If it seemed like 2018 was a particularly bad year for gun violence in schools, that’s because it was.

According to data from the US Naval Postgraduate School, there were 94 school gun violence incidents this year — a record high since 1970, which is as far back as the data goes, and 59 percent higher than the previous record of 59 in 2006.

The database counts “each and every instance a gun is brandished, is fired, or a bullet hits school property for any reason, regardless of the number of victims, time of day, or day of week,” according to the project. The project pulls from a variety of sources, including media reports and government agencies, to collect its data.

Separately, the project also tracks deaths. By this metric, 2018 was also the worst year on record. So far, 55 people, including the shooter, were killed in school gun violence. The second-worst annual death toll was 40 in 1993. (To put these numbers in context, there were nearly 39,000 gun deaths, including homicides and suicides, in the US in 2016.)

The great majority of incidents tracked involved handguns: 911 involved one or more handguns, 76 one or more rifles, 47 one shotgun, 33 a combination of weapons, and 230 unknown.

Other data backs up that 2018 was particularly bad for school gun violence. The database from Everytown, a gun control advocacy group, found 86 incidents so far this year in which “a firearm discharges a live round inside or into a school building or on or onto a school campus or grounds, as documented by the press.” That’s the highest number of incidents going back to 2013, when Everytown began tracking the data.

School shootings this year gave way to a new movement for stricter gun laws, with the March for Our Lives taking off after a gunman in Parkland, Florida, in February killed 17 people at Marjory Stoneman Douglas High School.

But it doesn’t seem that 2018 was a particularly abnormal year for mass shootings, regardless of whether they happened on school grounds. The Gun Violence Archive’s data, visualized by Vox in map form, indicates that there have been 328 mass shootings so far in 2018, or nearly one a day, resulting in 365 killed and 1,301 wounded. That’s roughly in line with recent years going back to 2015, which have averaged about one mass shooting a day. (The Gun Violence Archive defines mass shootings as any incident in which four or more people were shot but not necessarily killed, excluding the shooter, in a similar time or place, which differs from some other groups’ definitions.)

Meanwhile, gun homicides as a whole have trended down over the decades as crime has steadily dropped in the US — by roughly half since the 1990s.

It’s not clear why 2018 appeared to be a particularly bad year for gun violence in schools.

But at the international level, America has long been an outlier among developed nations when it comes to gun violence — and the experts and research suggest that the US’s abundance of firearms and extremely lax firearm laws have helped create its gun problem.

America’s gun problem, explained

America’s gun problem can be broken down into two parts.

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. (Although some states, like Massachusetts, have worked to change that.)

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.

Similarly, every country in the world has bigots, extremists, and disturbed people who may want to carry out mass atrocities. But in the US, it’s uniquely easy to do so because so many guns are readily available.

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, extremists, disturbed people, or anyone else 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.

There are also some evidence-based policies that could help outside the realm of gun control, including more stringent regulations and taxes on alcohol, changes in policing, and behavioral intervention programs.

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.