A serial killer is someone who kills at least three victims one by one in a series of sequential murders, with a form of psychological gratification as the primary motive. The need for psychological gratification distinguishes serial killers from other types of murderers, who usually murder for things like profit or revenge. Because an element of psychological instability is involved in serial killing, many serial killers are committed to mental institutions if and when they are caught, reflecting a general belief that they are not of sound mind.

While many people think of serial murder as a crime which emerged in the 20th century, it is actually quite ancient. Documents from places like China indicate that serial killers were operating well before the common era, and some particularly notorious serial killers like Erzsebet Bathory and Jack the Ripper date to periods before the 20th century. It is true, however, that the number of serial killings seems to have increased in the 20th century, perhaps due to increased population or social and cultural shifts.

As a general rule, someone does not start out life as a serial killer. The progress from ordinary individual to serial killer may begin early in life, but it proceeds in stages, beginning with crimes like arson, progressing to animal abuse, and then moving into the realm of human abuse. The reasons why people develop into serial killers are not known, although many show signs of childhood trauma and abuse.

Psychologists believe that all serial killers demonstrate some form of antisocial personality disorder, along with other psychiatric conditions which cause them to have a view of the world which is fundamentally different from that of other people. Many psychologists further divide serial killers into so-called “organized” serial killers, who put their high intelligence to use when planning their crimes,