The tragic shooting in Newtown, Connecticut on 14 December raises recurring questions. Who commits such atrocities and why? How can they be prevented? Despite their seemingly regular occurrence in the United States, the relative rarity of spree killings challenges attempts to understand them scientifically. Yet broad themes emerge if we examine published non-randomized samples that may help answer such questions.

Mass, or multiple, homicide has been defined as a single incident in which five or more victims are injured, at least three of them fatally.1 Mass murderers have been categorized according to their style: family annihilators, disciple murders, pseudo-commanders, disgruntled employees, and set and run killers.2 The events themselves can be divided into instrumental killings, victim specific killings, and massacres.3 Massacres are indiscriminate killings where victims are chosen by chance, situation, or peripheral affiliation. The autogenic massacre, which is distinct from the civil or military massacre, is carried out by an individual (or very occasionally more than one individual), serves …