Texas A&M Study Says Castle-Doctrine Laws Increase Homicides, Don't Deter Crime

In news sure to inspire furious bickering inside your television, a Texas A&M study has found that the castle doctrine -- on which Florida's controversial "Stand Your Ground" law is based -- does not deter crime and, in fact, increases murder rates.

The killing of Florida teen Trayvon Martin by neighborhood watch volunteer George Zimmerman in late February launched thousands of arguments about castle doctrine laws, which allow a person to use lethal force against an intruder in certain situations, provided they have a reasonable fear of death or serious bodily harm. Lawmakers in several states, including Texas, have debated revising their own self-defense laws.

"We found a 7 to 9 percent increase in homicides," says one of the study's authors, associate economics professor Mark Hoekstra. "That's significant. That's robust.

"We did comparisons in a bunch of different ways," he goes on. "We compared states that adopt [the law] to states that don't adopt. It doesn't matter if you control for things like policing or levels of incarceration. You can compare to only other states in the same region. It doesn't matter. At the end of the day, castle doctrine increased homicides by 7 to 9 percent."

Hoekstra and his co-author, grad student Chen Cheng, looked at 23 states where castle doctrine laws exist and found slight evidence that castle doctrine increases justifiable homicides committed by civilians by anywhere from 17 to 50 percent. That sounds like a lot. But the reality is that justifiable homicide is narrowly defined and exceedingly rare: according to the FBI, a killing can only be classified that way when someone kills another person who's committing a felony. Fewer than 200 deaths are classified that way each year.

Instead, the study found that castle doctrine increases total homicides, including murder and non-negligent homicide, by 7 to 9 percent, making a much larger 500 to 700 additional deaths per year. Hoekstra says they see three distinct possibilities that might account for the increase.

"One theory is that these are in some sense legitimate self-defense killings that just don't meet the strict definition of justifiable homicide," he says. "On the other hand, it could be that the increase in homicide is due to criminals escalating," say by carrying and using weapons more. "So one possible response to castle doctrine is for criminals to carry and use guns more frequently, for example. We could be picking up the effect of that. The third possibly is that otherwise non-lethal conflicts turn deadly because of castle doctrine. It's really, really difficult to distinguish between those three possibilities."

The study says that self-defense alone probably doesn't explain the numbers, though.

"We suspect that self-defense situations are unlikely to explain all of the increase, as we also find that murder alone is increased by a statistically significant 6 to 11 percent," they write." "This is important because murder excludes non-negligent manslaughter classifications that one might think are used more frequently in self-defense cases. But regardless of how one interprets increases from various classifications, it is clear that the primary effect of strengthening self-defense law is to increase homicide."

Any hope that criminals in castle doctrine states might be deterred from robbing you by the knowledge that you could be packing heat is also incorrect, he says.

"This is true not just of criminals, but of the general public: when it come to things that involve probabilistic thinking, people have a pretty hard time with it. 'What's the increase in the possibility that someone will defend themselves with lethal force against me?' It's tough to answer that in a super rigorous way. The idea that a criminal is going to do a really great job of answering that, and if they'd be able to make these calculations -- You're asking a lot of anybody to make that calculation."

The homicide increase also presents another issue for the researchers. How do you determine who died in a castle doctrine situation: the alleged criminal or the person allegedly defending themselves? The FBI data Hoekstra and Cheng studied doesn't show that kind of detail, and Hoekstra says it's crucial in figuring out what's driving the homicide increase. The answer, he says, is another study.

"The best idea I've come up with is to try to figure out if the people getting killed have criminal backgrounds," he says. "If you see an increase in people getting killed without criminal backgrounds then at least part of what it suggests is escalation." But, he concedes, "It's going to be difficult. I don't know how optimistic I am."

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