I’m using the book The Anatomy of Motive by John Douglas and Mark Olshaker to create believable antagonists. I’m in the middle of a discussion about arsonists, but the authors take a quick break from that to discuss the difference between signature and modus operandi. Then I’ll get back to arsonists.
(This signature courtesy of freedigitalphotos.net)
A signature is a trait an investigator would know or recognize as belonging to a single perpetrator. It does not change as the criminal gets better at his crimes. Modus operandi (or M.O.) is the means by which the crime is carried out. M.O. can evolve as the offender becomes more experienced, proficient, and successful at avoiding capture. Both are important to investigators, but there are differences that you, as a writer, need to know and understand.
“If you rob a bank at gunpoint, the gun is part of your M.O. True signature, on the other hand, is the aspect of the crime that emotionally fulfills the offender, and so it remains relatively the same. Torture, for example, is almost always a signature. No matter what crime an offender is committing, he doesn’t need to torture a victim to pull it off. He does so because of a sadistic emotional need. So, if an offender uses a gun to capture a victim so that he can torture her, you’ve got both an M.O. and a signature.”
Signature is the more important of the two in an investigation as it leads to the offender’s personality and motive. Policelink.monster.com lists two aspects of signature: aspect and behavior. “Aspect defines the theme or motive of an offense, and can include motivational categories such as profit, anger, retaliation, assertiveness, and sadism.” Behaviors are “committed to satisfy the emotional and psychological needs of the offender and usually define the theme of a crime.”
When you’re creating your antagonist, keep both these in mind. The M.O. is how your antagonist plans his crime in order to get away with it. Maybe it involves leaving a car running near the scene of the violence so he’ll have a quick getaway. Maybe it involves wearing a mask to hide his identity. Maybe it involves a bodysuit made of latex so he won’t leave forensic evidence behind (note: it’s REALLY hard to not leave forensic evidence behind, so if you go this route, research carefully or you’ll make mistakes).
The signature of your antagonist will be that thing or things he does to find emotional satisfaction from the crime. Maybe it involves choosing victims who look like his abusive mother. Maybe it involves urinating on his victim or the crime scene (disgusting, I know, but real-life offenders are known to do far worse. I’ll leave that one up to your imagination). Maybe he takes something from the scene of the crime as a souvenir. Whatever this signature is, once your investigator recognizes it as the signature, it will lead to an accurate profile and help catch the bad guy.