Mass murders are events where four of more individuals are killed without a cooling off period. The international distribution of these events indicates mass murders are commonly motivated by familial, psychogenic and instrumental factors. Legislation to restrict access to firearms was developed in Australia in 1996 after a mass homicide, and was associated with a distinct near elimination of mass murders. However, mass murders appear to be relatively frequent in China, with the common and effective use of knifes to dispatch multiple victims. It appears that restriction of the availability of automatic weapons contributed to the reduction of mass murders of the psychogenic type, but more research is needed to fully understand mass murders.