What we know about the Northern Arizona University shooting is tragic — but it's also an object lesson in how more guns leads to more shootings.

According to campus police, two groups of students were in a confrontation before 1:20 am at the parking lot of a residence hall in Northern Arizona University. At some point, 18-year-old freshman Steven Jones allegedly pulled out a gun and shot four of his peers. One is dead. Three are still being treated at the Flagstaff Medical Center, with at least some of them shot multiple times.

We don't know why the confrontation happened or how exactly it escalated into a tragic shooting. But what we do know is that it could only escalate into a shooting because someone had a gun.

In other developed nations, a student most likely wouldn't have a gun — they're less accessible and less abundant. So in another country, the confrontation would happen, and people might get hurt. But the chances of anyone getting killed or seriously injured would be greatly diminished — since it's likely no one would have a weapon as dangerous as a firearm in the first place.

The data backs this up. America has many more guns than other countries: According to a 2007 estimate, the number of civilian-owned firearms in the US was 88.8 guns per 100 people, meaning there was almost one privately owned gun per American and more than one per American adult. The world's second-ranked country was Yemen, a quasi-failed state torn by civil war, where there were 54.8 guns per 100 people.

America also has abhorrent levels of gun violence compared with other developed nations. The US has nearly six times the number of gun homicides as Canada, more than seven times as many as Sweden, and nearly 16 times as many as Germany, according to UN data 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.)

In fact, no other developed country comes close to the levels of gun violence, including suicides, that America has, as this chart from Tewksbury Lab shows:

These two issues — the numbers of guns and of gun deaths — are linked: Reviews of the empirical research by the Harvard School of Public Health's Injury Control Research Center have consistently found that when controlling for variables such as socioeconomic factors and other crime, places with more guns have more gun deaths.

The Arizona shooting is a clear example of what all this research shows: The shooting was only able to escalate because someone had a gun, which would be unlikely in other developed nations. And that means that if there's any hope America will prevent these types of tragedies in the future, and get levels of gun violence down to what other developed countries have, the US will at some point have to reduce the number of firearms — whether by restricting access to them or through a stricter policy like Australia's mandatory buyback program.

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