Jared Loughner, the young man charged in the shooting rampage that wounded Representative Gabrielle Giffords, was considered so threatening and mentally unstable that his community college in Tucson banned him from campus. But this judgment did not in any way impede Loughner's ability to buy a Glock semiautomatic pistol and several rounds of ammunition to go with it.

The Tucson shootings should prompt calls for more thorough background checks on gun buyers. It should spark calls for reinstating the ban on military-style assault weapons, which Congress allowed to expire in 2004. It should prompt Congress to limit the purchase of semiautomatic weapons—which can rip off dozens of shots within seconds—to police departments and the military. (What game hunter or target shooter needs to own an attack weapon?)

But when it comes to guns, Americans do not live in a rational universe. In the face of yet another gun massacre, Americans seem to think that there is only one answer: more guns.

In Arizona and across the nation, the defense of gun ownership no longer has much to do with the old idea of protecting the rights of hunters and hobbyists. Defenders of gun rights now portray gun ownership as necessary for self-defense and for deterring shooters like Loughner.

Within days of the shooting, the Arizona Citizens Defense League prepared legislation that would require the state to offer firearms training to politicians and their staff. The philosophy behind that bill was summed up by Arizona state legislator Jack Harper: "When everyone is carrying a firearm, nobody is going to be a victim."

Yet the Tucson murders reveal the folly of that approach. The gun Loughner used enabled him to get off an estimated 31 shots in a matter of seconds. No matter how fast the response by an armed civilian, it would not have prevented that assault. Furthermore, the act of taking up arms is likely to increase the chaos and endanger innocent lives, as Joe Zamudio discovered. Zamudio was at the Tucson shopping mall when he heard gunfire. He grabbed hold of the gun in his pocket and was ready to shoot a man he saw holding a gun, only to realize—seconds before pulling the trigger—that the man had wrestled the gun away from the real shooter. Zamudio also hesitated because he feared that in drawing his gun he might be mistaken for a second gunman.

The notion that the good guys will gun down the bad guys is a dangerous fantasy. When everyone carries a firearm, we can be sure that a lot more innocent people are going to be victims.

The debate over gun-safety laws is notoriously strewn with contested statistics about what gun laws do and don't accomplish. But there is one set of statistics to which the gun lobby has never had a cogent response: the ones showing that the murder rate per 100,000 people is 5.3 in the U.S., but only 0.47 for Canada and 0.07 for the United Kingdom. Unless one assumes that Americans are just angrier and more violent than other people, something in U.S. society is out of whack. It has a lot to do with our attitude toward guns.