But the criminologists and psychologists who study mass killings aren't so baffled. While news reports often define mass shootings solely by body count, researchers instead look at qualitative traits like the psychology of the perpetrator, his relationship to the victims and how he carries out the crime. Building on Dr. Dietz's seminal 1986 article on mass murder in the Bulletin of the New York Academy of Medicine, researchers have used these characteristics to develop a taxonomy of mass killing outside of warfare. The major types include serial, cult, gang, family and spree killings.
But it is another kind that dominates the headlines: the massacre or rampage shooting. Whereas the other types of mass murder usually occur in multiple incidents or in a concealed manner, massacres occur as a single, typically very public event.
In 2004, Paul E. Mullen, then the director of the Victorian Institute of Forensic Mental Health, wrote an illuminating study based in part on his personal interviews with rampage shooters who survived their acts. He notes that rampage shootings tend to follow a definite pattern, what he called a "program for murder and suicide." The shooter, almost always a young man, enters an area filled with many people. He is heavily armed. He may begin by targeting a few specific victims, but he soon moves on to "indiscriminate killings where just killing people is the prime aim." He typically has no plan for escape and kills himself or is killed by police.
Among the more pervasive myths about massacre killers is that they simply snap. In fact, Dr. Mullen and others have found that rampage shooters usually plan their actions meticulously, even ritualistically, for months in advance. Like serial killers, massacre killers usually don't have impulsive personalities; they tend to be obsessive and highly organized. Survivors typically report that the shooters appear to be not enraged but cold and calculating.
Central to the massacre pattern is the killer's self-styling. James L. Knoll IV, the director of forensic psychiatry at the State University of New York's Upstate Medical University, describes in a 2010 article how perpetrators often model themselves after commandos, wearing military dress or black clothing. Investigators usually find they had a lifelong fascination with weaponry, warfare, and military and survivalist culture. Their methodical comportment during the act is part of this styling.
Contrary to the common assumption, writes author Michael D. Kelleher in his 1997 book "Flash Point," mass killers are "rarely insane, in either the legal or ethical senses of the term," and they don't typically have the "debilitating delusions and insidious psychotic fantasies of the paranoid schizophrenic." Dr. Knoll affirms that "the literature does not reflect a strong link with serious mental illness."
Instead, massacre killers are typically marked by what are considered personality disorders: grandiosity, resentment, self-righteousness, a sense of entitlement. They become, says Dr. Knoll, " 'collectors of injustice' who nurture their wounded narcissism." To preserve their egos, they exaggerate past humiliations and externalize their anger, blaming others for their frustrations. They develop violent fantasies of heroic revenge against an uncaring world.
Whereas serial killers are driven by long-standing sadistic and sexual pleasure in inflicting pain, massacre killers usually have no prior history of violence. Instead, writes Eric W. Hickey, dean of the California School of Forensic Studies, in his 2009 book "Serial Murderers and Their Victims," massacre killers commit a single and final act in which violence becomes a "medium" to make a " 'final statement' in or about life." Fantasy, public expression and messaging are central to what motivates and defines massacre killings.
Mass shooters aim to tell a story through their actions. They create a narrative about how the world has forced them to act, and then must persuade themselves to believe it. The final step is crafting the story for others and telling it through spoken warnings beforehand, taunting words to victims or manifestos created for public airing.
What these findings suggest is that mass shootings are a kind of theater. Their purpose is essentially terrorism—minus, in most cases, a political agenda. The public spectacle, the mass slaughter of mostly random victims, is meant to be seen as an attack against society itself. The typical consummation of the act in suicide denies the course of justice, giving the shooter ultimate and final control.
We call mass shootings senseless not only because of the gross disregard for life but because they defy the ordinary motives for violence—robbery, envy, personal grievance—reasons we can condemn but at least wrap our minds around. But mass killings seem like a plague dispatched from some inhuman realm. They don't just ignore our most basic ideas of justice but assault them directly.
The perverse truth is that this senselessness is just the point of mass shootings: It is the means by which the perpetrator seeks to make us feel his hatred. Like terrorists, mass shooters can be seen, in a limited sense, as rational actors, who know that if they follow the right steps they will produce the desired effect in the public consciousness.
Part of this calculus of evil is competition. Dr. Mullen spoke to a perpetrator who "gleefully admitted that he was 'going for the record.' " Investigators found that the Newtown shooter kept a "score sheet" of previous mass shootings. He may have deliberately calculated how to maximize the grotesqueness of his act.
Many other perpetrators pay obsessive attention to previous massacres. There is evidence for a direct line of influence running through some of the most notorious shooters—from Columbine in 1999 to Virginia Tech in 2007 to Newtown in 2012—including their explicit references to previous massacres and calls to inspire future anti-heroes.
Aside from the wealth of qualitative evidence for imitation in massacre killings, there are also some hard numbers. A 1999 study by Dr. Mullen and others in the Archives of Suicide Research suggested that a 10-year outbreak of mass homicides had occurred in clusters rather than randomly. This effect was also found in a 2002 study by a group of German psychiatrists who examined 132 attempted rampage killings world-wide.
There is a growing consensus among researchers that, whether or not the perpetrators are fully aware of it, they are following what has become a ready-made, free-floating template for young men to resolve their rage and express their sense of personal grandiosity.
Whatever the witch's brew of influences that produced this grisly script, treating mass killings as a kind of epidemic or contagion largely frees us from having to understand the particular causes of each act. Instead, we can focus on disrupting the spread.
There is a precedent for this approach in dealing with another form of violence: suicides. A 2003 study led by Columbia University psychiatrist Madelyn Gould found "ample evidence" of a suicide contagion effect, fed by reports in the media.
A 2011 study in the journal BMC Public Health found, unsurprisingly, that this effect is especially strong for novel forms of suicide that receive outsize attention in the press.
Some researchers have even put the theory to the test. In 1984, a rash of suicides broke out on the subway system in Vienna. As the death toll climbed, a group of researchers at the Austrian Association for Suicide Prevention theorized that sensational reporting was inadvertently glorifying the suicides.
Three years into the epidemic, the researchers persuaded local media to change their coverage by minimizing details and photos, avoiding romantic language and simplistic explanations of motives, moving the stories from the front page and keeping the word "suicide" out of the headlines. Subway suicides promptly dropped by 75%.
This approach has been recommended by numerous public health and media organizations world-wide, from the U.K., Australia, Norway and Hong Kong to the U.S., where in 2001 a similar set of reporting guidelines was released jointly by the Centers for Disease Control and Prevention, the National Institute of Mental Health and the surgeon general. It is difficult to say whether these guidelines have helped, since journalists' adherence to them has been scattered at best, but they might still serve as a basis for changing the reporting of massacres.
How might journalists and police change their practices to discourage mass shootings? First, they need to do more to deprive the killer of an audience:
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Rampage shootings are fed by many other sources that also must be addressed, of course. Many shooters have suffered bullying, which inflicts a sense of powerlessness that their actions aim to overcome. Some (though not most) shooters have had prior contact with mental health services, and many give recognizable warnings beforehand to friends, family or teachers. Institutionally and individually, we must learn to take these signs seriously and report them to authorities. Massacres also would not be nearly so lethal without the widespread availability of guns and high-capacity magazines designed more for offense than for defense.
But, guns aside, these factors are more or less perennial problems of human life and cannot, alone, bear the blame for rampage shootings.
In coverage of these events, the focus on insanity particularly risks playing into the need of potential future shooters to convince themselves that the world rejects them, rather than the other way around. The minority who really are psychotic, or just act impulsively, are even more likely to draw their ideas from the cultural ether.
Even in the U.S., with our fierce commitment to a free and open press, there are precedents for voluntary media restrictions. Courts and journalists usually recognize an overriding public interest in protecting the privacy of sexual assault victims and minors involved in crimes, and sometimes even the reputations of the accused. Safety, too, can trump the public right to know. Few media outlets would publish the instructions for making a bomb. Promulgating the template for rampage shootings is in similar need of restriction.
In the days after the Newtown shooting, the blogger Rod Dreher pointed to the closing lines of Albert Camus's "The Stranger," about an alienated young man who commits a senseless murder: "As if that blind rage had washed me clean, rid me of hope…For everything to be consummated, for me to feel less alone, I had only to wish that there be a large crowd of spectators the day of my execution and that they greet me with cries of hate."
The massacre killer chooses to believe it is not he but the world that is filled with hatred—and then he tries to prove his dark vision by making it so. If we can deprive him of the ability to make his internal psychodrama a shared public reality, if we can break this ritual of violence and our own ritual response, then we might just banish these dreadful and all too frequent acts to the realm of vile fantasy.
Mr. Schulman is the executive editor of the New Atlantis: A Journal of Technology and Society.