“This study provides compelling confirmation that weaknesses in firearm laws lead to deaths from gun violence,” Daniel Webster, director of the Johns Hopkins Center for Gun Policy and Research and the study’s lead author, said in a news release. “There is strong evidence to support the idea that the repeal of Missouri’s handgun purchaser licensing law contributed to dozens of additional murders in Missouri each year since the law was changed.”

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The spike in murders only held for those committed with a gun and was consistent throughout the state. Neither Missouri’s border states nor the nation as a whole saw similar increases.

Police in border states that kept such laws reported a big spike in guns bought in Missouri that had been diverted to criminals. In 2009, Missouri exported 136 guns to neighboring Illinois and 78 to neighboring Kansas, according to data collected by the Bureau of Alcohol, Tobacco, Firearms and Explosives and compiled by Mayors Against Illegal Guns.

What they looked at

The researchers analyzed state murder data from the the Federal Bureau of Investigation’s Uniform Crime Reporting system between 1999 and 2012, controlling for changes to other potential influencers such as policing, incarceration, poverty and other changes to state law. They also looked at annual death-certificate data compiled by the Centers for Disease Control and Prevention, which revealed the spike in the firearm homicide rate.

What it could mean

The researchers argue that universal background checks, such as those proposed by President Obama, and licensing requirements reduce the availability of guns to criminals.