Researchers who've studied the effect of state "stand your ground" laws have found that states with such statutes have more homicides than states without them, NPR reports. "These laws lower the cost of using lethal force," says Mark Hoekstra, an economist with Texas A&M University who examined stand your ground laws. "Our study finds that, as a result, you get more of it."

Advocates for the laws, like Republican state Rep. Dennis Baxley, who sponsored Florida's version, says such legislation allows good people to defend themselves. Marion Hammer, a former president of the National Rifle Association, said the laws have been effective and are working exactly as designed. A committee analyzing the Florida statute has found no increase in violence as a result of the law. Because murder is a rare phenomenon, the numbers in any given state can be hard to analyze. It can be difficult, for example, to disentangle the effects of stand your ground statutes from other trends, such as natural fluctuations in the crime rate. Until now, there has been little attempt to study these laws rigorously at a national level.