The Ultimate Serial Killer Quiz

Robert Ressler, former director of the FBI Violent Criminal Apprehension Program, coined the term in the mid-1970s.

As opposed to mass murderers, who kill four or more people in a short period of time in the same place, serial killers murder three or more victims with "cooling-off" periods in between.

The FBI estimates there have been approximately 400 serial killers responsible for anywhere from 2,526 to 3,680 victims during the last century.

Although serial killers are classified based on either motive or organizational and social patterns, they don't always fall clearly into either category.

Gain killers are a type of process-focused serial killer who murder for a perceived profit.

More than 80 percent of serial killers are white men in their 20s or 30s. They tend to be intelligent and usually target white women.

Often, serial killers exhibit three behaviors in childhood known as the MacDonald triad: bed-wetting, arson and cruelty to animals.

Although you can never know with certainty why a person kills, childhood abuse, brain injury and insanity have all been attributed to serial murderers.

According to FBI profiler John Douglas, a signature "is a ritual, something the subject does intentionally for emotional satisfaction -- something that isn't necessary to perpetuate the crime."

The modus operandi reflects what the serial killer had to do to commit the crime and includes everything from luring and restraining the victim to the exact method of murder.