The Dilemma: The guy sitting next to you at the bar keeps insisting that John Wayne Gacy wasn’t a serial killer but a mass murderer, which is really creepy. But is he right?

People You Can Impress: Authors of true crime novels and suckers for semantics.

The Quick Trick: The creepy guy at the bar is full of it: Gacy was a serial killer because he committed many murders over a long period of time; mass murderers commit many murders all at once.

The Explanation:
The difference here is all about the details—but then, any CSI fan knows that the magic of police work is in the little things. According to the U.S. Department of Justice, Statistics Bureau (and yes, there really is such a thing), “mass murder” is a single event at one location involving the murder of four or more people. Kill three people at once, therefore, and you’re merely a homicidal jerk. Terrorism and government-sanctioned murder often are considered mass murder.

Serial killers, on the other hand, kill in a series of events. The killers usually don’t know their victims (the opposite is true with mass murderers), they almost always have “cooling off ” periods between murders, and they usually derive sexual excitement from the killings. To qualify as a serial killer, one needs three victims. It rather goes without saying, but serial killers tend to be pretty screwed up individuals. Although there are records of serial killers going back to at least 1400, the term wasn’t coined until the 1970s, when killers Ted Bundy and David “Son of Sam” Berkowitz were frequently in the news.

Serial Killers

GILLES DE RAIS (1404–1440): Once one of the richest men in France, Rais raped, tortured, and murdered between 80 and 200 boys—and a few girls—on the grounds of his various estates.

Long before there was Aileen “Monster” Wuornos, there was ELIZABETH “THE BLOODY LADY” Bathory (1560–1614). Some sources claim that Bathory, a Hungarian countess, tortured and killed 2,000 young girls (mostly peasants, but some lower gentry).

When it comes to British serial killers in the 19th century, Jack the Ripper gets all the press. But MARY ANN COTTON (1832–1873) was more prolific, killing as many as 21 people. Cotton probably poisoned four of her husbands, a variety of her friends and in-laws, and several of her own children with arsenic.

Mass Murderers

The term “going postal” has its roots in the case of one PATRICK SHERRILL, a disgruntled former postman who walked into the post office in Edmond, Ill. on August 20, 1986, and killed 14 employees before committing suicide.

On November 1, 1955, JACK GILBERT GRAHAM put his mother on a flight from Denver to Portland with a dynamite bomb in her suitcase. (Graham wanted her life insurance money.) The bomb exploded midair, killing all 44 people aboard.

This post was excerpted from the mental_floss book What’s the Difference? For more columns like this, click here.