To review, for those who weren't paying attention when the public spotlight shone on such laws in the aftermath of the killing by George Zimmerman of Trayvon Martin in Florida, such a law says, in essence, that one may use lethal force in self defense when s/he is anywhere s/he has a legal right to be.
Florida's "Stand Your Ground" law wasn't actually relevant to the case, as Zimmerman claimed self-defense as justification for the shooting without reference to the provisions of the law, but that was a legal distinction that was lost on the general public because it was largely ignored in the news reporting.
The NPR report relied heavily on a new study in the Journal of Human Resources by Cheng and Hoekstra from Texas A&M University. These investigators are economists who used the tools of social scientists to examine empirical data, comparing states that adopted new laws of this nature with others that did not, over time.
The first problem with the study, which is not a major flaw, is that it combines two different kinds of laws. The first is the Stand Your Ground law, and the second is the Castle Doctrine. The castle doctrine, simply, is the idea that one has no obligation to retreat from criminal assault in one's own home. The authors assert that the castle doctrine is a feature of English common law (which is historically true) and commonly applied in the US, sometimes in statute, sometimes in case law (which is a bit misleading, because there are some states - New York a prime example - in which the duty to retreat, even from one's own dwelling, remains).
So, the incorporation of the castle doctrine into statute and the enactment of stand your ground laws are combined for purposes of this study. The researchers then looked at the effect of such changes on homicide rates. The paper is long (43 pages) and, as one might expect of a study that incorporates statistical analysis and modeling, rather dense. But it is methodologically rigorous. Like any such study, it necessarily relies upon some assumptions that are subject to question or criticism, but the authors (to their credit) acknowledge this and (in some particulars) show how alternative assumptions affect the results of the analysis.
The central conclusion is that these laws have been associated with an 8% increase in homicides relative to states that did not adopt such laws. To be clear, that does not include killings that were reported to the national database as justifiable homicides, although the investigators note that only a small fraction (probably no more than 20%) of justifiable homicides are reported that way.
Using the 20% assumption, they estimate that about half the difference in killings between adopting and non-adopting states can be accounted for by justifiable homicide. They also acknowledge that if they had used a 10% assumption, which they grant is entirely within the realm of reasonable possibility, all of the additional homicides would then (statistically) be accounted for by the "justifiable" category. As a matter of definition, "justifiable" in this context means that lethal force was used to stop a felony in progress in a situation in which law enforcement or a prosecutor considered it proper for a civilian to do so.
[Oh, about this commonly accepted 20% assumption: we must realize it is based on the work of a criminologist whose methods are also very rigorous but who is perceived by pro-gun-control advocates as supportive of the right of armed self defense. Any time you're looking at work in this field, it is important to know whether the researchers have any biases, either real or perceived.]
So where does that leave us? What does it mean if homicides increased by 8%, which is significant, and half of them were justifiable? Is it bad to have more homicides whether they were justifiable or not? Did the justifiable homicides take some career criminals off the streets, thereby preventing future crimes? What about the half (and remember the underlying assumption) of homicides that may not have been justifiable? Who committed them, and why?
The challenging questions to answer are the ones about the myriad subtle effects of these laws. Do they increase the number of people who choose to own guns for personal protection? The number who carry guns as they go about their daily lives? The willingness of those who keep and bear arms to use lethal force in self defense? Do they lower the psychological threshold for using lethal force? How do they affect the inclination of those who are armed to do everything possible to deescalate conflict, which is the legally and ethically correct thing to do? Do they increase (or decrease) the likelihood that a verbal argument will escalate into a fistfight and then a shooting?
We can all speculate about the answers to these questions, and our answers will be strongly influenced by our own biases. But the answers to these questions are unknown.
It would be interesting to examine the difference in homicide rates following adoption of these laws on a more granular level. How many more homicides were committed by people having a previously clean record, in lawful possession of a gun (with a permit, if not in their own homes), with the killing ultimately found not to be justifiable? Although there is an abundance of data showing that holders of carry permits very rarely use their guns in the commission of crimes, we don't really know the answers to these questions. Zimmerman was acquitted in the killing of Martin, but even a cursory analysis of the incident as reported in the news suggests that there was plenty of opportunity for deescalation and avoidance of the shooting.
My own personal perspective on these laws is simple. If I ever (and I strongly prefer never) am forced to use lethal force in self defense, I want the burden of proof to be on the prosecutor to show that I acted unreasonably. If that cannot be shown according to the standard required under criminal law (reasonable doubt), I want to be shielded from civil liability, where the standard is much lower (preponderance of the evidence). The first part seems to me to be of self-evident necessity to someone who uses lethal force to preserve his own life. The second part may be every bit as important, because in a civil negligence case one has no access to a public defender, and even a successful defense is likely to be financially ruinous.
The broader question remains, I think, unanswered. Do these laws merely protect the intended victim of a crime from being victimized by the legal system, or do they lower people's threshold for using lethal force and make ours a more violent society? Cheng and Hoekstra have given us an interesting look at the data. They conclude that these laws do not deter crime, and they worry that the increase in the homicide rate might be the result of many killings that should not have occurred - and would not have occurred absent a change in the law. They are right to worry about that. We all should. But we cannot yet draw firm conclusions.
