Higher arrest or conviction rates and the death penalty reduce "normal" murder rates, say researchers; but the only policy that influences multiple-victim public shootings is the passage of laws allowing the carrying of concealed handguns. Public shootings are more sensitive than other violent crimes to concealed handguns, say John R. Lott Jr. and William M. Landes, of the University of Chicago Law School, and carry laws reduce both the number of shootings as well as their severity.

Between 1977 and 1995, 14 states adopted shall issue laws -- which give officials little discretion in issuing permits to qualified individuals. The number of states without shall issue laws declined from 43 to 29 and the percentage of the U.S. population in those states fell from 91.5 to 68 percent.

• States without shall issue laws consistently account for the overwhelming majority (often over 90 percent) of deaths and injuries.

• In states with shall issue laws, there was a sharp drop in multiple murders and injuries per 100,000 persons after the law's passage, with such murders falling by 89 percent and injuries by 82 percent.

• The drop occurred largely during the first full year after a state enacted its law.

• Overall, the decline was so large that there were zero multiple killings in three of the eight years after the passage of a law.

• A narrower sample of multiple shootings reported in the first section of the New York Times show a decline of 91 percent in the multiple shootings rate after a state adopted a shall issue law.

The most comprehensive study of laws permitting individuals to carry concealed weapons indicates they reduce overall murder rates by about 10 percent, with similar declines in other violent crimes. And contrary to a popular misconception, concealed handgun permit holders are virtually never involved in the commission of crime, let alone murder.

Source: John R. Lott, Jr. and William M. Landes, "Multiple Victim Public Shootings, Bombings, and Right-to-Carry Concealed Handgun Laws: Contrasting Private and Public Law Enforcement," John M. Olin Law & Economics Working Paper No. 73, April 1999, University of Chicago Law School.

For text http://papers.ssrn.com/sol3/papers.cfm?abstract_id=161637