Sunday, August 12, 2012
John J. Donohue III (pictured), Abhay Aneja and Alexandria Zhang (Stanford Law School , Stanford University and Johns Hopkins University) have posted The Impact of Right to Carry Laws and the NRC Report: The Latest Lessons for the Empirical Evaluation of Law and Policy on SSRN. Here is the abstract:
For over a decade, there has been a spirited academic debate over the impact on crime of laws that grant citizens the presumptive right to carry concealed handguns in public – so-called right-to-carry (RTC) laws. In 2005, the National Research Council (NRC) offered a critical evaluation of the “More Guns, Less Crime” hypothesis using county-level crime data for the period 1977-2000. 17 of the 18 NRC panel members essentially concluded that the existing research was inadequate to conclude that RTC laws increased or decreased crime. One member of the panel, though, concluded that the NRC's panel data regressions supported the conclusion that RTC laws decreased murder.
We evaluate the NRC evidence, and improve and expand on the report’s county data analysis by analyzing an additional six years of county data as well as state panel data for the period 1977-2006. We also present evidence using both a more plausible version of the Lott and Mustard specification, as well as our own preferred specification (which, unlike the Lott and Mustard model used in the NRC report, does control for rates of incarceration and police). While we have considerable sympathy with the NRC’s majority view about the difficulty of drawing conclusions from simple panel data models, we disagree with the NRC report’s judgment that cluster adjustments to correct for serial correlation are not needed. Our randomization tests show that without such adjustments the Type 1 error soars to 44-75 percent. In addition, the conclusion of the dissenting panel member that RTC laws reduce murder has no statistical support.
Our paper highlights some important questions to consider when using panel data methods to resolve questions of law and policy effectiveness. Although we agree with the NRC’s cautious conclusion regarding the effects of RTC laws, we buttress this conclusion by showing how sensitive the estimated impact of RTC laws is to different data periods, the use of state versus county data, particular specifications, and the decision to control for state trends. Overall, the most consistent, albeit not uniform, finding to emerge from both the state and county panel data models conducted over the entire 1977-2006 period with and without state trends and using three different specifications is that aggravated assault rises when RTC laws are adopted. For every other crime category, there is little or no indication of any consistent RTC impact on crime. It will be worth exploring whether other methodological approaches and/or additional years of data will confirm the results of this panel-data analysis.