For some people, the only way to explain serial murder is to say that serial killers are "insane." Some serial killers do plead "not guilty by reason of insanity" as a defense, but are all of them "insane" or even mentally ill? According to the U.S. Code, an insanity defense means that "at the time of the commission of the acts constituting the offense, the defendant, as a result of a severe mental disease or defect, was unable to appreciate the nature and quality or the wrongfulness of his acts. Mental disease or defect does not otherwise constitute a defense" [Source: U.S. Code].
Basically, a serial killer arguing "not guilty by reason of insanity" must prove that he did not understand right from wrong at the time that he killed. But it can be difficult to prove that he really did not understand that his actions would result in the death of the victims. Only two serial killers have successfully pled insanity. John Douglas, long-term head of the FBI's Investigative Support Unit, believes that serial killers "don't have a problem understanding what death means, and that they have the power to kill" [source: JohnDouglas.com].
Some serial killers have been diagnosed by psychologists and psychiatrists as psychopaths. The official term in the Diagnostic and Standard Manual of Mental Disorders, fourth edition (DSM-IV) is antisocial personality disorder (APD). According to the DSM-IV, a person with APD follows a pattern of "disregard and violation of the rights of others occuring since age 15 years." This pattern includes seven factors (three of which must be met for diagnosis), such as "failure to conform to social norms," "irritability and aggressiveness" and "lack of remorse" [source: Vronsky]. Psychopaths are not insane -- they do know right from wrong. But this diagnosis may explain their behavior during their killing cycles.
Some researchers theorize that serial killers have brain damage or other biological abnormalities that contribute to their actions. Damage to areas like the frontal lobe, the hypothalamus and the limbic system can contribute to extreme aggression, loss of control, loss of judgment and violence. Henry Lee Lucas, who was convicted of 11 murders, was shown to have extreme brain damage in these areas, probably the result of childhood abuse, malnutrition and alcoholism. Arthur Shawcross, another 11-time serial killer, was found to have had several brain injuries, including two skull fractures. While in prison, he suffered from headaches and often blacked out. Bobby Joe Long, convicted of nine murders, stated at one point, "After I'm dead, they're going to open up my head and find that just like we've been saying a part of my brain is black and dry and dead" [source: Scott].
In the next section, we'll see how law enforcement catches serial killers.