This week, a shooting attack at a grocery store in Boulder, Colo., left 10 people dead. Just days earlier, eight people were fatally shot in a rampage targeting spas in the Atlanta area.

As with almost every mass shooter in recorded U.S. history, both of the suspects in the recent attacks are men.

A staggering 98% of these crimes have been committed by men, according to The Violence Project, a nonpartisan research group that tracks U.S. mass shooting data dating back to 1966.

“Men just are generally more violent,” said the group’s president, Jillian Peterson, a forensic psychologist and professor of criminology and criminal justice at Hamline University. “There are many theories as to why that is.”

Researchers say that men, more than women, tend to externalize their problems and look for others to blame, which can translate into anger and violence. And when women do choose violence, guns are not typically their weapon of choice.

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If men vastly outnumber women as mass shooters, those perpetrators are often a model for the next male shooters who “see themselves in them,” Peterson said, a phenomenon she noted is particularly true in young, white men. Violence Project data show that white men are disproportionately responsible for mass shootings more than any other group.

“They study the perpetrators that came before them,” she said. “Many school shooters study Columbine, for example; other university shooters study the Virginia Tech shooting. And they really are kind of using those previous shootings as a blueprint for their own.”