A serial killer is a person who murders three or more people with a 'cooling off' period between each murder and whose motivation for killing is largely based on psychological gratification. One hypothesis is that all serial killers suffer from some form of Antisocial Personality Disorder. They are usually not psychotic, and thus may 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, or sex.
The term serial killer is said to have been coined by Michigan State University alumnus and FBI agent Robert Ressler in the 1970s. Serial killer entered the popular vernacular in large part due to the widely publicized crimes of Ted Bundy and David Berkowitz in the middle years of that decade.
