A 2003 study by North­eastern Uni­ver­sity researchers James Alan Fox and Jack Levin found that only 3 to 4 per­cent of all acts of homi­cide involve mul­tiple victims.

It’s impos­sible to rule out that an ordi­nary person might someday commit extreme, irra­tional vio­lence. But in most cases, a seem­ingly normal killer prob­ably isn’t really as normal as he might seem, explains Dr. Alan J. Lipman, a licensed clin­ical psy­chol­o­gist and founder and director of the Center for the Study of Vio­lence in Wash­ington, D.C.

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