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I'm Robert Siegel.

And among the big news stories of last year was the shooting of 17-year-old Trayvon Martin by George Zimmerman in Florida. That was back in February. And ever since, there's been heightened debate about Stand Your Ground Laws, which allow people to use deadly force to defend themselves. Social scientists are among those who've been studying the laws.

And NPR science correspondent Shankar Vedantam is going to tell us what they found.

SHANKAR VEDANTAM, BYLINE: If a stranger attacks you inside your own home, the law has always permitted you to defend yourself. On the other hand, if an altercation breaks out in public, the law requires you to try and retreat. At least, that's what it used to do.

In 2005, Florida became the first of nearly two dozen states to pass a Stand Your Ground law that removed the requirement to retreat. If you felt at risk of harm in a park or on the street, you could use lethal force to defend yourself.

Advocated for the law, like Representative Dennis Baxley, who sponsored Florida's law, said it allows good people to defend themselves.

STATE REPRESENTATIVE DENNIS BAXLEY: They're doing what they are supposed to do as a good citizen. They're stopping a violent act. And that's what I want the statute to do at the end of the day.

VEDANTAM: Marion Hammer, a former president of the National Rifle Association, told NPR 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.

Since murder is a rare phenomenon, the numbers in any given state can be hard to analyze. Until now, there has been little attempt to study these laws at a national level. At Texas A&M University, economist Mark Hoekstra recently decided to analyze national crime statistics to see what happens in states that pass Stand Your Ground laws. He found the laws are having an effect on the homicide rate.

MARK HOEKSTRA: Our study finds that homicides go up by seven to nine percent in states that pass the laws, relative to states that didn't pass the laws over the same time period. In contrast, we found no evidence of any deterrence effect over that same time period.

VEDANTAM: Hoekstra obtained this result by comparing the homicide rate in states before and after they passed the laws. He also compared states with the laws to states without the laws. Hoekstra said it was possible to quantify how many additional people were being killed as a result of Stand Your Ground laws.

HOEKSTRA: We find that there are 500 to 700 more homicides per year across the 23 states as a result of the laws.

VEDANTAM: That's out of about 14,000 homicides annually in the United States as a whole. Now, the fact more people are being killed doesn't automatically mean the law isn't working. Hoekstra said there were at least three possible explanations.

HOEKSTRA: It could be that these are self-defense killings. On the other hand, you know, the increase could be driven by an escalation of violence by criminals. Or it could be an escalation of violence in otherwise nonviolent situations.

VEDANTAM: But which is it? Hoekstra checked to see if police were listing more cases as justifiable homicides in states that passed Stand Your Ground laws. If there were more self-defense killings, this number should have gone up. He also examined whether more criminals were showing up armed.

In both cases, he found nothing. There were small increases in both numbers, but it was hard to tell if there was really any difference.

I asked Hoekstra where the increase in homicide was coming from.

HOEKSTRA: One possibility for the increase in homicide is that perhaps otherwise there would have been a fistfight. And now, because of Stand Your Ground laws, it's possible that those escalate into something much more violent and lethal.

VEDANTAM: In other words, crafters of these laws sought to give good guys more latitude to defend themselves against bad guys. But what Hoekstra's data suggests is that in real life conflicts, both sides think of the other guy as the bad guy. Both believe the law gives them the right to shoot.

In a separate analysis of death certificates before and after Stand Your Ground laws were passed in different states, economists at Georgia State University also found that states that passed the laws ended up with a higher homicide rate. This study also tracked the increased homicides by race. In contrast to the narrative established by the Trayvon Martin shooting, many people believe black men are likely to be the victims of Stand Your Ground laws, this analysis found the additional deaths caused by the laws were largely concentrated among white men.

The National Rifle Association, which has backed these laws, directed us to a critique of Hoekstra's paper. It was written by Howard Nemerov, a gun rights activist who often represents the NRA viewpoint on television. Nemerov's brief, which has not been reviewed by academic experts, said concerns about the law were flawed or overblown.

Stanford Law professor John Donohue, on the other hand, praised the study done by Texas A&M's Mark Hoekstra. Donohue has been studying crime and violence for two decades. He's working on his own independent analysis of Stand Your Ground laws. So far, he said, he's getting the same results Hoekstra did.

JOHN DONOHUE: The imperfect but growing evidence seems to suggest that the consequences of adopting Stand Your Ground laws are pernicious, in that they may lead to a greater number of homicides, thus going against the notion that they are serving some sort of protective function for society.

VEDANTAM: And in murder cases, Donohue said, the laws might end up being a refuge for some defendants.

DONOHUE: I've been hearing from defense lawyers around the country that if they happen to have a criminal defendant in a Stand Your Ground jurisdiction, pretty much no matter what happens, you can say, well, I shot the guy, but I felt threatened and had a reasonable basis for fearing injury to myself.

VEDANTAM: In Florida, George Zimmerman has maintained that he believed the unarmed Trayvon Martin posed a threat to his life, which is what prompted him to open fire. His lawyers have said they plan to use the Stand Your Ground defense at his upcoming trial.

Shankar Vedantam, NPR News.

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