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More Guns, More Time: Using Add-on Gun Laws to Estimate the Deterrent Effect of Incarceration . David S. Abrams June 6, 2006. Add-on Gun Law Background. Enhanced prison sentences for defendants using or in possession of a firearm while committing a felony.
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More Guns, More Time:Using Add-on Gun Laws to Estimate the Deterrent Effect of Incarceration David S. Abrams June 6, 2006
Add-on Gun Law Background • Enhanced prison sentences for defendants using or in possession of a firearm while committing a felony. • Over half the states currently have such laws, mostly adopted in 1970s and 1980s. • Large range in magnitude: from 1 to 30 years; most are between 1 and 5 years • Does not include mandatory minimums
Why Add-on Gun Laws? • Substantial heterogeneity in law adoption • Plausible exogeneity in adoption • Effected group already receives substantial sentence (mean 5.5 yrs, median 5 yrs for minimum robbery sentence) Impact can be interpreted as deterrent effect
Contributions • Event study methodology takes advantage of temporal variation in law introduction • Broad applicability due to length and breadth of data • Primary outcome, robbery rate, is of substantial interest • Numerous robustness checks and alternative specifications • Use of placebo laws to calculate correct standard errors
Methodology • Event Study: a - reporting agency, t - time period, s - state y - outcome of interest, usually log gun robberies per capita Dist - dummy variable that is 1 in state s if period t is exactly i periods after the effective date in that state, and zero otherwise State dummies (λs), year dummies (γt), state-specific time trends (ωst), and time varying state characteristics (xst) are included.
Methodology • Difference-in-Difference • Triple Difference
Data • Outcomes obtained from Uniform Crime Reports reported offenses from 1965-2002 • Data is from 521 most populous agencies, representing 40% of US population, and has been hand-cleaned • Gun law effective dates from Vernick and Hepburn (2003); dates of passage collected from state codes and state legislative journals
Robustness Checks • Other outcomes (total robberies, total assaults, burglaries) • Linear specification (instead of log) • Restricted comparison group (only states ever passing add-on laws) • State level data • Variation of population weighting, source of population data • Higher order time trends • Lagged dependant variables • Trend breaks
Conclusions • Significant deterrent effect of add-on gun laws leads to a 5% reduction in gun robberies • This reduction in crime does not appear to come from substitution to other types • Numerous robustness checks support basic findings