Home > modeling, news, statistics > Does making it easier to kill people result in more dead people?

Does making it easier to kill people result in more dead people?

February 17, 2014

A fascinating and timely study just came out about the “Stand Your Ground” laws. It was written by Cheng Cheng and Mark Hoekstra, and is available as a pdf here, although I found out about in a Reuters column written by Hoekstra. Here’s a longish but crucial excerpt from that column:

It is fitting that much of this debate has centered on Florida, which enacted its law in October of 2005. Florida provides a case study for this more general pattern. Homicide rates in Florida increased by 8 percent from the period prior to passing the law (2000-04) to the period after the law (2006-10).By comparison, national homicide rates fell by 6 percent over the same time period. This is a crude example, but it illustrates the more general pattern that exists in the homicide data published by the FBI.

The critical question for our research is whether this relative increase in homicide rates was caused by these laws. Several factors lead us to believe that laws are in fact responsible. First, the relative increase in homicide rates occurred in adopting states only after the laws were passed, not before. Moreover, there is no history of homicide rates in adopting states (like Florida) increasing relative to other states. In fact, the post-law increase in homicide rates in states like Florida was larger than any relative increase observed in the last 40 years. Put differently, there is no evidence that states like Florida just generally experience increases in homicide rates relative to other states, even when they don’t pass these laws.

We also find no evidence that the increase is due to other factors we observe, such as demographics, policing, economic conditions, and welfare spending. Our results remain the same when we control for these factors. Along similar lines, if some other factor were driving the increase in homicides, we’d expect to see similar increases in other crimes like larceny, motor vehicle theft and burglary. We do not. We find that the magnitude of the increase in homicide rates is sufficiently large that it is unlikely to be explained by chance.

In fact, there is substantial empirical evidence that these laws led to more deadly confrontations. Making it easier to kill people does result in more people getting killed.

If you take a look at page 33 of the paper, you’ll see some graphs of the data. Here’s a rather bad picture of them but it might give you the idea:

Screen Shot 2014-02-17 at 7.21.15 AM

That red line is the same in each plot and refers to the log homicide rate in states without the Stand Your Ground law. The blue lines are showing how the log homicide rates looked for states that enacted such a law in a given year. So there’s a graph for each year.

In 2009 there’s only one “treatment” state, namely Montana, which has a population of 1 million, less than one third of one percent of the country. For that reason you see much less stable data. The authors did different analyses, sometimes weighted by population, which is good.

I have to admit, looking at these plots, the main thing I see in the data is that, besides Montana, we’re talking about states that have a higher homicide rate than usual, which could potentially indicate a confounding condition, and to address that (and other concerns) they conducted “falsification tests,” which is to say they studied whether crimes unrelated to Stand Your Ground type laws – larceny and motor vehicle theft – went up at the same time. They found that the answer is no.

The next point is that, although there seem to be bumps for 2005, 2006, and 2008 for the two years after the enactment of the law, there doesn’t for 2007 and 2009. And then even those states go down eventually, but the point is they don’t go down as much as the rest of the states without the laws.

It’s hard to do this analysis perfectly, with so few years of data. The problem is that, as soon as you suspect there’s a real effect, you’d want to act on it, since it directly translates into human deaths. So your natural reaction as a researcher is to “collect more data” but your natural reaction as a citizen is to abandon these laws as ineffective and harmful.

Categories: modeling, news, statistics
  1. February 17, 2014 at 8:28 am

    Cathy, one interesting thing to note is that homicide here excludes killings that are deemed justified under stand-your-ground, so the rise in homicide must be coming from a less direct effect of the law. The answer, I think, lies in the fact that homicide can be motivated by pre-emption. SYG makes it more likely that someone will kill you, and hence increases your incentives to kill them first. More generally, it contributes to a climate of fear which makes people trigger happy out of self-preservation. I discussed the Cheng/Hoeksra paper here in light of this effect:



    • February 17, 2014 at 8:29 am

      Great point, thanks.


    • February 17, 2014 at 11:51 am

      …and if a certain percentage of gun carriers think the law allows more than it does, their attempts to “stand their ground” can also end up as part of the criminal stats.


    • Guest2
      February 18, 2014 at 5:46 pm

      Well, then that is what you need to provide evidence for — “a climate of fear which makes people trigger happy”. Not easy to do, but second-best is worse.


    • jerzy kaltenberg
      February 18, 2014 at 10:09 pm

      rajivsethi , Cathy, it seems that confirmation bias is setting in when i read “…homicide here excludes killings that are deemed justified under stand-your-ground, so the rise in homicide must be coming from a less direct effect of the law” .
      Please consider that states which have passed SYG legislation already had disproportionally high homicide rates; it may well be that SYG can only pass where the public support for the law is sufficient for cultural reasons or where voting population is disproportionally old & well off ( Florida). In any event, if you are _excluding_ the SYG killings, you can’t show reasonable correlation.


  2. February 17, 2014 at 9:20 am

    Geoffrey Canada had an excellent piece on “This American Life” about how his attitude on the street was completely transformed when he carried a gun. Growing up in the Bronx, he had learned how to avoid violence, but the gun changed all that. Here’s the link: http://www.thisamericanlife.org/radio-archives/episode/81/guns?act=2


  3. February 17, 2014 at 11:48 am

    Of course, big hope for the NRA is an endless upward spiral of more gun sales creating higher crime rates, creating more gun sales; etc…


  4. Christina Sormani
    February 17, 2014 at 11:49 am

    Sadly the people I know who support stand your ground laws have no interest whatsoever in whether it increases the homocide rate. I have given them an example of a scenario I have faced here as a crime victim in which five men came into my house to burglarize it. I hid with my family (including children) upstairs and called the police. The police arrived with sirens and the burglars fled. They were not caught but we were safe.

    My gun owning acquaintences all agree this is a scenario in which they would take out their gun and kill all the burglars. They do not care if I mention the chances the burglars would have shot back. Instead they point out the possibility that the burglars could have come upstairs and shot us with no motivation to do so. They say how police don’t respond quickly in their state. I point out that I would be sickened with my home if it was the sight of a massacre, far more than if I no longer had my expensive electronic equipment they were set of stealing.

    Ultimately these decisions are not data driven. The people who want these laws believe deep in their hearts that they should have the right to kill anyone who might harm them.


    • Guest2
      February 18, 2014 at 5:49 pm

      Sorry to hear about your experience — and that SYG supporters showed little if any sympathy. Perhaps their response was a kind of denial, and based on the fear they have.

      But, yeah: “Ultimately these decisions are not data driven.” Statisticians tend to forget this.


  5. Sarath
    February 17, 2014 at 3:27 pm

    “The problem is that, as soon as you suspect there’s a real effect, you’d want to act on it, since it directly translates into human deaths.”
    Why does that follow? We could save 30k+ lives a year by eliminating cars.


  6. February 17, 2014 at 5:34 pm

    Great illustration of correctly defining a cost function. Mathbabe and some commenters interpret this study as supporting elimination of SYG laws, apparently because of the high value they place on all human life. One commenter believes the laws supporters will not change their opinion based on this research – one reason could be that the law’s supporters believe that they perceive a benefit (possibly in terms of deaths of alleged burglars) which outweighs the deaths the law may possibly be causing.
    The difference between eliminating SYG and cars, in the same vein, is that the cost of removing cars is quite apparent, and potentially quite high. (also, if you eliminate cars by replacing them with, for example, horse and cart the amount of lives saved could be much lower than you expect)


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