A serial killer is traditionally defined as a person who has killed three or more people over a period of more than a month, with down time (a "cooling off period") between the murders, and whose motivation for killing is usually based on psychological gratification. Serial killers are not the same as mass murderers, nor are they spree killers, who commit murders in two or more locations with virtually no break in between; however, cases of extended bouts of sequential killings over periods of weeks or months with no apparent "cooling off" period or "return to normalcy" have caused some serial killer experts to suggest a hybrid category of "spree-serial killer".