By Ari N. Schulman, Wall Street Journal

Public shootings have become a familiar American spectacle in recent years, and two more occurred in recent weeks. The details are still unfolding, but so far these episodes seem to fit the general pattern. At Los Angeles International Airport, a young man entered a public area and started firing. Three days later, in the midst of intense media coverage of the first event, another young man did the same at a shopping mall in suburban New Jersey. One penned a note beforehand about his actions, the other a manifesto. Only the L.A. shooter appears to have meant to kill others, but both apparently planned to die in highly publicized blazes of terror.

Someday soon, we are likely to awake to news of yet another rampage shooting, one that perhaps will rival the infamous events at Columbine, Virginia Tech, Aurora and Newtown. As unknowable as the when and who and where of the next tragedy is the certainty that there will be one, and of what will follow: The tense initial hours as we watch the body count tick higher. The ashen-faced news anchors with pictures of stricken families. Stories and images of the fatal minutes. Reports on the shooter’s journals and manifestos. A weary speech from the president. Debates about guns and mental health.

Underlying this grim national ritual, and the pronouncements from all quarters that mass shootings are “senseless,” is the disturbing feeling that these acts are beyond our understanding. As the criminologist and forensic psychiatrist Park Dietz writes, we talk about these acts as if they arise from “alien forces.” So we focus our efforts on thwarting future mass shooters—catching them through the mental health system, or making it harder for them to get guns, or making it easier for others with guns to stop them. Some enterprising minds have even suggested that schoolchildren be trained to gang-rush them.

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.

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