Has the rate of random mass shootings in the United States increased? Over the past 30 years, the answer is definitely yes. It is also true that the total U.S. homicide rate has fallen by over half since 1980, and the gun homicide rate has fallen along with it. Today, Americans are safer from violent crime, including gun homicide, than they have been at any time since the mid-1960s.

Mass shootings, defined as four or more fatalities, fluctuate from year to year, but over the past 30 years there has been no long-term increase or decrease. But "random" mass shootings, such as the horrific crimes last Friday in Newtown, Conn., have increased.

Alan Lankford of the University of Alabama analyzed data from a recent New York Police Department study of "active shooters"—criminals who attempted to murder people in a confined area, where there are lots of people, and who chose at least some victims randomly. Counting only the incidents with at least two casualties, there were 179 such crimes between 1966 and 2010. In the 1980s, there were 18. In the 1990s, there were 54. In the 2000s, there were 87.

If you count only such crimes in which five or more victims were killed, there were six in the 1980s and 19 in the 2000s.

Why the increase? It cannot be because gun-control laws have become more lax. Before the 1968 Gun Control Act, there were almost no federal gun-control laws. The exception was the National Firearms Act of 1934, which set up an extremely severe registration and tax system for automatic weapons and has remained in force for 78 years.

Nor are magazines holding more than 10 rounds something new. They were invented decades ago and have long been standard for many handguns. Police officers carry them for the same reason that civilians do: Especially if a person is attacked by multiple assailants, there is no guarantee that a 10-round magazine will end the assault.

The 1980s were much worse than today in terms of overall violent crime, including gun homicide, but they were much better than today in terms of mass random shootings. The difference wasn't that the 1980s had tougher controls on so-called "assault weapons." No assault weapons law existed in the U.S. until California passed a ban in 1989.

Connecticut followed in 1993. None of the guns that the Newtown murderer used was an assault weapon under Connecticut law. This illustrates the uselessness of bans on so-called assault weapons, since those bans concentrate on guns' cosmetics, such as whether the gun has a bayonet lug, rather than their function.

What some people call "assault weapons" function like every other normal firearm—they fire only one bullet each time the trigger is pressed. Unlike automatics (machine guns), they do not fire continuously as long as the trigger is held. They are "semi-automatic" because they eject the empty shell case and load the next round into the firing chamber.

Today in America, most handguns are semi-automatics, as are many long guns, including the best-selling rifle today, the AR-15, the model used in the Newtown shooting. Some of these guns look like machine guns, but they do not function like machine guns.

Back in the mid-1960s, in most states, an adult could walk into a store and buy an AR-15 rifle, no questions asked. Today, firearms are the most heavily regulated consumer product in the United States. If someone wants to purchase an AR-15 or any other firearm, the store must first get permission for the sale from the FBI or its state counterpart. Permission is denied if the buyer is in one of nine categories of "prohibited persons," including felons, domestic-violence misdemeanants, and persons who have been adjudicated mentally ill or alcoholic.

Since gun controls today are far stricter than at the time when "active shooters" were rare, what can account for the increase in these shootings? One plausible answer is the media. Cable TV in the 1990s, and the Internet today, greatly magnify the instant celebrity that a mass killer can achieve. We know that many would-be mass killers obsessively study their predecessors.

Loren Coleman's 2004 book "The Copycat Effect: How the Media and Popular Culture Trigger the Mayhem in Tomorrow's Headlines" shows that the copycat effect is as old as the media itself. Johann Wolfgang von Goethe's 1774 classic "The Sorrows of Young Werther" triggered a spate of copycat suicides all over Europe. But today the velocity and pervasiveness of the media make the problem much worse.

A second explanation is the deinstitutionalization of the violently mentally ill. A 2000 New York Times study of 100 rampage murderers found that 47 were mentally ill. In the Journal of the American Academy of Psychiatry Law (2008), Jason C. Matejkowski and his co-authors reported that 16% of state prisoners who had perpetrated murders were mentally ill.

In the mid-1960s, many of the killings would have been prevented because the severely mentally ill would have been confined and cared for in a state institution. But today, while government at most every level has bloated over the past half-century, mental-health treatment has been decimated. According to a study released in July by the Treatment Advocacy Center, the number of state hospital beds in America per capita has plummeted to 1850 levels, or 14.1 beds per 100,000 people.

Moreover, a 2011 paper by Steven P. Segal at the University of California, Berkeley, "Civil Commitment Law, Mental Health Services, and U.S. Homicide Rates," found that a third of the state-to-state variation in homicide rates was attributable to the strength or weakness of involuntary civil-commitment laws.

Finally, it must be acknowledged that many of these attacks today unfortunately take place in pretend "gun-free zones," such as schools, movie theaters and shopping malls. According to Ron Borsch's study for the Force Science Research Center at Minnesota State University-Mankato, active shooters are different from the gangsters and other street toughs whom a police officer might engage in a gunfight. They are predominantly weaklings and cowards who crumble easily as soon as an armed person shows up.

The problem is that by the time the police arrive, lots of people are already dead. So when armed citizens are on the scene, many lives are saved. The media rarely mention the mass murders that were thwarted by armed citizens at the Shoney's Restaurant in Anniston, Ala. (1991), the high school in Pearl, Miss. (1997), the middle-school dance in Edinboro, Penn. (1998), and the New Life Church in Colorado Springs, Colo. (2007), among others.

At the Clackamas Mall in Oregon last week, an active shooter murdered two people and then saw that a shopper, who had a handgun carry permit, had drawn a gun and was aiming at him. The murderer's next shot was to kill himself.

Real gun-free zones are a wonderful idea, but they are only real if they are created by metal detectors backed up by armed guards. Pretend gun-free zones, where law-abiding adults (who pass a fingerprint-based background check and a safety training class) are still disarmed, are magnets for evildoers who know they will be able to murder at will with little threat of being fired upon.

People who are serious about preventing the next Newtown should embrace much greater funding for mental health, strong laws for civil commitment of the violently mentally ill—and stop kidding themselves that pretend gun-free zones will stop killers.