To see if there are any public policies that reduce the carnage of public mass shootings, researchers at the University of Chicago compiled data on all multiple-victim public shootings that occurred in the United States from 1977 to 1999.

These were incidents in which at least two people were killed or injured in a public place, excluding gang wars or shootings during another crime, such as robbery.

• The United States averaged more than 20 such shootings annually, with an average of 1.5 people killed and 2.5 wounded per incident.

• The researchers examined the effects of different gun laws, such as waiting periods, as well the frequency and level of punishment, on the occurrence of these tragedies.

• They found no consistent deterrent effect from higher arrest and conviction rates, prison sentences or the death penalty -- in fact, the perpetrators are usually killed in the attack or commit suicide.

• To be effective, policies must deal with what motivates these criminals, which is to kill and injure as many people as possible.

Some appear to do it for the publicity, which is itself related to the amount of harm they inflict. Thus the best way to stop these attacks is to enact policies that can limit the carnage.

The only policy they found that effectively accomplishes this is the passage of right-to-carry laws. Some 32 states now give adults the right to carry concealed handguns as long as they do not have a criminal record or a history of significant mental illness.

• When states passed such laws during the 23 years studied, the number of multiple-victim public shootings declined by a dramatic 67 percent.

• Deaths and injuries from these shootings fell on average by 78 percent.

Source: John R. Lott Jr., "What we can do after Wakefield?" Boston Globe, December 28, 2000.