When Sacramento, California, police officers on March 18 confronted Stephon Clark in his grandmother’s backyard, they appeared to believe that he was holding a gun. In the dark of night, they opened fire — shooting 20 rounds and hitting the 22-year-old eight times, mostly from the back, according to an autopsy commissioned by the family.

It turned out, though, that the officers had made a huge mistake: What they thought was a firearm was actually a cellphone.

This kind of situation isn’t new. Officers have shot people after mistaking wrenches and badges for guns. Cops have shot people thinking that they’re reaching for a firearm when they’re really pulling up loose-fitting shorts. Police have shot multiple people thinking that a toy gun was a real firearm.

Behind all these incidents lies what seems to be a constant fear that a gun may be present.

According to criminal justice and policing experts, police have good reason to be fearful. The US has a tremendous amount of civilian-owned guns — far more than any other country in the world. Based on recent estimates, there are more firearms in America than there are people. That presents a constant potential threat to police.

“Police officers in the United States in reality need to be conscious of and are trained to be conscious of the fact that literally every single person they come in contact with may be carrying a concealed firearm,” David Kennedy, a criminologist at John Jay College, told me. “That’s true for a 911 call. It’s true for a barking dog call. It’s true for a domestic violence incident. It’s true for a traffic stop. It’s true for everything.”

This is one potential reason, experts said, that the US has far more police shootings than other developed nations. A 2015 analysis by the Guardian found that “US police kill more in days than other countries do in years.” Between 1990 and 2014, police in England and Wales shot and killed 55 people.

In just the first 24 days of 2015, the US surpassed that toll of fatal police shootings. The differences are not explained by population, since the US is nearly six times as populous as England and Wales, but, based on the Guardian’s count, has hundreds of times the fatal police shootings.

So far, however, there hasn’t been much empirical research on the question of whether more guns in America lead to more police shootings — in large part because the US has long done a poor job tracking killings by cops, making them difficult to study.

Vox and John Roman, a senior fellow at NORC at the University of Chicago, decided to put this to the test. Our results are suggestive: They indicate that weaker gun laws and higher rates of gun ownership do, at the very least, correlate with more killings by police officers (including shootings and other incidents of lethal force).

That suggests that while America has to address a whole host of issues to bring down its levels of police killings — from department-level policies to systemic racism — it also may be prudent to start thinking of police killings as inherently linked to America’s gun problem in general.

Police killings correlate with state gun ownership rates and gun control laws

For this analysis, Roman and I analyzed data for police killings to see if there was any correlation with gun control laws and gun ownership rates across states.

Roman combed through the police killing data from the Police Violence Report, verifying it with other reports and the Washington Post’s database, to get an idea of how many police killings there were in 2017. We then compared police killing rates for each state with the state’s population (based on Census data), a composite score for state’s gun control laws (based on a National Rifle Association database), and gun ownership rates (based on a 2013 national survey).

The results: There is a correlation between killings by police officers and states’ gun control laws and gun ownership rates. The stronger the gun control laws, the fewer police killings. The higher the gun ownership rates, the more police killings. (You can see the raw data here and the comparison data here.)

This is a crude analysis, not a peer-reviewed study.

So caveats apply: Correlation is not causation. The NRA score doesn’t differentiate between the strength of different gun control laws, instead assigning one point to each of the laws included in the database. Also, the survey used for state-level gun ownership may not have picked up a large enough sample size for some low-population states, such as Hawaii. And the analysis didn’t deploy any rigorous controls to rule out other possible factors.

But the findings are echoed by a 2017 study, published in the American Journal of Public Health, that found that stronger gun laws are associated with fewer fatal police shootings. “When we controlled for sociodemographic factors, states in the top quartile of legislative strength had a 51% lower incidence rate than did states in the lowest quartile. Laws aimed at strengthening background checks, promoting safe storage, and reducing gun trafficking were associated with fewer fatal police shootings,” the researchers concluded in the study.

“One of the most reasonable take-homes is that … when police are less often in situations in which they might reasonably fear that a gun is going to be pulled, they’re less likely to react with the use of lethal force,” Aaron Kivisto, the study’s lead author, told me.

This would not only apply to cases like Clark’s, where police turned out to be wrong, but also to justified police shootings in which the perpetrator really did pose a deadly threat to cops or others. By mitigating the chances anyone has a gun, stricter gun laws may make all sorts of dangerous encounters less likely — and that could, based on Kivisto’s study, save a lot of lives.

So while guns don’t explain the whole story, killings by police officers do appear to be associated with gun laws and gun ownership rates. At the very least, this topic warrants more thorough empirical work.

The findings align with the claims of criminal justice experts, who have told me that one reason the US’s criminal justice system is so punitive — including through police shootings — is in part a result of genuinely higher levels of deadly gun violence.

US police have to constantly fear guns

As a breakthrough analysis by UC Berkeley’s Franklin Zimring and Gordon Hawkins in the 1990s found, it’s not even that the US has more crime than other developed countries. This chart, based on data from Jeffrey Swanson at Duke University, shows that the US is not an outlier when it comes to overall crime:

Instead, the US appears to have more lethal violence — and that’s driven in large part by the prevalence of guns.

“A series of specific comparisons of the death rates from property crime and assault in New York City and London show how enormous differences in death risk can be explained even while general patterns are similar,” Zimring and Hawkins wrote. “A preference for crimes of personal force and the willingness and ability to use guns in robbery make similar levels of property crime 54 times as deadly in New York City as in London.”

This is in many ways intuitive: People of every country get into arguments and fights with friends, family, and peers. But in the US, it’s much more likely that someone will get angry at an argument and be able to pull out a gun and kill someone.

This is something that police in the US have to constantly fear. In day-to-day work that is often inherently confrontational, it’s a lot easier in the US for those confrontations to turn deadly.

“It’s an obvious, common-sense observation,” Tracey Meares, a policing expert at Yale Law School, told me. “In situations where police officers say, ‘I was in fear for my life,’ and later substantiate that with, ‘I thought the person had a gun,’ the reasonableness of someone’s assumption that somebody could possibly have a gun is naturally related to the prevalence of guns in the environment.”

Kennedy of John Jay College called it “a very, very powerful dynamic” in America. For his policing research, Kennedy has traveled all around the developed world, from the US to Canada to Europe to Japan. “In none of these other settings do police officers feel this way or do the police have these sorts of conversations and concerns,” he told me.

That’s not to say that people in other countries are always unarmed. In some places, like Glasgow, Scotland, they’ve had big problems historically with knife crime. But the reality is non-firearms are simply much less likely to carry a deadly threat. (Indeed, Meares pointed out that UK police are often trained to disarm people armed with knives through communication, their bare hands, and nonlethal weapons — something that is simply much more difficult with guns, given the much higher risks.)

So for US police, the prevalence of guns creates a unique problem. In short, Kennedy said, “it affects every single contact they have with members of the public.” This is not just a matter of perception, he added, but “an objective fact” of being a police officer in the US. And it “changes how people behave” — not just police, but civilians who also, particularly in historically neglected neighborhoods, have to be always cognizant of the heightened risk of gun violence.

There is research that suggests police should be wary of the abundance of firearms out there. A 2015 study, published in the American Journal of Public Health, concluded that every 10 percent increase in firearm ownership correlated with 10 additional officers killed in homicides over the 15-year study period.

Other studies have also linked more access to guns to more gun deaths, including homicides, suicides, and domestic violence. For example, a 2013 study, also published in the American Journal of Public Health, found that, after controlling for multiple variables, each percentage point increase in gun ownership correlated with a roughly 0.9 percent rise in the firearm homicide rate.

Police officers have to constantly keep all of this in mind in their work. The end result is they are more guarded, more fearful, and, in some cases, perhaps more likely to shoot.

Guns are not the only explanation — but they are a part of it

As with the link between guns and gun deaths more broadly, gun ownership rates and gun control laws are not the only contributors to killings by police officers. Other research, for example, has linked police shootings to structural racism, which speaks to the vast racial disparities seen in police use of force. Policies at the department level — which can often encourage escalation instead of deescalation — play an important role as well. And laws that give police wide latitude to use force may be part of the issue too.

But experts and the limited research so far suggest that the abundance of guns and gun laws in America likely play a role. So stricter gun laws could help reduce killings by police officers.

We already know that restrictions on firearms lead to fewer deaths in other areas. A 2016 review of 130 studies in 10 countries, published in Epidemiologic Reviews, found that new legal restrictions on owning and purchasing guns tended to be followed by a drop in gun violence — a strong indicator that restricting access to firearms can save lives.

We don’t have similar definitive proof that there is a causal mechanism between stricter gun laws and fewer police shootings. But at the very least, the possibility should be a part of the broader discussion about the disproportionate amount of police violence in America.