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Every time there's a mass shooting, the question comes up: how did the shooter get the gun? Are better gun control policies needed?

But there's a shadow death toll from guns that is usually left out of these discussions -- one that's even bigger than homicides. For years, evidence has mounted that the biggest threat from guns isn't people using them to kill others, but those who turn guns on themselves. There were 16,000 homicides in 2013 and at the same time more than 40,000 suicides, half of which were committed with a gun, according to the Centers for Disease Control and Prevention.

A new study from researchers at the Johns Hopkins Bloomberg School of Public Health suggests that the discussion about the effect of handgun policies shouldn't just be about public safety, but also about public health. By tracking suicide rates in two states that changed their policies -- one that made handguns easier to obtain and one that made it harder -- they were able to see that these policies may be more far-reaching than most people think.

"I think it’s been difficult for suicide to get inserted into this conversation," said Daniel Webster, who directs the Johns Hopkins Center for Gun Policy and Research. "When we think about what people want from government policy, typically it’s not policies to keep us from harming ourselves. We expect government to do things to protect us from others. But I think that so many people are affected by suicide of a loved one, and that they should recognize that these types of policies that are designed maybe with the principal intention to reduce criminal violence also protect loved ones."

Published in the journal Preventive Medicine, Webster's study examined changes in two states' policies, suicide data, and a model to estimate what the suicide rate would have been in both states if the policy hadn't been implemented.

Missouri, which began requiring permits to purchase handguns starting in 1921, repealed the law in 2007. Webster and colleagues estimated a 16 percent increase in suicides with a firearm. Other studies have also found that repeal was followed by an increase in homicides with guns and in the number of guns diverted to criminals.

Connecticut went in the other direction. In 1995, the state implemented a mandatory requirement for a permit to purchase a handgun after buyers passed a background check. The suicide rate by firearm decreased by 15.4 percent, the researchers estimated.

The researchers argue that background checks will often flag risk factors for suicide, such as previous offenses involving drugs and alcohol, or a history of violent crimes. The impulse to commit suicide often does not last long, meaning that if a person doesn't have access to an easy way to commit the act in a brief period, they may not do it. But the research can't rule out that other factors played a role in the change in firearm suicide rates.

The question remains whether people will simply use some other means; maybe taking away guns doesn't actually affect the problem. The researchers found no change in suicide rates by non-firearm methods in Missouri -- a sign that there might be a causal connection between the policy and the surge in suicides by gun. But the evidence from Connecticut was less clear.

"This leaves us wondering, or uncertain that at least some of the decline observed in firearm suicides may have to do with factors that affect suicide more generally, not specific to firearms," Webster said.