do you think of that — crime dropped as murders increased?

HM: Well, murders and shootings are going up. Overall crime — and I'd like to see violent crime separated out — is driven by property crime. It's a rate that's eight times higher than violent crime.

Property crime can be going down while the type of inner-city violence of these mindless drive-by shootings, where they're mowing down children across the country, is going up, because police are not engaging in the type of discretionary stops that is the best way to deter people from carrying guns.

GL: On that point, I wanted to ask you about New York City in particular. A New York Times report showed that crime and shootings there dropped in 2015 — even though, again, murder numbers went up. If you were expecting a Ferguson effect, you would expect to see it in New York City of all places, where the mayor was perceived as an outspoken critic of police tactics at first. But it seems like it's a much more mixed picture.

HM: Well, shootings were going very high in the first half of 2015, and [Mayor Bill] de Blasio and [Police Commissioner Bill] Bratton were terrified. Bratton's response was to start the "Summer All Out" program months earlier, which pulls every available cop out of existing duties, desk duties, and puts them out in the streets to engage in command presence — at enormous overtime costs to the taxpayers.

New York has the luxury of having a massive police force, so they managed to quell the shooting outbreak and brought down the rate of increase for homicides by flooding the hot-spot shooting zones with officers. And then the cooler weather pulled that back.

But if he didn't have the luxury of being able to put that manpower on the dots, there is no way the shooting spree would have leveled off as it did.

GL: It seems to me, though, that the fact that New York City officials were able to do that pushes back against the idea that a Ferguson effect was causing more shootings, because if what you're saying is correct, the cops were clearly able to do their jobs —even if they claimed to have lower morale.

HM: Well, they were mostly there, standing on corners. Again, I hear from officers that they're reluctant to get out of cars and engage a pedestrian that they see in a corner. But there's no question that command presence — just being there — has been shown again and again in studies to deter crime.

And New York, again, has the numbers of cops. Other cities can't do that, and they're going to be much more reliant on actual proactive stops to try and deter gun slinging.

GL: The other aspect of this is the focus on homicide numbers in particular. Brennan pointed out, and I've heard this from criminologists as well, that homicide numbers are very noisy from year to year — meaning they fluctuate a lot. We see that in, for example, Chicago, where numbers go up and down — sometimes as high as 10 percent in one year — even as the overall trend is downward.

Couldn't something like that be happening this year, where local trends — maybe even some sort of Ferguson effect — are temporarily pushing the numbers up but the overall trend downward continues?

HM: Well, it may continue going back on a downward trend if officers feel like they're backed up politically again. But the rise in homicides has been so striking in so many cities — and against what the trends have been for years nationally — that I think there needs to be an explanation.

FiveThirtyEight is showing a 16 percent increase in 60 US cities. That is a very significant increase — again, one that comes after erasing what would have been a merely 5 percent drop in violent crime in the first six months of 2014. So you're looking at more likely a 20 percent increase. I'm not aware of that kind of national homicide increase or violent crime increase in one year.

GL: You talk about the Ferguson effect and policing's effects on crime. But if we look at the past few years of the drop in crime, most of the research shows that it's just not police that pushed crime down, but other explanations, even something like lead abatement. Don't you think other factors besides policing could be playing a role in the crime increase and the previous crime drop as well?

HM: I'm persuaded of Franklin Zimring's analysis of the New York crime drop that led the country in 1994, when crime dropped 12 percent in New York City when it was 1 percent in the rest of the country, and the next year it was down 16 percent when it was flat in the rest of the country.

The type of data-driven policing, with accountability imposed on police commanders and a fanatical attention to crime patterns breaking out before they really became entrenched, spread across the country. Zimring has shown that nothing else in New York City — the usual market basket of root cause of crimes — changed. The only thing that changed was the policing techniques that explained a crime drop in New York that was twice as long and twice as steep as the national average.

That technique that set New York apart spread across the rest of the country, albeit without the manpower to back it up and the same kind of cutthroat department ethos that was extraordinarily pressure-inducing on commanders in New York.

To me, I don't see other factors changing to the same extent as policing did. I would throw in there, as well, the incarceration buildup, which I think did also contribute to crime going down.

GL: That may be true for New York, but the rest of the country seems different. The Brennan Center report released earlier this year looked at all these theories for why crime dropped, and I talked to criminologists about that report and other theories it may have missed. They have all sorts of ideas for why crime dropped that go beyond policing, including lead abatement and technology and video games keeping people indoors.

Couldn't these other trends explain the crime drop as well? And, similarly, couldn't some other factors that we haven't even accounted for or just don't know yet similarly explain any homicide rise this year?

HM: Is lead abatement unabated this year? Are there fewer video games keeping people indoors?

GL: I'm not suggesting that. But just like we're not sure why crime plummeted across the country for years, perhaps there are other factors that we just don't know about yet that may explain why homicides are rising now.

HM: Well, you know, I certainly am not placed to deny the possibility of an explanation that is more powerful than policing and incarceration. But I am not persuaded to the same extent by the other arguments.

I think there's a plausibility to video games making people more self-engrossed and keeping them out of the streets, but that just does not explain what we saw in New York, where you had a revolution in policing that produced immediate results with nothing else changing. The youth demographics in New York didn't change, poverty rates didn't change, and conservative's favorite root cause — out-of-wedlock parenting — didn't improve. And yet the city had an extraordinary turnaround in its crime situation.

GL: And in your view, any reverse of that would explain why crime is now rising again?

HM: Yeah. I believe proactive policing works.

I also believe that that's what many members of communities want. I can't go to a policing meeting in South Central Los Angeles or Harlem where I don't hear people saying that they want the dealers off the streets, the kids off their stoops who are smoking weed. That's what the cops are hearing.

So when they are enforcing low-level misdemeanor laws and quality-of-life laws, they are responding to what the community is asking them to do.

GL: Still, there's something to Black Lives Matter. There are a lot of people who really feel that police are overly aggressive in their neighborhoods. How do people protest that if, as you claim, it demoralizes police officers and emboldens criminals?

HM: What's leading to this effect is not peaceful protest. It's the hostility and venom that is directed at officers now when they're trying to engage in lawful activity. When they're trying to make an arrest and they're routinely surrounded by people jeering at them, cursing at them, throwing things at them, that's the sort of thing that makes officers think twice about initiating an activity that is not being brought to their attention by virtue of a 911 call and virtue of an actual victim asking for assistance.

As often as I can, I say that officers need constant training, reinforcement, reminders that they have to treat everybody courteously and respectfully. There's no question that officers can develop hardened, obnoxious, condescending attitudes — and that needs to be softened all the time.

But let's be honest that part of the reason they develop that street hardness is because they are treated by some people in the community pretty poorly. Some of those people are criminals who resent police interfering in their livelihoods.

I'm not saying that those people in any way make up the bulk of the current protest movement, and I'm saying that officers do have to make sure they're behaving respectfully and appropriately as representatives of the law, but I think it's incredibly disingenuous to suggest that what's been going on in the past year and a half is peaceful protests as opposed to extraordinary hostility police — second-guessing of officers, sweeping claims of officers being racists and killer cops, and demands that officers back off of proactive policing.

I would be interested if you could explain to me the double standard of people who criticize the police for being too aggressive but also criticize police backing off that kind of policing. Aren't the officers doing exactly what the movement is asking them to do — cutting back on pedestrian stops, cutting back on overpolicing for what you say are petty crimes? Police are political. They are human beings. They are responding to the message they're getting.

GL: I don't think the worry is that police are no longer going after petty crimes, but that police aren't going after any crimes in general when they engage in a slowdown.

HM: Well, the cops are responding to 911 calls. If somebody's getting robbed, they're running to that robbery scene and sometimes getting shot themselves in the process. But what they're doing less of is self-initiated activity that does not already have a victim, but they're trying to prevent someone else from becoming a victim.

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