A serial killer is someone who murders three or more people with a 'cooling off' period between each murder, whose motivation for killing is largely for psychological gratification.[1] All serial killers suffer from some form of Antisocial Personality Disorder.[2] They are usually not psychotic, and thus appear to be quite normal and often even charming, a state of adaptation which Hervey Cleckley calls the "mask of sanity." There is sometimes a sexual element to the murders. The murders may have been completed/attempted in a similar fashion and the victims may have had something in common, for example occupation, race, sex, etc.