Their crimes are committed as a result of a compulsion that, in many but not all cases, has roots in the killer's (often dysfunctional) youth, as opposed to those who are motivated by financial gain (e.g., contract killers) or ideological/political motivations (e.g., terrorism, democide). Many times this compulsion is linked to the individual's sexual drive.

Some research suggests damage to certain regions of the brain (hypothalamus, the temporal lobe, and/or the limbic brain) or genetic traits passed down from abusive mothers can also play a part in the behavior of serial killers.

Defining serial murder

The term serial killer is widely believed to have been 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 that decade.

The term allows criminologists to distinguish those who kill several people over a long period of time from those who kill several people during a single event (mass murderers). A third type of multiple killer is a spree killer.