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. All serial killers suffer from some form of Antisocial Personality Disorder. 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.