When Writing a Whodunit, Think of Dear Old MOM (Motive, Opportunity and Means)

A writer friend of mine once landed a book contract where the publisher asked for a “complex crime” at the core of the story. My friend contacted me, worried. “I’ve never written a crime! Can you give me any advice?” “Sure,” I said. “Think MOM. Motive, Opportunity and Means.”

In US criminal law, MOM encapsulates three sides of a crime necessary to convince a jury of guilt in a criminal proceeding. Did the defendant have a motive to commit the crime? Did the defendant have an opportunity, or chance, to accomplish the deed? Did the defendant also have the ability (means)?

Let’s look at some ways a fictional sleuth might use MOM in a story:

Conduct witness interviews. There’s the direct questions a sleuth might ask, and which we often hear in movies, such as “Where were you at nine o’clock on the night of April 12, Miss Smith?” (opportunity). But also think about your sleuth asking questions that delve into a suspect’s character (motive), history of violence or peacefulness (means/motive or lack of means/motive), or knowledge about using a certain type of weapon (means). A sleuth might also interview other people who’ve seen that suspect use the same type of weapon or conduct certain violent acts.

Examine the murder weapon. Let’s say your sleuth wants to prove the killer was someone other than the person charged with the crime. Your sleuth might looks for clues that show lack of means on the murder weapon (such as bloody hand imprints that are larger than the defendant’s or a strand of hair stuck in blood that’s a different color than the defendant’s).

Recreate the homicide event. Your sleuth might reconstruct the event at the scene of the crime to prove a person had access to a weapon (means) as well as opportunity. For example, the reconstruction might show how easily a suspect could have reached for the murder weapon. Or, conversely, that the suspect wasn’t tall enough to reach the weapon, strong enough to lift it, or maybe even literate enough to have read the instructions on how to use the weapon. As a lawyer, Abraham Lincoln once reconstructed a crime scene to prove that a witness couldn’t possibly have seen what she claimed to have seen because there wasn’t ample lighting to clearly see at the time the incident occurred.

Find an alternate suspect. Your sleuth might research other people who had motive, opportunity, and means to commit a crime. For example, the sleuth might analyze someone’s character for motive (such as his/her history of outbursts toward the victim), look for clues tying another person to the murder weapon (for example, his/her knowledge of how to use that weapon), or establish someone had opportunity (by analyzing a person’s timeline).

A last point to keep in mind: a court cannot convict based solely on motive, opportunity and means. A lawyer must provide convincing proof of all three. Obtaining this proof is, of course, what your sleuth (a detective, private investigator, amateur sleuth) has been doggedly investigating, with the help of MOM, throughout the course of your story.

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