The term "serial killer" was coined either by FBI agent Robert Ressler or by Dr. Robert D. Keppel in the 1970s (the credit for the term is still disputed); "serial killer" entered the popular vernacular in large part due to the well-publicized crimes of Ted Bundy and David Berkowitz ("Son of Sam") in the middle years of the decade. The term allows criminologists to distinguish those who claim victims over a long period of time from those who claim multiple victims all at once (mass murderer). A third type of multiple killer is a spree killer.

The following are brief definitions of these three types:

Serial Killer
A serial killer is someone who commits three or more murders over an extended period of time with cooling-off periods in between. In between their crimes, they appear to be quite normal, a state which Hervey Cleckley and Robert Hare call the "mask of sanity." There is frequently—but not always—a sexual element to the murders.

Mass Murderer
A mass murderer, on the other hand, is an individual who kills three or more people in a single event and in one location. The perpetrators sometimes commit suicide, meaning knowledge of their state of mind and what triggers their actions is often left to more speculation than fact. Mass murderers who are caught sometimes claim they cannot clearly remember the event.

Spree Killer
A spree killer commits multiple murders in different locations over a period of time that may vary from a few hours to several days. Unlike serial killers, however, they do not revert to their normal behavior in between slayings.

All of the above types of crimes are usually carried out by solitary individuals. There have been examples in all three categories whereby two or more perpetrators have acted together. Author Michael Newton states that this happens in about a third of the cases. Lee Boyd Malvo and John Muhammad are prime examples. Both are known for the Beltway sniper attacks

Serial killers are generally, but not always, male. Noted female exceptions include Aileen Wuornos, Myra Hindley and Erzsébet Báthory. Serial killers are specifically motivated by a variety of psychological urges, primarily
power and sexual compulsion. They feel inadequate and worthless, often owing to humiliation and abuse in childhood or the pressures of poverty and low socioeconomic status in adulthood, and their crimes give them a feeling of power, both at the time of the actual killing and also afterwards for power-control killers. The knowledge that their actions terrify entire communities and often baffle police adds to this sense of power. This motivational aspect separates them from contract killers and other multiple murderers who are motivated by profit. For example, in Scotland during the 1820s, William Burke and William Hare murdered people in what became known as the "Case of the Body Snatchers." They would not count as serial killers by most criminologists' definitions, however, because their motive was financial. Of course, people do things for multiple motivations.

This ignores the other, more miniscule serial killer catagories, visionary and missionary types, and barely covered the lust killer. The above stated definition covers only power-control killers, and hedonistic killers. A visionary killer is motivated tocommit their series of murders by delusional visions and/or thoughts (e.g Richard Chase). These are highly disorganized generally and lie in delusion rather than sexual urges. The missionary killer has an object, such as the Zebra Killers or KKK members, their series of kills has an actual object and is not rooted in sexual urges. The hedonistic lust killer does not care generally about the attention or the actions
pre-mortem, but wish to kill solely for use of the body post-mortem like Ed Gein.

In many cases, a serial killer will plead not guilty by reason of insanity in a court of law. This defense is almost uniformly unsuccessful. In most U.S. jurisidictions (i.e., the states), the legal definition of insanity is still generally based upon the classic common law "right or wrong" test delineated by an English court in the 1843 M'Naughten case. The M'Naughten rule, as it's generally known in the legal profession, hinges upon
whether the defendant knows the difference between right and wrong at the time of the offense. With serial killers, extensive premeditation, combined with lack of any obvious delusions or hallucinations that would hinder the defendant's ability to elude detection after commiting multiple murders, make this defense extremely difficult. The United States Bureau of Justice Statistics defines a serial killing as: "[involving] the killing of several victims in three or more separate events." This definition is especially close to that of a spree killer, and perhaps the primary difference between the two is that a serial killer has a cooling-off period. They will commit a murder and temporarily feel sated until they feel their homicidal urges resurface. The time period between murders can vary between a few days to several years and will often decrease the longer the offender goes uncaught. For example, Jeffrey Dahmer murdered his second victim nine years after his first, but his last eight victims were murdered in a span of just seven months. Spree killers, on the other hand, do not have a cooling-off period and are in a state of constant hunting until they are caught or killed, even though their murder spree may sometimes extend to a period of several months.

Serial killers frequently have extreme sadistic urges. Ones who lack the ability to empathize with the suffering of others are frequently called psychopathic or sociopathic, terms which have been renamed among professional psychologists as antisocial personality disorder. Some serial killers engage in lust and torture murder, loosely defined terms involving, respectively, mutilation for sexual pleasure and killing
victims slowly over a prolonged period of time.

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