On average, there are about 20 mass mur­ders every year, and the trend has been steady since the 1970s, said James Alan Fox, a pro­fessor of crim­i­nology at North­eastern Uni­ver­sity in Boston and author of five books on mass murder.

The typ­ical mass mur­derer is a socially iso­lated white man in his 30s or 40s. He has no crim­inal record, but his life has been marked by failure and frus­tra­tion, and he decides that his family, boss or co-​​workers must pay with their lives, Fox said.

The rarest form of mass murder is the com­pletely random shooting,” Fox said. “Those per­pe­tra­tors tend to be younger (in their 20s). They are more likely to have pro­found mental health issues, as opposed to the older guy who is quite sane, knows exactly what he is doing, and just decides that life is miserable.”

The random shooter, Fox said, feels “the whole world is unfair, someone has to pay, and it doesn’t matter who.”

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