Jack Levin, co-director of the Brudnick Center on Violence at Northeastern University, said so many variables are at play when examining homicides that it’s “almost impossible to give a definitive explanation for the dramatic decline.”

But he said the aging population definitely makes a difference. “There’s a larger number of older people, the baby boomers are in their late 50s and 60s, and they simply don’t commit the large number of violent street crimes that young people do,” Levin said.