Places with higher gun ownership rates also have higher firearms-related deaths, a new study finds.

In the study, published today (Sept. 20) in the American Journal of Medicine, researchers analyzed gun ownership rates, crime rates and deaths from firearms across 27 developed countries around the world.

"The U.S. has the highest gun ownership rates and also has the highest rate of firearm-related deaths," said study co-author Dr. Sripal Bangalore, a cardiologist at the New York University School of Medicine. [5 Biggest Gun Control Milestones in History]

Despite the prevalence of high-profile mass shootings, such as the recent Navy shipyard shooting, where the attacker showed signs of mental illness, the prevalence of mental illness in a society is only weakly correlated with gun-related deaths.

Contentious issue

Gun rights advocates argue that having more guns in society makes people safer by deterring crime and allowing law-abiding citizens to defend themselves against would-be attackers. Gun control proponents argue that guns lead to more violence, not less.

Past studies have shown that gun owners are much likelier to be shot with their own weapons than they are to use it to thwart a crime. Other research has shown that gun laws are linked with lower rates of firearms deaths.

But teasing apart causal factors can be difficult: After all, more people may feel compelled to purchase a gun in a crime-ridden neighborhood, but that doesn't mean the guns themselves cause the violence. And states that pass gun control measures may simply have different cultures than those that enshrine gun rights in their laws.

Cross-country analysis

After several high profile shootings, such as the Newtown, Conn., school shootings, Bangalore and his colleagues wanted to see whether guns actually make people safer, or whether inadequately treated mental health issues played a role.

The team looked at the fraction of people who owned guns across 27 developed nations, including the United States, Switzerland, Finland, Australia and Japan.

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Gun ownership was lowest in Japan and highest, by far, in the United States.

Gun ownership rates were strongly correlated with higher death rates from firearms.

In contrast, the incidence of major depression was only weakly linked to firearms-related deaths. (Data on other conditions such as schizophrenia were not widely available.)

And crime didn't seem to be correlated at all with gun ownership rates. That suggests purchasing a gun doesn't have an effect on overall crime rates, which include both violent and non-violent crimes.

"We can show that guns don't make a nation safer," Bangalore told LiveScience.

The study "provides some very convincing evidence that firearms-related deaths are very strongly correlated with prevalence of guns," said Dr. Eric Fleegler, a health services researcher at Boston Children's Hospital, who was not involved in the study.

Causation difficult

Still, it's difficult to say that gun ownership actually causes more gun violence.

The current study lumped together suicides and homicides.

And countries are very different.

"There are many factors that could influence both gun ownership and homicide rates," said Dr. Michael Siegel, a community health researcher at Boston University School of Public Health, who has separately found that states with more guns have higher murder rates.

One clue for causality comes from Australia, where tighter restrictions on gun ownership were instituted in 1996 and gun-related deaths fell dramatically, Bangalore said.

But the only way to untangle the links between gun ownership and violence is to do studies that track both over time, and that research has been limited because the Centers for Diseases Control and Prevention and the National Institutes of Health don't fund gun violence research, Siegel told LiveScience.

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