What makes a serial killer?
In the past thirty years, multiple definitions of serial murder have been used by law enforcement, clinicians, academia, and researchers. While these definitions do share several common themes, they differ on specific requirements, such as the number of murders involved, the types of motivation, and the temporal aspects of the murders. To address these discrepancies, attendees at the Serial Murder Symposium examined the variations in order to develop a single definition for serial murder.
Previous definitions of serial murder specified a certain number of murders, varying from two to ten victims. This quantitative requirement distinguished a serial murder from other categories of murder (i.e. single, double, or triple murder).
Most of the definitions also required a period of time between the murders. This break-in-time was necessary to distinguish between a mass murder and a serial murder. Serial murder required a temporal separation between the different murders, which was described as: separate occasions, cooling-off period, and emotional cooling-off period.
Generally, mass murder was described as a number of murders (four or more) occurring during the same incident, with no distinctive time period between the murders. These events typically involved a single location, where the killer murdered a number of victims in an ongoing incident (e.g. the 1984 San Ysidro McDonalds incident in San Diego, California; the 1991 Luby’s Restaurant massacre in Killeen, Texas; and the 2007 Virginia Tech murders in Blacksburg, Virginia).
There has been at least one attempt to formalize a definition of serial murder through legislation. In 1998, a federal law was passed by the United States Congress, titled: Protection of Children from Sexual Predator Act of 1998 (Title 18, United States Code, Chapter 51, and Section 1111). This law includes a definition of serial killings:
The term ‘serial killings’ means a series of three or more killings, not less than one of which was committed within the United States, having common characteristics such as to suggest the reasonable possibility that the crimes were committed by the same actor or actors.
Although the federal law provides a definition of serial murder, it is limited in its application. The purpose of this definition was to set forth criteria establishing when the FBI could assist local law enforcement agencies with their investigation of serial murder cases. It was not intended to be a generic definition for serial murder.
The Symposium attendees reviewed the previous definitions and extensively discussed the pros and cons of the numerous variations. The consensus of the Symposium attendees was to create a simple but broad definition, designed for use primarily by law enforcement.
One discussion topic focused on the determination of the number of murders that constituted a serial murder. Academicians and researchers were interested in establishing a specific number of murders, to allow clear inclusion criteria for their research on serial killers. However, since the definition was to be utilized by law enforcement, a lower number of victims would allow law enforcement more flexibility in committing resources to a potential serial murder investigation.
Motivation was another central element discussed in various definitions; however, attendees felt motivation did not belong in a general definition, as it would make the definition overly complex.
The validity of spree murder as a separate category was discussed at great length. The general definition of spree murder is two or more murders committed by an offender or offenders, without a cooling-off period. According to the definition, the lack of a cooling-off period marks the difference between a spree murder and a serial murder. Central to the discussion was the definitional problems relating to the concept of a cooling-off period. Because it creates arbitrary guidelines, the confusion surrounding this concept led the majority of attendees to advocate disregarding the use of spree murder as a separate category. The designation does not provide any real benefit for use by law enforcement.
The different discussion groups at the Symposium agreed on a number of similar factors to be included in a definition. These included:
• one or more offenders
• two or more murdered victims
• incidents should be occurring in separate events, at different times
• the time period between murders separates serial murder from mass murder
In combining the various ideas put forth at the Symposium, the following definition was crafted:
Serial Murder: The unlawful killing of two or more victims by the same offender(s), in separate events.