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Do mass shootings cause more mass shootings? Research is divided
Two mass shootings in California last week that killed dozens of people in public spaces, as well as a third attack days earlier that killed six family members in their home, have again raised questions about whether such violent incidents inspire new ones.
Data on mass public shootings going back to 1966 suggests such attacks are becoming more frequent, said James Densley, co-founder of the Violence Project, a nonprofit research center.
But it's not yet clear if shootings that come close together in time are intertwined.
"You find even in random simulations, there are times when there's a cluster and times when there are not," said James Alan Fox, a criminology professor at Northeastern University who serves on U.S. TODAY's Board of Contributors.
Are mass shootings happening more often?
The frequency of fatal mass shootings has increased from 23 a year in 2010 to 27 a year through the end of 2022, based on five-year averages of data gathered by U.S. TODAY, The Associated Press and Northeastern University.
That includes incidents in which at least four people were fatally shot, excluding the shooter, and a variety of motives—from hate crimes to family violence and gang-related attacks.
The number of incidents categorized as caused by hate, terrorism or indiscriminate violence, which tend to draw significant public attention, has increased from two a year to five. Similarly, attacks in public places like schools, hospitals and malls have increased from seven a year to 10 a year.
What are trends in gun violence generally?
Overall, the number of U.S. gun deaths soared to records levels during the COVID-19 pandemic, and firearms became the leading cause of death among children and teens.
There are shootings every day in the United States, and about 20,000 people have been fatally shot each year since 2020, according to the nonprofit Gun Violence Archive. About another 20,000 have died by firearm suicide, according to the nonprofit.
When have there been strings of high-profile mass shootings?
Densley, of the Violence Project, pointed to several examples of mass shootings that happened close together in recent years.
- August 2019: A gunman killed 23 people at a Walmart in El Paso, Texas. The next day, a gunman killed nine people in Dayton, Ohio. At the end of the month, a gunman killed seven people in Midland and Odessa, Texas.
- March 2021: A gunman killed eight people at Atlanta-area spas. Days later, a gunman killed 10 people at a supermarket in Boulder, Colorado. Days after that, a gunman killed four people at an office in Orange County, California.
- Summer 2022: In May, there were mass shootings at a supermarket in Buffalo, New York, and an elementary school in Uvalde, Texas. In June, it was a hospital in Tulsa, Oklahoma. In July, a Fourth of July parade in Highland Park, Illinois.
- November 2022: A gunman opened fire at an LGBTQ nightclub in Colorado Springs, Colorado, killing five people. Two days later, a store manager killed six people at a Walmart in Chesapeake, Virginia.
Do mass shootings prompt more mass shootings?
Available studies have reached different conclusions on whether one mass shooting raises the likelihood that others will follow soon after.
Research on mass shootings is limited, in part because of a congressional amendment that for more than two decades barred federal public health funding from being used to study gun violence.
In 2015, a team of researchers at Arizona State and Northeastern Illinois universities analyzed the U.S. TODAY/AP/Northeastern database for trends. It found the probability of a new fatal mass shooting increased for 13 days after each incident, an effect that held up when looking at school shootings, in particular.
Densley said the Violence Project has noticed an "anecdotal" increase in clusters of high-profile mass shootings but is working on an analysis to determine whether the apparent clusters are statistically significant.
Fox, meanwhile, said the clustering some people see in lists of mass shootings is the result of pure chance. He said the 2015 study didn't take into account important differences in types of shootings and the level of attention they receive.
A 2021 study by Fox and others found no short-term "contagion" effect linked to newspaper and television coverage of mass shootings.
What about longer-term effects on other shooters?
In public health research, contagion measures whether one event increases the likelihood of a new one in the immediate aftermath, similar to how the spread of virus can quickly escalate case counts.
Copycat attacks, in contrast, are not triggered by the timing of the latest killing seen on cable news. Instead, those shooters are motivated by a previous event, often years prior. For instance, some shootings—such as the attack on Columbine High School in Colorado in 1999—have been mentioned by later killers as part of their motive.
Researchers said it's rare to be able to trace such a motive directly but note evidence suggests teens can be particularly influenced.
Are there ways to prevent copycat killings?
Public campaigns like Don't Name Them and No Notoriety demand that the news media not publish the identity or personal details of mass killers. The campaigns argue that such attention inspires other attackers seeking fame.
Fox disagrees with the premise, saying it's the shooting itself—not the shooter—that could cause contagion. In the El Paso Walmart shooting that targeted Latinos, he said, white supremacists "applauded" the killing without knowing much about the gunman.
Fox acknowledges, however, that some news outlets go overboard and provide details about the shooter that shift the focus from the harm caused.
Sherry Towers, a data scientist at Arizona State University who led the 2015 study on shooting "contagion," said news media coverage reflects a broader appetite for information about mass shootings.
"The public is actually reading the details obsessively, more than they need to inform themselves. It's almost as a form of lurid entertainment. It's something we as a society need to look at."
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