The United States owns way, way more guns per capita than the rest of the world. And the best research on gun violence suggests that's probably contributing to our homicide problem — as exemplified by Wednesday's horrific shooting at Marjory Stoneman Douglas High School in Florida.
Here's a map of firearm ownership around the world, using 2012 data compiled by the Guardian. The United States has nearly twice as many guns per 100 people as the next closest country, Yemen — 88.8 guns per 100 as opposed to 54.8 in Yemen:
Now, gun ownership doesn't translate directly to more homicides. For instance, the United States has more than 12 times as many guns per person as Honduras, but the 2012 US gun homicide rate per 100,000 people (2.97) was 1/22 of Honduras's (68.43). That's because while guns make murder easier, internal instability or weak governance, or especially a recent history of internal conflict, can also contribute to this sort of violence.
But when you compare the United States with nations like Britain and Japan, it becomes clear that firearm ownership contributes to America's murder problem. The American firearm homicide rate is about 20 times the average among Organization for Economic Cooperation and Development countries (excluding Mexico).
Harvard researchers Daniel Hemenway and Matthew Miller examined 26 developed countries and checked whether gun ownership correlated with murder rates. They found that "a highly significant positive correlation between total homicide rates and both proxies for gun availability." They also didn't find much evidence that a higher rate of gun murders led to lower rates of other kinds of murder (i.e., stabbings).
Interestingly, these results tended to hold true even when you exclude the United States and its super-high homicide and gun-ownership rates. "More guns are associated with more homicides across industrialized countries," Hemenway and Miller conclude. Another study, by Berkeley's Franklin Zimring and Gordon Hawkins, found that the US has crime rates comparable to those in similarly developed countries, but much higher rates of lethal violence — owing in significant part to our high rates of gun ownership.
Data from inside the United States suggests the same thing.
A recent, highly sophisticated study found that once you control for general crime rates and other confounding factors like poverty, "each 1 percentage point increase in proportion of household gun ownership" translated to a 0.9 percent increase in homicides. A meta-analysis — study of studies — found a strong consensus among researchers that access to guns correlated with higher homicide rates in the United States.
So it seems very, very likely that the US's exceptional rate of firearm ownership is contributing to its exceptionally high murder rate. That's one reason why, despite the fact that the homicide rate is falling sharply both inside the United States and around the world, America has a much bigger homicide problem than the rest of the developed world.