What explains America's extraordinary levels of gun violence among developed nations? For one, the country has an extraordinary amount of guns.

America has 4.4 percent of the world's population, but almost half of the civilian-owned guns around the world — enough to arm every adult in the country:

Whether at the state or country level, reviews of the studies by the Harvard School of Public Health's Injury Control Center have consistently found that places with more guns have more deaths after controlling for variables like socioeconomic factors and other crime. "Within the United States," David Hemenway, the Injury Control Research Center's director, wrote in Private Guns, Public Health, "a wide array of empirical evidence indicates that more guns in a community leads to more homicide."