In the US, researching the interaction between regulations and gun violence is incredibly challenging. A patchwork of state and local regulations complicates finding populations that can be used to track any changes in response to new legislation. And, since 1996, federal funding for the research has been nearly nonexistent.

Nevertheless, the researchers that have found money to study the subject sometimes do get a bit of a break. One of those occurred in 2007, when the state of Missouri repealed regulations on the purchase of guns that had been in place since 1921. Daniel Webster of the Johns Hopkins School of Public Health described the consequences of this change at the meeting of the American Association for the Advancement of Science.

The US limits on gun research date back to a 1993 study funded by the CDC, which found that guns in the home were associated with an increased risk for homicide. After an attempt to eliminate the CDC's National Center for Injury Prevention failed, Congress simply eliminated all the money that had been used to fund gun research there and directed that “none of the funds made available for injury prevention and control at the Centers for Disease Control and Prevention may be used to advocate or promote gun control." Those prohibitions remain in place.

Even were money for gun research easy to obtain, it's difficult to do well. Each state has its own set of firearm regulations, and historic, cultural, and demographic differences among the states make interstate comparisons treacherous. Rates of violence tend to vary from year-to-year as well, often for reasons that are independent of access to weaponry.

As a result, we have an extremely limited picture of how gun regulations influence violence in the US. That's why the events in Missouri provide a rare opportunity. The population stayed largely the same as the regulations changed, and the change came at a time when the national murder rate was dropping at a steady pace.

Since the 1920s, Missouri had been operating under a permit-to-purchase system, where would-be gun buyers would have to see local law enforcement for a background check and general vetting. If a person passed the check, they'd be given a permit that allowed them to buy guns. In 2007, that law was changed so that any required background checks were performed at the time of purchase, and buyers would be approved immediately after completion. It passed as part of a package that included stand-your-ground legislation.

As the national murder rate continued to trend downward, Missouri's held steady in the wake of the changes. Webster also said that the age adjusted gun homicide rate in Missouri went up by 25 percent. In eight other states, there were no significant changes in this figure and, in aggregate, the rate in these states went down by 2.2 percent. Changes in murders committed without firearms were not statistically significant.

Another change came in the dynamic of the guns seized after crimes. Prior to the change in the law, most of the guns recovered at crimes had been in circulation for a while; after, there were many more that had been in circulation for less than a year. The number of guns seized just three months after purchase doubled.

Webster showed polling results that indicate that the vast majority of Americans, including gun owners and NRA members, are in favor of background checks for people interested in purchasing guns. But his data suggests that how the checks are handled can make a big difference in what ultimately happens with the weapons.