In the last week, more than 30 people have died in three separate mass shootings in Gilroy, El Paso and Dayton, Ohio. We believe that analyzing and understanding data about who commits such massacres can help prevent more lives being lost.

For two years we've been studying the life histories of mass shooters in the United States for a project funded by the National Institute of Justice, the research arm of the U.S. Department of Justice. We've built a database dating back to 1966 of every mass shooter who shot and killed four or more people in a public place, and every shooting incident at schools, workplaces and places of worship since 1999. We've interviewed incarcerated perpetrators and their families, shooting survivors and first responders. We've read media and social media, manifestos, suicide notes, trial transcripts and medical records.

Our goal has been to find new, data-driven pathways for preventing such shootings. Our data do reveal four commonalities among the perpetrators of nearly all the mass shootings we studied.

First, the vast majority of mass shooters in our study experienced trauma and exposure to violence at a young age. The nature of their exposure included parental suicide, physical or sexual abuse, neglect, domestic violence and/or severe bullying. The trauma was often a precursor to mental health concerns, including depression, anxiety, thought disorders or suicidality.

Second, practically every mass shooter we studied had reached an identifiable crisis point in the weeks or months leading up to the shooting. They often had become angry and despondent because of a specific grievance. For workplace shooters, a change in job status was frequently the trigger. For shooters in other contexts, relationship rejection or loss often played a role.  

Third, most of the shooters had studied the actions of other shooters and sought validation for their motives. People in crisis have always existed. But in the age of 24-hour rolling news and social media, there are scripts to follow that promise notoriety in death. Societal fear and fascination with mass shootings partly drive the motivation to commit them. Hence, as we have seen in the last week, mass shootings tend to come in clusters. They are socially contagious. Perpetrators study other perpetrators and model their acts after previous shootings. Many are radicalized online in their search for validation from others that their will to murder is justified.

Fourth, the shooters all had the means to carry out their plans. Once someone decides life is no longer worth living and that murdering others would be a proper revenge, only means and opportunity stand in the way of another mass shooting. Is an appropriate shooting site accessible? Can the would-be shooter obtain firearms? In 80% of school shootings, perpetrators got their weapons from family members, according to our data. Workplace shooters tended to use handguns they legally owned. Other public shooters were more likely to acquire them illegally.

Potential shooting sites can be made less accessible with visible security measures such as metal detectors and police officers. And weapons need to be better controlled, through age restrictions, permit-to-purchase licensing, background checks, safe storage campaigns and red-flag laws – measures that help control firearm access for vulnerable individuals or people in crisis.

Another step is to try to make it more difficult for potential perpetrators to find validation for their planned actions. Don't read or share killers' manifestos and other hate screeds posted on the internet.  

Proactivity needs to extend also to the traumas in early life that are common to so many mass shooters. Those early exposures to violence need addressing when they happen with ready access to social services and high-quality, affordable mental health treatment in the community.

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