Florida’s “stand your ground” law is coming under attack after Saturday’s acquittal of George Zimmerman for murder and manslaughter charges in the death of Trayvon Martin. Stand your ground laws expand the legal justification for the use of lethal force in self-defense. Since Florida passed its law in 2005, 20 other states passed similar laws between 2006 and 2009. A recent paper by Cheng Cheng and Mark Hoekstra of Texas A&M University presents empirical evidence that these laws “increase homicides by a statistically significant 8 percent, which translates into an additional 600 homicides per year” in the states (including Florida) that passed “stand your ground” laws from 2005 to 2009. Moreover, they find that these laws do not appear to deter burglaries, robberies or aggravated assaults.
The Cheng and Hoekstra study compares crime rates before and after laws were passed in “treatment” states to crime trends in the “control states” where there was no change in the law. This is a standard practice in social science policy evaluation studies. Cheng and Hoekstra control for differences in policing and law enforcement, changes in economic conditions, and changes in the demographic composition of states and provide several different empirical specifications of their model. Their careful study suggests that their key results are robust to alternative specifications of the empirical model.
I remain skeptical of the Cheng and Hoekstra results for three reasons:
- The raw decline in homicides per 100,000 residents between 2000 and 2011 is very similar for states that did and did not enact “stand your ground laws” from 2005 to 2009. The Cheng and Hoekstra result relies on percentage changes in homicide rates. The percentage decline was smaller in “stand your ground” states because they had higher homicide rates in 2000.
- Cheng and Hoekstra did not present results for rapes. The raw decline in the number of rapes per 100,000 residents between 2000 and 2011 is larger for states that enacted “stand your ground laws.” In other words, a simple comparison of crime rates suggest that “stand your ground” laws prevented more than 1500 rapes per year in the 21 states that passed these laws.
- Homicide rates and trends differ substantially across states which makes this kind of policy evaluation quite difficult. For example, I compare homicide trends in Nevada, ground zero for the real estate bust, and North and South Dakota where the economy has been booming because of natural gas and oil exploration. A consistent estimate of the impact of “stand your ground” must control for cross-state differences in homicide levels and trends.
Homicides Declined by Similar Amounts in States That Did and Didn’t Pass “Stand Your Ground” Laws
The following chart compares annual population-weighted average homicide rates across two groups of states, those that passed “stand your ground laws” between 2005 and 2009 and those with no change in the law since 2000.
In states that would eventually pass “stand your ground” laws the average homicide rate was 27% higher in 2000 than it was in states that would not change their law. The reasons for these cross-state differences in homicide rates are likely to persist regardless of a change in the law.
Homicide rates fell in both groups of states, but the timing of the decline was somewhat different. Homicide rates started declining about two years earlier in states with no change in the law. A simple comparison of three-year average homicide rates from 2000-02 to 2009-11 indicates:
- The average homicide rate in states that passed a “stand your ground law” declined by 0.79 per 100,000 residents between 2000-02 and 2009-11.
- The average homicide rate in states with no change in the law declined by 0.89 per 100,000 residents between 2000-02 and 2009-11.
Homicide rates fell less in states that passed a “stand your ground law.” The magnitude of the difference is about 1 homicide per year per 1,000,000 residents. There were 7,272 homicides in 2011 in the 21 states that passed “stand your ground.” The simple calculations above suggest that there might have been 135 fewer homicides in these 21 states had the law not been passed. This difference is neither practically or statistically significant.
Cheng and Hoekstra estimate a much larger effects of the law, about 4.5 times as large as the raw difference in homicide rates presented above. They contend that “stand your ground” was responsible for an additional 600 homicides in 2011. Their effect is larger in part because the percentage decline in homicide rates was larger in states that did not pass the law (because these states already had a lower homicide rate). In addition their methodology attempts to control for other factors that could explain homicide trends. It is not clear which of these factors caused the regression-adjusted results to be larger than the simple comparisons of mean homicide rates. They also attempt to exploit differences in the timing of changes in the law to measure it’s effects. Unfortunately 13 of the 21 states that passed “stand your ground” did so in 2006 and 4 more in 2007, so it is difficult to ascertain whether the impact of the law occurs with a lag.
Rapes Declined More in States That Passed “Stand Your Ground” Laws
Cheng and Hoekstra attempted to estimate the deterrent effect of “stand your ground” on burglary, robbery and aggravated assault crime rates. They found no effect. They did not, however, attempt to measure the impact of these laws on rapes. Had they done so they would have seen the following pattern:
The rate at which rapes occurred was 28% higher in 2000 in states that would eventually pass “stand your ground” relative to the states that would not change their law. Rapes declined in both groups of states between 2000 and 2011. A simple comparison of three-year average rates from 2000-02 to 2009-11 indicates:
- In states that passed a “stand your ground law” rapes declined by 5.9 per 100,000 residents between 2000-02 and 2009-11.
- In states with no change in the law rapes declined by 4.7 per 100,000 residents between 2000-02 and 2009-11.
The decline in rapes was more substantial in states that passed a “stand your ground law.” The magnitude of the difference is about 1.2 rapes per year per 100,000 residents or more than 10 times the impact of the law on homicides as described earlier. Taken at face value this implies a reduction of 1,579 rapes due to the “stand your ground” law in 2011. Despite the larger decline in rapes in “stand your ground” states, the percentage decline is slightly larger (16% compared to 15.6%) in states with no change in the law because these states started with a lower crime rate in 2000.
State Difference in Crime Trends
The purpose of the policy evaluation study is to impute state-specific crime rates but for the passage of the “stand your ground” law. This is an extremely difficult task and requires an understanding of why crime trends differ from one state to another. As the previous charts have shown murders and rapes were generally declining in states regardless of whether a “stand your ground” law was passed. It is clear that “stand your ground” was more likely to be enacted in higher crime rate states. A similar correlation between state-specific crime trends and the passage of “stand your ground” would invalidate the empirical approach used by Cheng and Hoekstra to evaluate this law.
As an example of the difficulties encountered when comparing cross-state differences in crime trends consider the case of Nevada and the Dakotas. The following chart shows that the homicide rate in Nevada fell by 43% between 2006 and 2011 after increasing from 2000 to 2006. In contrast the homicide rate in North and South Dakota (combined) increased by 265% between 2000 and 2011.
South Dakota and North Dakota passed stand your ground laws in 2006 and 2007 while Nevada did not change its law. Clearly there are many other important differences between these states and the economic conditions in these states have changed substantially over the past decade. Nevada suffered more than any state from the real estate collapse and recession. Between 2006 and 2011 its unemployment rate jumped from 4.2% to 13.2%. Between 2006 and 2011 North and South Dakota benefitted economically from an energy boom with some of the lowest unemployment rates in the U.S. The boom also brought big population growth and an increase in crime rates.
The comparison of Nevada and the Dakotas illustrates why one should be skeptical of studies that compare percentage changes in homicide rates across states. In the Dakotas in 2000 there were only 0.8 homicides per 100,000 residents per year. Thus even a massive percentage increase in the homicide rate in the Dakotas led to an increase of about 2.2 homicides per 100,000 residents per year by 2011. In contrast the 43% decline in the homicide rate in Nevada between 2006 and 2011 led to a decrease of about 3.9 homicides per 100,000 residents per year. It makes more sense for a policy evaluation study to compare changes in homicide rates per 100,000 residents than percentage changes in state-specific rates, because even modest percentage declines in high crime rates states will save more lives than very large percentage changes in lower crime rate states. Finally, it is worth noting that even after the large percentage increases in homicides in the Dakotas and the more modest percentage declines in homicides in Nevada, the homicide rate in Nevada remains 74% higher than in the Dakotas in 2011.
Although it is tempting to evaluate the impact of “stand your ground” laws by comparing the trend in homicides in states that passed laws since 2005 to trends in states with no change in the law, one should do so with great caution. First, homicide rates differ widely among states and there are equally important underlying differences in crime trends among states. “Stand your ground” laws were passed in states with higher violent crime rates, on average. This means that roughly equal declines in homicides per 100,000 residents in states with and without changes in the law translated into smaller percentage reductions in homicide rates in “stand your ground” states. I consider this to be weak evidence that “stand your ground” laws increase homicides because it presumes that in the absence of the law homicides per 100,000 residents would have declined proportionately more in the states with the highest homicide rates.
This post also presented new evidence that rapes per 100,000 residents declined more substantially in states that passed “stand your ground” laws. While I am skeptical of cross-state comparisons in crime trends, if one believes the evidence that “stand your ground” increased homicides one should also believe that rapes were reduced by more than 10 times as much as the increase in homicides after the passage of “stand your ground.”