Most of this research—and there have been several dozen peer-reviewed studies—punctures the idea that guns stop violence. In a 2015 study using data from the FBI and the Centers for Disease Control and Prevention, for example, researchers at Boston Children’s Hospital and Harvard University reported that firearm assaults were 6.8 times more common in the states with the most guns versus those with the least. Also in 2015 a combined analysis of 15 different studies found that people who had access to firearms at home were nearly twice as likely to be murdered as people who did not.

This evidence has been slow to accumulate because of restrictions placed by Congress on one of the country’s biggest injury research funders, the CDC. Since the mid-1990s the agency has been effectively blocked from supporting gun violence research. And the NRA and many gun owners have emphasized a small handful of studies that point the other way.