"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."

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.

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.

The study is at http://www.ajpmonline.org/article/S0749-3797(15)00072-0/abstract