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Study: Why some mass killings and school shootings seem to be contagious

Police tape surrounds the parking lot behind the AME Emanuel Church in Charleston, S.C., where nine people were shot dead. (AP/Stephen B. Morton)

Every mass killing or school shooting leads to a debate over whether publicizing the tragedies influences violent crime. A new statistical study has found that such events do, in fact, seem to be contagious.

Researchers from Arizona State University, in a study published Thursday in PLOS One, are the latest to add to the long-standing debate over whether media can plant ideas for "copycat murderers," and they focus mostly on the math of the issue.

They created a model to analyze high-profile tragedies, and found that mass killings — events with four or more deaths — and school shootings create a "period of contagion" that lasts an average of 13 days.

Under this model, for every three mass killings, an average of one more happens. And for every four or five school shootings — including those with no fatalities — another shooting takes place.

[Has there been one school shooting a week since Sandy Hook?]

Previous studies have indicated that suicide can spread through social groups, with the news of one death provoking others to end their own lives. The phenomenon is especially common among adolescents -- and in the age of social media, widely-publicized suicides can spread the effect much further than the rumor mill at a high school.

[Suicide contagion and social media: The dangers of sharing ‘Genie, you’re free’]

"It occurred to us that mass killings and school shootings that attract attention in the national news media can potentially do the same thing, but at a larger scale," Sherry Tower, an ASU professor and the study's author, said in a statement. "While we can never determine which particular shootings were inspired by unconscious ideation, this analysis helps us understand aspects of the complex dynamics that can underlie these events."

The study comes after a widely publicized shooting at a black church in Charleston, S.C., where 21-year-old Dylann Roof allegedly killed nine African Americans including the Rev. Clementa Pinckney, a state senator.

Following the tragedy, six black churches have burned, three of which are being investigated as arsons.

On average, mass killings involving firearms happen once every two weeks in the United States, according to the study, and school shootings happen once every month. The researchers also found that these tragedies happen more often in states with high gun ownership.

For instance, a dozen mass shootings made headlines in 2012, including high profile cases in a Colorado movie theater, a Sikh temple in Wisconsin and an elementary school in Newtown, Conn.

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