Researchers controlled for different variables, including property crime rates, the percentage of people ages 15 to 29, racial demographics, percent of people in urban areas, and poverty rates.
The federal data the study relied on had some limitations: The gun ownership data was limited to certain years, and it may have underreported police deaths. But the analysis, researchers said, passed several sensitivity tests that were fully posted online.
The results aren't really surprising. Previous research found that places with more guns and more access to guns — barring other variables that can influence crime — tend to have more homicides.
For example, 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.
Guns are not the only factor that contribute to violence. (Other factors include, for example, poverty, urbanization, and alcohol consumption.) But when researchers control for other confounding variables, they have found time and time again that America's high levels of gun ownership are a major reason the US is so much worse in terms of gun violence than its developed peers.
"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," UC Berkeley's Franklin Zimring and Gordon Hawkins wrote in a breakthrough analysis in 1999. "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."
So the study on guns and police simply follows up on this established link between more guns and more gun deaths — clarifying that more guns mean more police officer deaths, too.
Perhaps some Americans think that guns should still remain easily accessible, because the right to bear arms is too important to limit or lose. But an increasing amount of research shows that this comes with a grim downside: more lives lost.
Commentary by German Lopez, a writer at Vox.com.
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