What's the Difference Between a Killer's Signature and M.O.?

True crime shows, documentaries, and podcasts are everywhere these days, not to mention all the crime-focused movies and TV shows—like NBC's Law & Order: SVU, CBS's CSI: Crime Scene Investigation, and Netflix's Mindhunter. And you've probably heard terms like signature and M.O. being thrown around a lot without much explanation as to what they mean, or how they're different.

If you're confused about the difference between them, well, you’re not alone. As former FBI agent and behavioral analyst John Douglas notes in his book Mindhunter: Inside the FBI's Elite Serial Crime Unit (which the Netflix series is based on), "Both [signature and modus operandi] are extremely important concepts in criminal investigations analysis, and I have spent many hours on witness stands of courtrooms throughout the country trying to get judges and juries to understand the distinction between them."

Douglas, who was recently in New York to promote his new book, The Killer Across the Table: Unlocking the Secrets of Serial Killers and Predators with the FBI's Original Mindhunter, out now, helped break down the difference between signature and M.O. for us.

M.O. stands for Modus Operandi, and, according to Douglas, it's a learned, dynamic behavior. "When a criminal starts perpetrating crimes, if something doesn't go right, he's got to perfect the M.O.," he tells Mental Floss. "He's got to get it better and better." In other words, unless an offender executes the perfect crime his first time out, he'll continue to tweak his M.O. as he goes. The method of committing the crime is modified for success. That's why, Douglas says, "you shouldn't link cases together strictly by modus operandi. … You don't do that because those characteristics could fit people that have nothing to do with the case as well."

But what you can use to link crimes together is an offender's signature, a term that Douglas says he coined. "A signature is a ritual—something [that] is done that is not necessary to perpetrate that particular crime," he says. "The signature is the ritual that is unique to the offender, and that's what you're looking for."

To demonstrate what he means, Douglas uses sports as an example. "It's like a baseball batter [who], before a ball comes in, does rituals," like touching his hat or cleats. "Or shooting a basketball: bounce it three times, [do a certain move], take the shot. It's not necessary to get it in the hoop or hit the ball, but in his mind he's got to do it. He's got to do it this way."

In Mindhunter, Douglas acknowledges that "the differences between M.O. and signature can be subtle." To demonstrate just how subtle, he compares two robbery cases. Both robbers made their captive undress; one "posed them in sexual positions, and took photographs of them" while the other did not take photos.

The latter made his hostages undress "so the eyewitnesses would be so preoccupied and embarrassed that they wouldn't be looking at him and so couldn't make a positive ID later on," Douglas writes. That's an example of M.O. The former robber is an example of a signature, because it wasn't something the offender had to do to rob the bank—and actually put him at risk of being caught, because he was in the bank longer. "It was something he clearly felt a need to do," Douglas writes.

Because the signature is unique to the offender, Douglas says that you can use it in trials: "A case in Washington state, the subject was posing the victims after he killed [them]. And all that was allowed for me to testify to."

There's one challenge with signatures, though. "You can only see it when it starts showing up in repetitive crimes," Douglas says. "You can't look at a single case and say, 'Oh, this was the signature.' Say the victim is posed—that may end up being the signature, but you've got to compare it to something, later on."

As criminology professor Scott A. Bonn, Ph.D., points out in a post for Psychology Today, "While every crime has an M.O., not all crimes have a signature." Now, whether you're listening to a true crime podcast, in search of serial killer gifts, or watching an episode of Mindhunter, you'll know the difference.

This article was originally published in 2019 and has been updated in 2022.