NEW YORK — The relentless series of mass killings across the globe poses a challenge for experts trying to analyze them without lapsing into faulty generalizations. Terms like contagion and copycat killing apply in some cases, not in others, they say, and in certain instances perpetrators' terrorist ideology intersects with psychological instability.
Some of the attacks, such as the coordinated assault on multiple targets in Paris last November, were elaborately planned operations by Islamic State adherents. However, they may have contributed to some of the other attacks by troubled individuals with no established ties to the militant group.
J. Reid Meloy, a San Diego-based forensic psychologist who has served as a consultant to the FBI's Behavioral Analysis Program, said some of the attackers appear to have identified with Islamic State as an outlet for their own seething emotions.
"In virtually every one of these cases, there was a deeply held personal grievance — loss, anger, humiliation," Meloy said. "When they come across Islamic State material, they're stimulated by that. They can take their personal grievance worldwide."
Meloy said two different syndromes could be surfacing in the series of attacks — contagion, in which one attack rapidly inspires imitation attacks, and copycat incidents, in which an individual seeks to emulate a previous perpetrator.
In Germany, for example, the deadliest of four recent attacks was carried out by an 18-year-old German-Iranian who killed nine people in Munich. Police said the young man had researched previous mass attacks, including the rampage in Norway by Anders Behring Breivik that killed 77 people exactly five years before the Munich attack.
A 2015 study by researchers at Arizona State University found significant evidence of a contagion effect in the United States. According to the study, the likelihood of new attacks rose significantly over a two-week period after any widely publicized mass killing or school shooting.
The study's lead author, professor Sherry Towers, said contagion likely played a role in the recent spate of killings in Europe, particularly those carried out by individuals.
"Planned, coordinated terrorist attacks are in a different class compared to lone wolf killings, since logistical matters likely play more of a role in the timing," Towers said in an email. "Lone wolf attacks by people who might have extremist leanings, but no solid connections to terrorist cells, and also might have other mental issues, are the types of events that would likely show more of a contagion effect."
The attacker who killed 84 people in Nice, France, on July 14th by driving through a holiday crowd was described as a psychologically troubled and violent man, not linked directly to Islamic State. But what had been a history of domestic violence and petty crime took on darker implications with his decision to use a truck as a killing machine, as called for in Islamic State and al-Qaida propaganda.
"We've got this situation where it seems like almost any public act of violence can be attributed to the Islamic State regardless of how nebulous a connection there is to the group," said Daniel Schoenfeld, an analyst with the Soufan Group, a security consulting firm.
Max Abrahms, a terrorism analyst who teaches political science at Northeastern University, has been using the term "loon wolf" to depict individuals whose attacks are as much the product of mental instability as of any form of radical ideology.
"They're seeing others do this and replicating their behaviors," said Abrahms, who suggested the phenomenon will be troublesome for counterterrorism investigators.
"Historically, governments were looking for people who seemed to be undergoing radicalization," he said. "Now, we're looking at people committing similar acts, but in some cases with no evidence they were being radicalized and maybe were being driven by mental instability."
Brian Jenkins, a senior adviser with the RAND Corporation and author of numerous books and articles on terrorism and security, said the emerging trends are the subject of ongoing research by experts, and may prove frustrating for members of the public.
"There's a certain degree of comfort with categorization — if we can put this guy in the terrorism bin or the mentally disturbed bin," he said. "The problem is, people are really complicated."
He said researchers are trying to determine why certain unstable people might be attracted by Islamic State ideology.
Islamic State "advertises atrocities," he said. "Normal people would look and say, 'Oh my God.' We're talking about those who are attracted to those images. Those images become, in a sense, an invitation to action."
Jenkins says there's a strong possibility that Islamic State-inspired violence in Europe could worsen before it eases, given the likelihood that military setbacks in Syria and Iraq will prompt many foreign fighters to return to their home countries.
"They will have difficulty becoming assimilated," he said. "Some are going to be frustrated and angry and will carry out acts of violence."
Suggested countermeasures vary — ranging from tougher policing and detention policies to more robust social-support programs. This week, some leading French media outlets pledged to stop publishing the names and images of attackers linked to the Islamic State group to prevent individuals from being inadvertently glorified.
However, Scott Atran, an anthropologist and terrorism researcher who has taught in France, Britain and the United States, worries that a contagion of mass killings in democratic nations is spreading beyond any organized political or ideological agenda, including that of the Islamic State.
"The big concern has to be that savage violence is adopted by other groups, leading to a quasi-anarchy like we haven't seen before, at least not in our lifetimes," he said in an email.
Associated Press writer Lori Hinnant in Paris contributed to this report.