A high-profile shooting, like the June 17 crime that left dead nine members of a historically black church in Charleston, South Carolina, is typically followed by calls for greater gun control, along with counter arguments that the best way to stop gun crimes is with more guns.

"The one thing that would have at least ameliorated the horrible situation in Charleston would have been that if somebody in that prayer meeting had a conceal carry or there had been either an off-duty policeman or an on-duty policeman, somebody with the legal authority to carry a firearm and could have stopped the shooter," presidential candidate Mike Huckabee said in a Fox News interview on June 19.

A new study, however, throws cold water on the idea that a well-armed populace deters criminals or prevents murders. Instead, higher ownership of guns in a state is linked to more firearm robberies, more firearm assaults and more homicide in general. [5 Milestones in Gun Control History]

"We found no support for the hypothesis that owning more guns leads to a drop or a reduction in violent crime," said study researcher Michael Monuteaux, an epidemiologist and professor of pediatrics at Harvard Medical School. "Instead, we found the opposite."

More guns, more gun crime

Numerous studies have found that gun ownership correlates with gun homicide, and homicide by gun is the most common type of homicide in the United States. In 2013, for example, there were 16,121 total homicides in the United States, according to the Centers for Disease Control and Prevention (CDC), and 11,208 of those were carried out with a firearm. (Gun suicides outpace gun homicides by far; in 2013, the CDC recorded 21,175 suicides by firearm, about half of all suicides that year. Contrary to popular belief, suicide is typically an impulsive act, psychiatrists say. Ninety percent of people who attempt suicide once will not go on to complete a suicide later, but a suicide attempt using a gun is far more lethal than other methods.)

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Monuteaux and his colleagues wanted to test whether increased gun ownership had any effect on gun homicides, overall homicides and violent gun crimes. They chose firearm robbery and assault, because those crimes are likely to be reported and recorded in the Federal Bureau of Investigation (FBI) Uniform Crime Report.

Along with that FBI data, the researchers gathered gun ownership rates from surveys in the CDC's Behavioral Risk Factor Surveillance System, an ongoing, nationally representative survey in which participants answered questions about gun ownership in 2001, 2002 and 2004. Using those years and controlling for a slate of demographic factors, from median household income, population density, to age, race and more, the researchers compared crime rates and gun ownership levels state by state.

They found no evidence that states with more households with guns led to timid criminals. In fact, firearm assaults were 6.8 times more common in states with the most guns versus states with the least. Firearm robbery increased with every increase in gun ownership except in the very highest quintile of gun-owning states (the difference in that cluster was not statistically significant). Firearm homicide was 2.8 times more common in states with the most guns versus states with the least. [Private Gun Ownership in the US (Infographic)]

The researchers were able to test whether criminals were simply trading out other weapons for guns, at least in the case of homicide. They weren't. Overall homicide rates were just over 2 times higher in the most gun-owning states, meaning that gun ownership correlated with higher rates of all homicides, not just homicide with a gun. The results will be published in a forthcoming issue of the American Journal of Preventive Medicine.

Pinpointing causation

The results do need to be interpreted with caution — this study method proves that more guns are linked to more gun crime and overall homicide, but not that access to guns directly causes this criminal uptick, said study researcher David Hemenway, the director of the Harvard Injury Control Research Center.

"This study suggests that it's really hard to find evidence that where there are more guns, there are less crimes, but you can easily find evidence that where there are a lot more guns, there are a lot more gun crimes," Hemenway told Live Science.

It's possible that people stockpile guns in response to higher levels of crime. The researchers tried to tease out whether this was the case by testing whether gun ownership levels were a prerequisite for crime or a response to higher crime levels. Though they still couldn't prove causation, they did find that higher gun ownership levels preceded crime increases, not the other way around.

"It's difficult to imagine how the hypothesis that increased ownership reduces criminal behavior could be valid, given our findings," Monuteaux said.

Other researchers have tried to explore this question in different ways. Boston University researcher Michael Siegel and colleagues found in a 2013 study published in the American Journal of Public Health that over 30 years, gun ownership levels correlated with firearm homicides, such that the higher the gun ownership rate, the higher the firearm homicide rate.

However, Siegel said, it was possible that when people noticed the gun homicide rate going up around them, they went out to purchase guns for protection. To see if the idea held water, the researchers repeated the study, but differentiated between the stranger firearm homicide rate and the nonstranger firearm homicide rate.

They found something striking. Firearm ownership was not related to the number of stranger firearm homicides — cases where someone is killed by a stranger.

But when more people owned guns, the nonstranger firearm homicide rate rose — cases where someone is killed by someone they know.

"It wouldn't make sense to argue that people only go out to buy guns if the nonstranger homicide rate goes up, but not if the stranger homicide rate goes up," Siegel told Live Science. The data, he said, points to a picture in which confrontations between families, friends, bosses and acquaintances become lethal in the presence of guns.

"The types of fatalities that occur with nonstrangers are often situations where the presence of a gun makes all the difference in the world," Siegel said. "Having guns available makes the difference between having a fatal confrontation and a nonfatal confrontation."

Lingering questions

Despite the political firestorm over firearms, some questions about guns are settled science, Hemenway said. He's made a side project of surveying active firearm researchers on the literature in an attempt to learn what areas of research have reached a consensus, and which remain open.

What's known? One, the presence of a gun in the home increases the risk of suicide in that home. "That relationship we really know, no doubt about it," Hemenway said.

Second, the research also confirms that more access to guns means more firearm homicides, Siegel added. Research on whether other weapons replace guns when guns are unavailable suggests that they do not: Overall homicide rates, not only gun homicides, creep up when guns are in the picture. A 2014 study published in the journal Injury Prevention, for example, found a 0.7 percent increase in overall homicides for every 1 percent increase in household gun ownership. [Fight, Fight, Fight: The History of Human Aggression]

The devil, however, is in the details, which often remain unexamined.

"We know so little about gun training, we know so little about gun theft, we know some about self-defensive gun use but not really much," Hemenway said. He and his colleagues are working on studies about accidental gun deaths in children, about who kills police and whom police kill, and they'd like to research gun deaths in the elderly and gun intimidation events, in which a person brandishes a gun to scare another.

Also unclear are what policies work best to lower the number of firearms available, Siegel said. He and his colleagues are tackling that question now.

Another recent study highlighted just how little researchers know. In July 2013, researchers published a paper in the open-access journal PLOS ONE, attempting to mathematically model the trade-off between increased gun crimes with gun ownership and gun use for self-protection. Because the available data isn't comprehensive enough, the researchers weren't able to make specific policy recommendations, study researcher Dominik Wodarz of the University of California, Irvine, told Live Science.

"What this really does, this model, is it identifies what parameters are important, which should be measured," Wodarz said. The hope is to motivate future studies on factors like how many people own guns legally versus illegally, how likely someone is to die if there is a shooting, and how many people carry their guns around on a regular basis.

"The model essentially said that reducing the amount of guns would be beneficial with the data we have, but this is not something that we say should inform policy," he said.

How — or if — gun research will inform policy remains an open question. After federally funded research in the 1980s and 1990s began to reach a consensus that firearms in the home were linked to higher chances of violent death in the home, the National Rifle Association (NRA) lobbied successfully for an end to federal funding of firearms research. The prohibition had a chilling effect on the field. After the elementary school shooting in Newtown, Connecticut, in 2013, President Obama issued an executive order lifting the ban on funding gun research, but little has changed in the two years since that order, scientists in the field say. Congress has to earmark the money for such research, and has not made that cash available to the CDC. The National Institute of Justice and National Institutes of Health have limited funding for gun research, but there is very little federal money available, Hemenway said.

Nor do decision makers necessarily care about science-based policy: Hemenway recalls presenting his research to a group of congressional representatives and having one declare that he didn't care what the data had to say.

"One of the bad things the gun lobby has done is they've said, 'it's us or them, and you've got to choose sides,'" Hemenway said. "That makes it so people choose sides, and then they look for confirmatory data instead of trying to see what the world is really like."

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