What defines a serial killer? According to the FBI, a serial killer is someone who commits at least three murders over more than a month with an emotional cooling off period in between.
This definition, however, is problematic for a variety of reasons, one being that it doesn't fit the common understanding of the term. Suppose a gang member ends up shooting three people in street fights with members of other gangs over a ten-year period with plenty of cooling down periods. Strictly speaking, this gang member is a serial killer by the above definition, but he would not fit the common conception of what a serial killer is.
A serial killer very commonly has a deviant sexual motive. The National Institute of Justice provides a definition of serial killing that is closer to the common conception. According to them, it involves committing two or more murders with a psychological motive and sadistic sexual overtones. On this conception, serial killing can be understood as a type of sex crime—a monstrous version of normal male sexuality.
There are multiple problems with this definition as well. One is that it only seems to apply to male serial killers (and perhaps a few females). Female serial killers often are not motivated by sexual gratification but rather by a twisted sense of love, sympathy, or altruism. A stereotypical example of a female serial killer is a nurse who kills her suffering patients because she wants to end their suffering.
But the sex-crime definition of a serial killer also is too narrow to capture the common conception of serial killing even for males. The Zodiac killer in the San Francisco Bay area was not driven by any obvious sexual movies but rather by his ability to generate fear and terror in the Bay area and become famous.
Finally, the idea of serial killers being sexually motivated does not always extend to killers who suffer from psychosis. There may not be a sexual motive underlying the killings of a person targeting his victims because an internal hallucinatory voice tells him that he must do this to stay alive.
So how to understand the concept of a serial killer? It appears that the concept is best understood as a prototype concept. Prototype theory as developed by the American psychologist Eleonor Rosch and colleagues is a theory of concepts that deviates from a traditional view that takes concepts to be analyzable in terms of necessary and sufficient conditions. For example, for a number to be even it is necessary and sufficient that it is divisible by the number two. The traditional theory works well for mathematical concepts but is not super-promising for most non-mathematical concepts.
Prototype theory is an extension of 20th century Austrian philosopher Ludwig Wittgenstein’s well-known theory of family resemblance. Wittgenstein’s legendary example is that of a game. Wittgenstein thought that no definition could be given of the concept of a game that would capture both professional sports and child’s play. As a result of this, he suggested that something is a game, if it resembles the most evident types of games closely enough, for example, soccer games, Trivial Pursuit or hide-and-seek.
Rosch and her colleagues suggested a theory of how we classify the world around us. On their view, the world doesn’t come divided up into categories. Our basic understanding of the world, which is necessary for all decision-making and action, consists in placing things into categories. As they put it:
The world consists of a virtually infinite number of discriminably different stimuli. One of the most basic functions of all organisms is the cutting up of the environment into classifications by which non-identical stimuli can be treated as equivalent. (“Classification of Real-World Objects: Origins and Representations in Cognition”, p. 383)
We use prototypes to understand the world, they argue. Prototypes are things that most clearly fall under a given concept according to our ordinary understanding of things. Soccer games are prototypes for game, chairs, and sofas are prototypes for furniture, robins are prototypes for birds and men are prototypes for humans (unfortunately). Whether something falls under the concept is determined by its resemblance to the prototypes. Because love seats resemble sofas, they fall under the category furniture.
Whether or not something falls under a concept is a matter of degree. For example, when two-hundred Americans were asked to rank examples of furniture in terms of how good examples they were, the following items scored highest: Chair/sofa, couch/table, easy chair, dresser, rocking chair, coffee table, rocker, love seat, chest of drawers, desk and bed. At the very end of the list we find things like: Rug, pillow, wastebasket, sewing machine, stove, refrigerator and telephone. The latter items do not fall under the concept of furniture to a very high degree. Most items belong to more than one category. For example, a telephone can be both an electrical device and a piece of furniture, even if it’s more of an electrical device than it is a piece of furniture. A prototype concept does not have determinate boundaries. There are items that definitely belong to a category and items that definitely do not. A chair clearly belongs to furniture; a gorilla clearly does not. But some items do not clearly belong to a category and do clearly not belong to the category. For example, there is no determinate answer to the question of whether a walk-in closet or a carpet does or does not belong to the category of furniture.
Among the prototypical serial killers that stand out in the mind of the public are: (1) Ted Bundy, targeting young college-age women with particular hair styles, resembling the girlfriend to which he was most attached, (2) Jeffrey Dahmer, targeting young men at bars and luring them back to his apartment under false pretenses of being a photographer in need of models, and (3) the Zodiac killer, targeting mostly couples owing to a relationship failure and a desire for revenge, fame, and mind games. Other serial killers may be so-called because they share a lot of features in common with the prototypes—along the lines of Rosch's theory.
The prototype definition of a serial killer allows for a killer to be more or less of a serial killer depending on how closely they resemble the prototypes. Just as we extend the term "furniture" to items like book shelves and shoe racks, which are not prototypes, we extend the notion of a serial killer to people who kill because they hear voices demanding them to kill or a nurse who kills her patients out of mercy. The latter two examples may not be perfect examples of serial killers but they fit the concept well enough to be included.
Rosch,, E., "Classification of Real-World Objects: Origins and Representations in Cognition", pp. 212–222 in Johnson-Laird, P.N. & Wason, P.C., Thinking: Readings in Cognitive Science, Cambridge University Press, (Cambridge), 1977.