­The Zodiac Killer. John Wayne Gacy. The BTK Killer. Ted Bundy. Son of Sam. Jeffrey Dahmer­. The names and pseudonyms of these killers are burned into the collective consciousness of Americans, thanks to massive coverage in newspapers, books, films and TV specials. Many of those who have been captured appeared average -- attractive, successful, active members of the community -- until their crimes were discovered.

This kind of killer doesn't just "go crazy" one day and kill a lot of people. He doesn't kill out of greed or jealousy. So what makes a person not only murder, but murder multiple people over periods of days, weeks and years? There's a special name for these types of murderers: serial killers. In this article, we'll learn about what makes them tick.

The term "serial killer" was coined in the mid-1970s by Robert Ressler, the former director of the FBI's Violent Criminal Apprehension Program. He chose "serial" because the police in England called these types of murders "crimes in a series" and because of the serial films that he grew up watching. Prior to this, these types of crimes were sometimes known as mass murders or stranger-on-stranger crime.

The FBI defines a serial killer as one who murders three or more victims, with "cooling-off" periods between each murder [source: U.S. Code]. This sets them apart from mass murderers, who kill four or more people at the same time (or in a short period of time) in the same place, and spree killers, who murder in multiple locations and within a short period of time. Serial killers usually work alone, kill strangers and kill for the sake of killing (as opposed to crimes of passion).

According to a recent FBI study, there have been approximately 400 serial killers in the United States in the past century, with anywhere from 2,526 to 3,860 victims [source: Hickey]. However, there's no way to really know how many serial killers are active at any point in time -- experts have suggested numbers ranging from 50 to 300, but there's no evidence to support them.

Serial murders also appear to have increased over the past 30 years. Eighty percent of the 400 serial killers of the past century have emerged since 1950 [source: Vronsky]. Why this is happening is a question of some debate; there is no answer, just as there is no simple answer as to why some people become serial killers.

In the next section, we'll look at some classifications of serial killers in use by criminal researchers and profilers so we can begin to understand this phenomenon.

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