ROCHESTER, Minn. — As forensic phenomena with searing overtones for public health in the United States, the mass shootings that took 29 lives in El Paso, Texas, and Dayton, Ohio, last week share a familiar set of characteristics, say researchers in Minnesota. The incidents are also officially now on the rise.

"It's the shootings that take place in public places in which four or more victims are killed with a gun in the absence of any other criminal activity," says Grant Duwe, a researcher at the Minnesota Bureau of Corrections and national authority on mass murder in the United States. "These are the incidents that have long galvanized the public. They have triggered debates over gun control, mental health, school safety, workplace violence policies, video games, the list goes on and on."

Duwe, author of Mass Murder in the United States: A History, draws a distinction between mass shootings, which tend to occur in the home or in the course of a crime, and mass public shootings.

"When we are just looking at mass public shootings," he says, "the data through 2016 show that the incidence had not increased over the previous 30 or 40 years. Up until then, what had changed was the severity. The killings had become deadlier, with more persons being killed, and that was true even before the mass shooting in Las Vegas took place. What the most current data reveal, however, and this is through 2018 which is the latest data available, is that not just the severity but the incidence of these incidents has increased as well."

Since first emerging in the mid 1970's, mass public shootings had averaged about four a year, he says. "But then in 2017 there were seven, and in 2018 there were 10, which is the most we've ever had in a given year. If we're looking at trends in crime over time, the per capital incidence has increased."

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Mass murder with all its terrible repercussions is as old as America, Duwe says. After years of decline, starting in the 1970's, modern rates of mass murder began to resemble those found during the Great Depression, and before that, the wave of mass murders in late 19th century.

"In the 19th century it was more of the haves who were murdering the have-nots," he says. "The mass murders occurred in connection with the post-Civil War south, where there was a lot of racial terrorism. We also saw it with labor strikes of the period, where a lot of recent immigrants were murdered.

"What changed in the 20th century is that those who perceive themselves as have-nots are murdering those who they now see as the haves." In the case of the killings in El Paso, he says, while much is still unknown, the shooter apparently targeted immigrants. "Even the people who are carrying out these types of attacks don't perceive themselves as the haves. In most instances, they perceive themselves as have-nots," and as attacking people "who the perpetrator believes don't deserve to have what they have."

The thinking has been termed "aggrieved entitlement," by forensic scientists. Research on school shooters by Peter Langman has paired it with a preoccupation with personal failures, perceived humiliation, romantic rejection and a host of other assorted failures to measure up according to the value system of masculinity. On the far end of the spectrum, these motivations can become persecutory delusions, suicidal fantasies that can become homicidal. It's almost exclusively associated with men.

"I'm not sure what the solution is for the fact that we see 98% of mass public shooters are male," says Duwe, who calls the variable "the elephant in the living room."

"As criminologists we've known this for decades, and nothing is done about it...some folks have talked about it in terms of toxic masculinity. That could be a way to describe it. It warrants more scrutiny."

Writing on in an oped published Monday in the Los Angeles Times, Jillian Peterson and James Densley of The Violence Project in St. Paul, a nonpartisan think tank and database of mass public shootings funded by the Department of Justice, listed four common characteristics in their study of of the perpetrators: childhood trauma, a recent personal crises, a fascination with other shooters and access to firearms.

"We all can slow the spread of mass shootings," they wrote. "Don't like or share violent content. Don't read or share killers' manifestos and other hate screeds posted on the internet."

The researchers expressed skepticism about school lock downs, which they said "hand potential shooters the script for mass violence," and called for the better control of weapons, "through age restrictions, permit to purchase licensing, universal background checks, safe storage campaigns and red-flag laws, measures that help control firearm access for vulnerable individuals or people in crisis."