The common defense to criticisms of racial disparities in police use of force takes the form of a question: "But what about black-on-black crime?" The conservative argument is that black people are disproportionately killed by gun violence in their own communities, so perhaps they should worry about that before they worry about what police are doing.

On Tuesday night, Fox News's Mike Tobin put the black-on-black crime question to a protester in Chicago, Brendan Glover, after the release of a video showing the police shooting of Laquan McDonald. Here's how it played out, as shown in the video above posted by Media Matters:

Mike Tobin: Essentially the question he's getting to is why is there not outrage when there's black on black violence. Brendan Glover: Because crime is going to happen. Wherever you go, crime happens. And the people that are here to serve and protect are a part of that problem. So it becomes a problem when you can't even call 911 and feel like you're safe — to protect you from people that are committing crimes. That's the point I'm trying to make.

Glover essentially argued that while crime can be shocking (earlier he and Tobin pointed to the murder of a 9-year-old as one example of a horrible local incident), it's also disturbing that black communities are genuinely scared of calling for help when that crime happens, because they're by and large dealing with police forces that disproportionately shoot and kill black Americans. In other words, everyone expects violent criminals to exist, and that's obviously tragic and dangerous — but at the very least people shouldn't have to believe that their government-sanctioned public protectors are also dangerous.

But it's not just that police brutality makes people feel less safe. This distrust actually makes it harder to really address black-on-black violence.

The criminal justice system both underpolices and overpolices black communities

As journalist Jill Leovy argued in her recent book Ghettoside, the abhorrent levels of black-on-black homicide and the police harassment of black communities are actually two sides of the same coin:

Like the schoolyard bully, our criminal justice system harasses people on small pretexts but is exposed as a coward before murder. It hauls masses of black men through its machinery but fails to protect them from bodily injury and death. It is at once oppressive and inadequate.

Under this argument, black Americans are exposed to both neglect and overpolicing by the criminal justice system. The question isn't whether police use of force in black communities or black-on-black crime is a bigger problem, but rather how we got to a point in the criminal justice system in which both are problems.

On one hand, the criminal justice system is highly active in black communities but typically with a focus on petty offenses. Stop and frisk in major US cities targets drugs and other nonviolent crimes. The brutal arrest and death of Eric Garner, a black man in New York City, came about after he allegedly sold untaxed cigarettes. Courts and police in Ferguson, Missouri, focused on low-level crimes to raise revenue through fines and court fees from black residents.

On the other hand, black communities are typically neglected when they're hit by a terrible crime such as murder. In her book, Leovy outlines the inadequate resources Los Angeles relies on to solve black-on-black murders — the elite homicide unit, the robbery-homicide division, typically focuses on celebrity cases, massacres, and arson murders but rarely pays any attention to black-on-black violence, leaving it instead to understaffed local divisions. This is typical across the US: Investigations have found, for instance, that black homicides are less likely to be solved than those that involve white victims.

"Police can't do the job on their own — they need the community to help them"

What's worse, these issues work together to make both problems worse. Leovy writes that a big hurdle to solving murders in crime-ridden areas is that witnesses aren't willing to cooperate with police — they're too scared, or they just don't trust cops.

As Charis Kubrin, a criminologist at the University of California Irvine, previously told me, "People don't feel like they can go to the police even if they've witnessed crimes, because they don't trust the police, and there's antagonism there. And police can't do the job on their own — they need the community to help them."

So when police are harassing predominantly black neighborhoods for petty offenses, they're making it less likely that the residents of that same neighborhood cooperate with police in future, more serious cases. And the unsolved murders can lead to more murders: People are more likely to take matters into their own hands — and resort to violence — if they no longer believe that the criminal justice system will protect them, as Leovy details in her book.

It's not just that many people in black communities feel like they can't trust police, then, but that the distrust actually feeds into more violence within those communities.

Watch: Why it's important to film the police