DISCUSSION
This study provides compelling evidence that the repeal of Missouri's PTP handgun licensing law, which required all handgun purchasers to pass a background check even for purchases from private sellers, contributed to a sharp increase in Missouri’s homicide rate. Our estimates suggest that the law was associated with an additional 55 to 63 murders per year in Missouri between 2008 and 2012 than would have been forecasted had the PTP handgun law not been repealed.
Our analyses ruled out several alternative hypotheses to explain the relatively large and highly statistically significant increase in firearm homicides in Missouri following the repeal of its PTP handgun licensing law. We controlled for changes in unemployment, poverty, policing levels, incarceration rates, trends in crime reflected in burglary rates, national trends in homicide rates, and several kinds of other laws that could affect homicides. That Missouri’s sharp increase in firearm homicides was unique within the region, specific to firearms, and was observed in metropolitan jurisdictions across Missouri, suggests that unmeasured unique local circumstances (e.g., gang activity, changes in social norms) are unlikely to have biased our estimates of the impact of the policy change. Estimates of the effects of the repeal of Missouri's PTP handgun law were similar for firearm homicides and total homicides using death certificate data for 43 states through 2010, and for murders and non-negligent manslaughters using police reports for 11 all 50 states through 2012. This suggests that the data source and time period studied are unlikely to have biased the findings.
Causal inferences from quasi-experimental studies can be strengthened with empirical evidence supporting the proposed causal chain between the intervention, mediators, and the outcomes under study. Handgun purchaser licensing and universal background checks are hypothesized to affect homicide rates by reducing gun diversions to criminals and other prohibited groups. The evidence that Missouri’s increase in firearm homicides was fueled by the state’s repeal of its PTP law is bolstered by data indicating that the repeal was immediately followed by a two-fold increase in the percentage of crime guns that were recovered by police soon after the guns’ retail sales and an unusually large increase in the percentage of Missouri’s crime guns that had been purchased from Missouri gun dealers. These finding are consistent with prior research showing that states that regulated handgun sales by unlicensed sellers had fewer guns diverted to criminals shortly after in-state retail sales,8 and that states with the most comprehensive handgun sales laws including PTP licensing requiring direct interface with law enforcement have proportionately fewer guns used in crime that were originally sold by in-state retailers.
Having a large percentage of crime guns originate from out-of-state sales, as was the case prior to Missouri’s repeal of its PTP law, is indicative of a restricted supply of guns available to criminals from in-state sources. Restrictions from local suppliers increases prices in the underground gun market and attracts suppliers from states with fewer legal impediments to gun diversion. The weakening of Missouri’s gun laws may have also contributed to gun trafficking to border states that regulate handgun sales by all sellers via PTP licensing. The number of guns sold in Missouri and later recovered by police in Illinois and Iowa, two border states with 12 handgun purchaser licensing laws, increased 37 percent (from 133 to 182) from 2006 (just before Missouri’s PTP law was repealed) to 2012 when the overall number of crime guns recovered by police in those states actually declined by 6 percent.
A potential threat to the validity of our estimate of the impact of the repeal of Missouri’s PTP law is confounding by the simultaneous adoption of a Stand Your Ground Law in Missouri. Controlling for the effects of SYG laws across all states, our estimate of the effect of the repeal of Missouri’s PTP law on homicide rates declined slightly, but was still substantial and statistically significant at p<.001. A separate analysis of justifiable homicide data from the FBI's Uniform Crime Reports revealed that there were approximately 3 additional justifiable homicides per year in Missouri following the adoption of the state's Stand Your Ground law above pre-SYG-law levels – less than 1 percent of the total number of gun homicides during 2008-2010.
Critics could question the use of a relatively short pre-repeal baseline period used for this study. Using more longitudinal observations can potentially produce more accurate forecasts of the counterfactual in interrupted time-series impact studies. However, the period from 1985 through 1998 included dramatic increases and decreases in US homicide rates. Experts believe these changes were driven by factors that could not be directly measured (e.g., dynamics of the crack cocaine market, changes in social norms) and thus controlled for statistically, and these unmeasured forces appear to have been uneven across states. Such conditions pose considerable challenges for deriving unbiased estimates of policy impacts. By limiting the analyses to the relatively stable period of 1999-2012, we minimized the potential for omitted variable bias that would have likely been introduced by including data from this earlier time period.
The generalizability of our findings to other states with PTP handgun laws is unknown. Data from a recent cross-sectional study indicated that PTP licensing laws and universal background check requirements were associated with lower homicide rates after controlling for other population risk factors;9 however, the lack of longitudinal data weaken causal inference from that study. We caution, however, that passage of a PTP handgun licensing law with mandatory background checks and record-keeping for all handgun sales may not result in as immediate and large a reduction in firearm homicides as occurred in reverse when Missouri’s law was repealed. Although our findings indicate that Missouri benefited from the protective effects of its PTP law before the law’s repeal, the beneficial effects of new laws of this type may be more gradual as enforcement practices are put in place, awareness of the law increases, and the stock of guns available in the underground market is depleted. Additional methodologically rigorous research of the impact of other laws of this type is warranted.
Acknowledgements
This research was funded by a grant from The Joyce Foundation to the Johns Hopkins Center for Gun Policy and Research.
Death By Gun: Top 20 states With Highest Rates
https://www.cbsnews.com/pictures/death-by-gun-top-20-states-with-highest-rates/10/
Image Note: US patent 6101918, William Akins, "Method and apparatus for accelerating the cyclic firing rate of a semi-automatic firearm", published Aug 15, 2000, assigned to William Akins. Bump stocks, gunstocks designed to enable bumpfire, exist to allow semi-automatic weapons to mimic the firing speed of fully automatic weapons.
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