Nine of the 25 mass shooters in recent years had psychiatric problems, and others had 'limited conflict resolution skills', say researchers from Clemson University.
They say that mass school shootings—defined as any shooting in which more than four people are killed—have become an epidemic this century; 66 people have been killed in the 18 years of this century, which is more than the total number killed in mass shootings throughout the 20th century.
Aside from psychiatric problems, another common pattern has been that all the shooters were young white males, aged between 11 and 18.
"One alarming trend is that the overwhelming majority of 21st-century shooters were adolescents, suggesting that it is now easier for them to access guns, and that they more frequently suffer from mental health issues or limited conflict resolution skills," said lead researcher Antonis Katsiyannis.
A ban on assault rifles and closer background checks could help reduce the incidents of mass shootings, the researchers say.