Confirming "more guns, less crime".

• Author: Plassmann, Florenz
• Position: Examination of Ian Ayres and John Donohue's evidence that right-to-carry gun laws increase crime

INTRODUCTION I. WHAT DOES THEIR EVIDENCE SHOW? A. Is There a "Robbery Effect"? B. Murder Rates C. Rapes and Aggravated Assaults D. Critiques of Year-by-Year Breakdown of the Law's Impact E. The Estimated Benefits from the Law II. COUNTY-LEVEL DATA FROM 1977 TO 2000 A. Advantages and Disadvantages of Different Types of Data B. Extending the Data to 2000 C. Is the Adoption of a Right-to-Carry Law Endogenous? D. Poisson Estimates III. EVALUATING SOME GENERAL CLAIMS MADE BY AYRES AND DONOHUE A. Has Previous Work, Acknowledged Both the Costs and Benefits of Guns? B. Possible Bad Effects of Concealed Handgun Laws C. Other Concerns: The Risks to Police, Accidents, and the Problem with Untrained Permit Holders D. Are There "Initial Jumps in Crime"? E. Can Cocaine Use Explain the Results? F. Measurement Error in County-Level Data G. Is the U.S. Murder Rate "Exceptional"? H. Should Philadelphia Be Treated Differently from the Rest of Pennsylvania After 1989? CONCLUSION FIGURES FIGURE 1A: AYRES AND DONOHUE'S ESTIMATED IMPACTS ON ROBBERY (ALL THE YEAR-BY-YEAR ESTIMATES THAT IAN AYRES AND JOHN DONOHUE REPORT) FIGURE 1B: AYRES AND DONOHUE'S ESTIMATED IMPACTS ON MURDER (ALL THE YEAR-BY-YEAR ESTIMATES THAT IAN AYRES AND JOHN DONOHUE REPORT) FIGURE 1C: AYRES AND DONOHUE'S ESTIMATED IMPACTS ON RAPE (ALL THE YEAR-BY-YEAR ESTIMATES THAT IAN AYRES AND JOHN DONOHUE REPORT) FIGURE 1D: AYRES AND DONOHUE'S ESTIMATED IMPACTS ON AGGRAVATED ASSAULT (ALL THE YEAR-BY-YEAR ESTIMATES THAT IAN AYRES AND JOHN DONOHUE REPORT) FIGURE 2: SHOWING WHAT HAPPENS WHEN YOU FIT AN INTERCEPT SHIFT AND A TREND TO A NONLINEAR TREND FIGURE 3: EXAMINING THE LAST STATES TO ADOPT THE RIGHT-TO-CARRY LAW FIGURE 4A: VIOLENT CRIME: WEIGHTED LEAST SQUARES ESTIMATES USING COUNTY-LEVEL DATA FROM 1977 TO 2000 FIGURE 4B: MURDER: WEIGHTED LEAST SQUARES ESTIMATES USING COUNTY-LEVEL DATA FROM 1977 TO 2000 FIGURE 4C: RAPE: WEIGHTED LEAST SQUARES ESTIMATES USING COUNTY-LEVEL DATA FROM 1977 TO 2000 FIGURE 4D: ROBBERY: WEIGHTED LEAST SQUARES ESTIMATES USING COUNTY-LEVEL DATA FROM 1977 TO 2000 FIGURE 4E: AGGRAVATED ASSAULT: WEIGHTED LEAST SQUARES ESTIMATES USING COUNTY-LEVEL DATA FROM 1977 TO 2000 FIGURE 4F: PROPERTY CRIMES: WEIGHTED LEAST SQUARES ESTIMATES USING COUNTY-LEVEL DATA FROM 1977 TO 2000 TABLES TABLE 1: COST-BENEFIT ANALYSES OF AYRES AND DONOHUE'S COUNTY-LEVEL REGRESSIONS: FIVE-YEAR AVERAGE ESTIMATE OF NET COSTS/BENEFITS TABLE 2: COST-BENEFIT ANALYSES OF COUNTY- AND STATE-LEVEL REGRESSIONS IN DONOHUE'S BROOKINGS PAPER: FIVE-YEAR AVERAGE ESTIMATE OF NET COSTS/BENEFITS TABLE 3A: COMPARISON OF THE THREE DUMMY SPECIFICATIONS USED BY AYRES AND DONOHUE (OUR MODEL INCLUDES REGION-SPECIFIC YEAR DUMMIES AND USES 1977-2000 DATA) TABLE 3B: LIMITED SET OF DEMOGRAPHICS AND LAGGED PER CAPITA PRISON POPULATION, 1977-2000 DATA TABLE 4: EARLY AND LATE ADOPTERS, FULL SET OF DEMOGRAPHICS, 1977-2000 DATA TABLE 5: TEST WHETHER THE ADOPTION OF SHALL-ISSUE LAWS IS ENDOGENOUS TO THE CRIME RATE TABLE 6: STATE-BY-STATE BREAKDOWN: SPLINE MODEL, 1977-2000 COUNTY-LEVEL DATA, WITH REGIONAL YEAR FIXED EFFECTS TABLE 7: POISSON ESTIMATES: ALL SOCIOECONOMIC VARIABLES AND THE ARREST RATE OF VIOLENT CRIME TABLE 8: ALL SOCIOECONOMIC VARIABLES AND THE ARREST RATE OF VIOLENT CRIME APPENDIX TABLE 1: REPORTING THE RESULTS ON VIOLENT CRIME RATES FROM STUDIES CRITICAL OF RIGHT-TO-CARRY LAWS (USING THE NATIONAL COEFFICIENTS FROM THE MOST CRITICAL STUDIES THAT EXAMINED THE CHANGE IN CRIME RATES BEFORE-AND-AFTER THE PASSAGE OF RIGHT-TO-CARRY LAWS) APPENDIX TABLE 2: ANALYSIS WITH YEAR-SPECIFIC TREND DUMMIES (OUR MODEL INCLUDES REGION-SPECIFIC YEAR DUMMIES AND USES 1977-2000 DATA) APPENDIX TABLE 3: LIMITED DEMOGRAPHIC VARIABLES AND ADDING LAGGED PRISON POPULATION, 1977-2000 APPENDIX TABLE 4: SPLINE MODEL, 1977-2000 STATE-LEVEL DATA, REGIONAL YEAR FIXED EFFECTS INTRODUCTION

Quite a few empirical papers have examined the impact of right-to-carry laws on crime rates. Most studies have found significant benefits, with some finding reductions in murder rates twice as large as the original research. (1) Even the critics did not provide evidence that such laws have increased violent crime, accidental gun deaths, or suicides. (2)

Unlike previous authors, Ian Ayres and John Donohue claim to have found significant evidence that right-to-carry laws increased crime. However, they have misread their own results. The most detailed results they report--following the change in crime rates on a year-by-year basis before and after right-to-carry laws were adopted--clearly show large drops in violent crime that occur immediately after the laws were adopted. Their hybrid results showing a small increase in crime immediately after passage are not statistically significant and are an artifact of fitting a straight line to a curved one. When one examines a longer period--from 1977 to 2000--even this type of result disappears.

Ayres and Donohue's efforts have been valuable in forcing others to reexamine the evidence, extend the dataset over more years, and think of new ways to test hypotheses, and we appreciate their efforts. (3) They are both highly regarded and well-known for their research, such as claiming that the legalization of abortion can account for half the drop in murder during the 1990s. (4) Unfortunately, their research on this issue inaccurately describes the literature and also fails to address previous critiques of their work. For example, Ayres and Donohue claim that "[w]hen we added five years of county data and seven years of state data, allowing us to test an additional fourteen jurisdictions that adopted shall-issue laws, the previous Lott and Mustard findings proved not to be robust." (5) All their tables report results for "Lott's Time Period (1977-1992)" and compare those estimates with the "Entire Period (1977-1997)." Yet, whatever differences in results arise, they are not due to the inclusion of more data for a longer period. Their paper gives a misleading impression as to how much their research extends the data period, since Lott's book and other work examined both the county and state data up through 1996. (6) Ayres and Donohue's work thus extends the county-level data by one year, from twenty to twenty-one years.

Part I of this Response reviews some of Ayres and Donohue's claims and shows that even their own estimates imply fairly consistently large annual benefits from reducing crime. We then extend the U.S. county-level data to 2000 in Part II, and, consistent with previous work, find large benefits from states adopting right-to-carry laws. As others have already found, the results are not sensitive to the inclusions of particular control variables, such as demographic measures. Finally, Part III provides direct responses and corrections to several specific claims made by Ayres and Donohue.

1. WHAT DOES THEIR EVIDENCE SHOW?

   The most general specifications show the year-by-year changes in crime rates before and after the enactment of a right-to-carry law. Ayres and Donohue provide this breakdown for state-level data from 1977 to 1999. Their state-level data show the crime rates in the first year after the law was passed, the second year, and so on. While we disagree with some of their assumptions, their results provide a very useful starting point as their results stake out one side of the debate.

   John Donohue has another paper addressing these issues published by the Brookings Institution. (7) This paper presents the year-by-year changes for county-level data from 1977 to 1997. The county-level estimates report the crime rates in two-year intervals, and a separate dummy variable measures the combined effects whenever the state has had the law for eight or more years. (8) One of the county estimates includes a separate state time trend for each state. Although our principal focus is on Ayres and Donohue's joint paper, we will refer to Donohue's Brookings paper several times in this Part because the papers are very similar and because several important results (like the county-level year-by-year breakdowns) are only reported in the Brookings paper.

   Note also that the county and state estimates use two different definitions of the implementation of state right-to-carry laws, with the county-level data using a "corrected" version of the dates that Lott and Mustard used from Kopel and Cramer, and the state-level data using definitions supplied by Vernick and Hepburn. (9)

   1. Is There a "Robbery Effect"?

      Robbery is a good place to start our inquiry because it is committed in public more than any other crime, and should be the crime most likely to decline if the Lott and Mustard story of deterrence has any plausibility. (10) [T]he failure of the model to show a drop in robbery[] cast[s] doubt on the causal story that they advance. (11) Ayres and Donohue have consistently argued in several papers that robbery is the key result upon which the deterrence by right-to-carry laws is based. (12) In contrast, Lott has argued many times that there is no a priori reason to believe that the benefits of right-to-carry laws are larger for robbery than for other violent crimes. (13) Putting that debate aside, the robbery results presented by Ayres and Donohue present a very clear, consistent story. (14) The state-level analysis shows that robbery rates continued rising, though at a slower rate, for the first two years after the law was passed. After that, however, robbery rates in right-to-carry states fell relative to non-right-to-carry states for the next nine years, and then remained fairly constant through year seventeen. The two sets of county-level estimates are even more dramatic. Robbery rates in right-to-carry states were rising until the laws were passed and then fell continually after that point. The pattern is very similar to that shown earlier by Lott in examining county-level data from 1977 to 1996. (15)

      The changes are also very large. By the time the law has been in effect for six years, the...