Regression to the Mean, Murder Rates, and Shall-Issue Laws

摘要

The relationship between state murder rates and the liberalization of conditions under which a citizen can obtain a permit to carry a concealed weapon (shall-issue laws) is controversial and important for policy. Many analyses have been done during the last decade, but regression to the mean has been ignored with the exception of two papers which concluded that it did not matter. We consider state murder rates for 1976-2001 and compare relative murder rate slopes (relative to the U.S. murder rate) for the five years following state adoption of shall-issue laws to the five years preceding for the 25 states becoming shall-issue in 1981-1996. We find strong evidence for regression to the mean. Using both a random and a fixed effects model, we compare analyses ignoring the regression effect via a paired t-test to those controlling for it by conditioning on the pre shall-issue slopes. We find that controlling for regression to the mean changes the sign of the estimated intervention effect on murder rate slopes from negative to positive, has strong impact on statistical significance, and gives no support to the hypothesis that shall-issue laws have beneficial effects in reducing murder rates.