From the attacks of 9/11 to Monday's school shooting, after every mass murder there is an overwhelming urge to "do something" to prevent a similar attack.

But since Adam ate the apple and let evil into the world, deranged individuals have existed.

Most of the time they can't be locked up until it's too late. It's not against the law to be crazy -- in some jurisdictions it actually makes you more viable as a candidate for public office.

It's certainly not against the law to be an unsociable loner. If it were, Ralph Nader would be behind bars right now, where he belongs. Mass murder is often the first serious crime unbalanced individuals are caught committing -- as appears to be in the case of the Virginia Tech shooter.

The best we can do is enact policies that will reduce the death toll when these acts of carnage occur, as they will in a free and open society of 300 million people, most of whom have cable TV.

Only one policy has ever been shown to deter mass murder: concealed-carry laws. In a comprehensive study of all public, multiple-shooting incidents in America between 1977 and 1999, the inestimable economists John Lott and Bill Landes found that concealed-carry laws were the only laws that had any beneficial effect.

And the effect was not insignificant. States that allowed citizens to carry concealed handguns reduced multiple-shooting attacks by 60 percent and reduced the death and injury from these attacks by nearly 80 percent.

Apparently, even crazy people prefer targets that can't shoot back. The reason schools are consistently popular targets for mass murderers is precisely because of all the idiotic "Gun-Free School Zone" laws.

From the people who brought you "zero tolerance," I present the Gun-Free Zone! Yippee! Problem solved! Bam! Bam! Everybody down! Hey, how did that deranged loner get a gun into this Gun-Free Zone?

It isn't the angst of adolescence. Plenty of school shootings have been committed by adults with absolutely no reason to be at the school, such as Laurie Dann, who shot up the Hubbard Woods Elementary School in Winnetka, Ill., in 1988; Patrick Purdy, who opened fire on children at Cleveland Elementary School in Stockton, Calif., in 1989; and Charles Carl Roberts, who murdered five schoolgirls at an Amish school in Lancaster County, Pa., last year. Continued...