An 1829 illustration of British serial killer William Burke murdering Margery Campbell.
A serial killer is typically a person who murders three or more persons, with the murders taking place over more than a month and including a significant period of time between them. While most authorities set a threshold of three murders, others extend it to four or lessen it to two.
Psychological gratification is the usual motive for serial killing, and many serial murders involve sexual contact with the victim. The Federal Bureau of Investigation (FBI) states that the motives of serial killers can include anger, thrill-seeking, financial gain, and attention seeking, and killings may be executed as such. The victims may have something in common; for example, demographic profile, appearance, gender or race. Often the FBI will focus on a particular pattern serial killers follow. Based on this pattern, this will give key clues into finding the killer along with their motives.
Although a serial killer is a distinct classification that differs from that of a mass murderer, spree killer, or contract killer, there exist conceptual overlaps between them. Some debate exists on the specific criteria for each category, especially with regard to the distinction between spree killers and serial killers.
Holmes & Holmes 1998, Serial murder is the killing of three or more people over a period of more than 30 days, with a significant cooling-off period between the murders The baseline number of three victims appears to be most common among those who are the academic authorities in the field. The time frame also appears to be an agreed-upon component of the definition.
Petherick 2005, p. 190 Three killings seem to be required in the most popular definition of serial killing since they are enough to provide a pattern within the killings without being overly restrictive.
Flowers 2012, p. 195 in general, most experts on serial murder require that a minimum of three murders be committed at different times and usually different places for a person to qualify as a serial killer.
Schechter 2012, p. 73 Most experts seem to agree, however, that to qualify as a serial killer, an individual has to slay a minimum of three unrelated victims.
^Hough & McCorkle 2016, p. [...] Serial killing has been defined by different researchers or groups as either two or more, three or more or even four or more people killed over at least one month with a cooling off period between each of the murders.
^Geberth 1995, p. ? "The base population was 387 serial murderers, who killed (under various motivations), three or more persons over a period of time with cooling-off periods between the events. The author identified 232 male serial murderers who violated their victims sexually".