It’s been a horrible few months for mass shootings.

There’s the Thousand Oaks, California, bar shooting on Wednesday. There was the Tallahassee, Florida, yoga studio shooting last week. There was the Pittsburgh synagogue shooting late last month. There was the Florence County, South Carolina, shooting in which police officers were shot in October. There was the downtown Cincinnati shooting in September.

There were also a bunch of mass shootings that didn’t get much attention. The Gun Violence Archive considers events in which four or more people, excluding the shooter, were shot (but not always killed) in the same time period and place as mass shootings. By this count, there have been 311 mass shootings in 2018, resulting in 314 killed and 1,270 wounded. That’s almost a mass shooting a day.

After these horrific events, politicians and pundits will latch onto all sorts of explanations for why these keep happening: It’s mental illness. It’s misogyny. It’s anti-Semitism. It’s some other form of extremism or hate.

In individual shootings, these all of course can play a role. But when you want to explain why America sees so many of these mass shootings in general, none of these factors in individual shootings give a satisfying answer. Only one thing does: America’s easy access to guns.

America does not have a monopoly on mental health issues, bigots, or extremists. What is unique about the US is that it makes it so easy for people with these issues to obtain a gun.

America’s gun problem

It comes down to two basic problems.

First, America has uniquely weak gun laws. Other developed nations at the very least require one or more background checks and almost always something more rigorous beyond that to get a gun, from specific training courses to rules for locking up firearms to more arduous licensing requirements to specific justifications, besides self-defense, for owning a gun.

In the US, even a background check isn’t a total requirement; the current federal law is riddled with loopholes and snared by poor enforcement, so there are many ways around even a basic background check. There are simply very few barriers, if any, to getting a gun in the US.

Second, the US has a ton of guns. It has far more than not just other developed nations, but any other country period. Estimated for 2017, the number of civilian-owned firearms in the US was 120.5 guns per 100 residents, meaning there were more firearms than people. The world’s second-ranked country was Yemen, a quasi-failed state torn by civil war, where there were 52.8 guns per 100 residents, according to an analysis from the Small Arms Survey.

Both of these factors come together to make it uniquely easy for someone with any violent intent to find a firearm, allowing them to carry out a horrific shooting.

This is borne out in the statistics. The US has nearly six times the gun homicide rate of Canada, more than seven times that of Sweden, and nearly 16 times that of Germany, according to United Nations data for 2012 compiled by the Guardian. (These gun deaths are a big reason America has a much higher overall homicide rate, which includes non-gun deaths, than other developed nations.)

The research, compiled by the Harvard School of Public Health’s Injury Control Research Center, is also pretty clear: After controlling for variables such as socioeconomic factors and other crime, places with more guns have more gun deaths. Researchers have found this to be true not just with homicides, but also with suicides (which in recent years were around 60 percent of US gun deaths), domestic violence, and violence against police.

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.

Researchers have found that stricter gun laws could help. 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 guns can save lives. A review of the US evidence by RAND also linked some gun control measures, including background checks, to reduced injuries and deaths.

That doesn’t mean that bigots and extremists will never be able to carry out a shooting in places with strict gun laws. Even the strictest gun laws can’t prevent every shooting.

And guns are not the only contributor to violence. Other factors include, for example, poverty, urbanization, alcohol consumption, and the strength of criminal justice systems. But when researchers control for other confounding variables, they have found time and time again that America’s loose access to guns is a major reason the US is so much worse in terms of gun violence than its developed peers.

So America, with its lax laws and abundance of firearms, makes it uniquely easy for people to commit massacres. Until the US confronts that issue, it will continue seeing more gun deaths than the rest of the developed world.

For more on America’s gun problem, read Vox’s explainer.