The Detective’s Trinity: Means, Motive, and Opportunity

By David Griffin Brown

Almost all mystery fiction is built around three investigatory goals: means, motive, and opportunity. This Sherlockian trinity has been handed down through a century of detective novels. Give your suspects two of each and your killer all three, and your mystery novel will write itself. Maybe.

Except real detectives will tell you this isn’t how murder gets solved. David Simon spent a year embedded with Baltimore detectives for his book Homicide: A Year on the Killing Streets. He found that while motive is interesting, it’s rarely useful. More often, detectives close cases with witnesses and confessions, and the fastest route to a confession is a suspect who can’t explain away their means and opportunity, not why they may or may not have offed someone. Find the how, Simon’s detectives will tell you, and the who tends to follow.

So why did this trinity become the backbone of an entire genre, how do you actually build a mystery around it, and what can the divide between fiction and police work teach us about character? Let’s take these one at a time.

Holmes Got Us Partway There

Means, motive, and opportunity started out as courtroom shorthand, a way to explain circumstantial evidence to a jury. Arthur Conan Doyle is largely responsible for pulling that shorthand into fiction.

But Doyle wasn’t nearly as fair with his readers as the genre he inspired would later demand. Take The Adventure of the Dancing Men. Holmes decodes a cipher, sends off a batch of telegrams to police in New York, and confirms a killer’s identity using information the reader doesn’t see until the end. In The Hound of the Baskervilles, it’s even worse. Holmes vanishes from the narrative for a huge chunk of the novel, allowing Watson to blunder around in the fog. Then Holmes finally shows up and explains all the evidence that his clueless (literally) partner missed. So Doyle may have established the trinity, but then he used his detective’s superior access to information to keep readers in the dark.

The Golden Age Mystery Rules

By 1928, this had become enough of a problem that writers started soapboxing against it. S.S. Van Dine, an American art critic writing detective novels under a pseudonym, published twenty rules for the genre. His first and most important: the reader must have equal opportunity with the detective to solve the case, with every clue plainly stated. He went on to ban romance subplots, plot-rescuing secret passages, secret societies, hypnotism, and telepathy.

A year later, Ronald Knox, a Catholic priest with a mystery-writing side hustle, published his own ten commandments. The criminal has to be introduced early in the story, though the reader can’t have access to their private thoughts. (Which is really about concealing motive without obscuring opportunity.) No undiscovered poisons or gadgets requiring pages of scientific explanation at the climax. (In other words, keep it plausible.) No accident may ever hand the detective the solution. No unexplained intuition may turn out to be right. And no twins allowed.

Some of these rules look absurd now, but the underlying logic holds up. A mystery is a promise, and means and opportunity are the measurable parts of that promise a reader can verify. Motive is the part that gets to stay hidden a little longer.

Building Your Trinity: A Mystery Writer’s Guide

Each piece of the trinity plays a different role in your manuscript.

Means is the most mechanical of the three, and also the one most likely to get you into trouble if you’re not careful. Means has to be plausible without needing a whack of scientific explanation, as per Knox, but it also has to be interesting. Sue Grafton handles this well in A Is for Alibi, where the killer spikes the victim's allergy medication with oleander. Poison is a fairly ordinary means, but with this clever twist, the killer is long gone by the moment of death, which creates a handy alibi in the process.

Poe went the opposite direction in The Murders in the Rue Morgue, making the means so bizarre (a razor-wielding orangutan) that no reasonable reader could guess it, while still (technically) keeping it fair, since Dupin walks through every physical clue that leads to this hairy conclusion. Both approaches work because the means is consistent with the internal logic of the story and story world. What doesn’t work is a means that requires the reader to accept new information that they weren’t given a chance to notice.

Opportunity is all about access, and it can work well when applied to a swath of suspects. Christie’s Murder on the Orient Express hands opportunity to nearly everyone aboard—which makes perfect sense by the end since the murder was a collaboration. Still, you don’t need your murderer to be an entire trainload of people. Casting a wide net with opportunity gives your detective more leads to chase down.

Another fun trick you can play is to falsify opportunity. Christie gives us a good case study in And Then There Were None, where Justice Wargrave obscures opportunity by faking his own death midway through the novel. Once the other guests believe he’s dead, no one can implicate him as the killer. Same goes for the reader.

Motive is the most important element of the trinity, and you want to come up with more than one. First, there will be an apparent motive, which will draw the detective’s focus and create your most powerful misdirection for readers. Later, when the true motive is revealed, the entire tableau of evidence will reconfigure, and suddenly a character who could never have been implicated by the false motive is now looking pretty damn guilty.

Blake Snyder of Save the Cat! fame built this concept into his breakdown of the mystery genre, which he refers to as the Whydunnit rather than the Whodunnit. While the whole point of the investigation may be to figure out who the murderer is, the most structurally important question (and the source of the twist) comes down to motive.

In Magpie Murders, Anthony Horowitz gives us several false motives before the true one surfaces. Once Susan Ryeland realizes Alan Conway’s death wasn’t a suicide, she discovers that Donald Leigh resents Conway for stealing his novel, and James Taylor stands to lose everything now that he's been cut from the will. But the true motive belongs to someone who was never on her radar. It turns out the publisher Charles Clover killed Conway to stop him revealing, in an upcoming interview, an anagram buried in his own manuscript that would have humiliated the mystery genre and sunk the publishing house.

In The Name of the Rose, Umberto Eco spends most of the novel handing readers plausible motives that all turn out to be wrong. Heresy, monastic politics, sexual shame, and Inquisitor Bernard Gui's public persecutions all suggest the killer is driven by religious or personal scandal. There’s even a false confession under torture to further confuse matters. The true motive belongs to Jorge of Burgos, who has been killing to keep a single lost manuscript out of circulation, a treatise on comedy he believes would corrode faith itself if it ever got out. Because laughter is evil?

Gillian Flynn pulls off an especially fun trick in Gone Girl. The reader is led to believe Nick's affair is the reason Amy has vanished (and is presumed dead). But at the midpoint, it turns out that Amy is alive and well, and the affair was actually her motive to frame Nick for murder, not his motive to kill his wife.

What Real Detectives Can Teach You About Character

Roughly half of the murders committed in America each year go unsolved. Up here in Canada, we manage to solve about 75% of them. The figure in both countries was over 90 percent in the 1960s. But the number that perhaps matters more for novelists is that about 80% of murders are committed by someone the victim knows. Which is why solving a murder usually has less to do with clever deduction than with mapping relationships.

Investigators call this core technique victimology. Before you can find an offender, you evaluate why a particular victim was targeted. A real detective doesn't start with a list of suspects and rank them according to means, motive, and opportunity. They begin with the victim and reconstruct their life until the offender shakes out.

Compare that to how most mysteries are structured, including the classic ones. Christie hands you a train car full of supposed strangers, or an island of ten people with no apparent connection between them. That's compelling, but the stranger homicide is the rarer and much harder case to solve in real life. The far more common real-world case, the one that actually gets solved, is the intimate one: a spouse, an employee, a neighbour, a friend.

If you want your detective's methods to feel grounded rather than lifted from Holmes, give them a victimology scene before you give them a suspect list. Who did the victim call in their last week? What routines didn’t they follow? Who were they most likely to be in conflict with? Kinsey Millhone does a version of this in A Is for Alibi. She spends far more time excavating who Laurence Fife actually was, his affairs, his enemies, his blackmail arrangement with Sharon Napier, than she does assembling a murder board.

The second thing real investigation offers you is the value of witnesses and confession. Cases don't close because a detective deduces the sneaky truth, but rather because someone decides to go to the police about something they saw or heard. Or with a secret they just can’t keep quiet about.

In violent circles, cooperating with police can be dangerous. Sharon Napier in A Is for Alibi is a good example of this: she knows something, she's weighing whether telling Kinsey is worth the risk, and she ends up murdered before she can say a word. That's perhaps a sharper approach than the all-too-typical red herring of suspects lying to conceal an affair.

These two real-world tools can give you an edge on the Holmesian playbook. Means, motive, and opportunity are still important structural elements to building a mystery, but victimology and reluctant witnesses can help you build stronger characterization into the mix.

In Conclusion: We’ve Arrived at Character

Literary theorist Tzvetan Todorov argued that a mystery is always two stories in one: the crime itself, and the investigation. Means, motive, and opportunity belong entirely to the hidden story. The truth of the crime is generally going to stayed buried until the final chapter, and the detective and the killer both need serious poker faces to keep the trick working. A detective who talks through every possible theory will tip the reader off. And a killer with guilt stamped across their face isn’t helpful either.

But we have, by now, seen pretty much every method, every motive, and every reversal the trinity can produce. The hidden story has been told in all possible configurations. So the genre's remaining unexplored territory isn't so much the crime anymore, but the people on either side of it. Give your detective doubt, exhaustion, a personal stake that complicates their objectivity. Give your victim a healthy dose of social dysfunction. Give your killer such a compelling motive that readers feel they are almost justified in their crime. In other words, bring nuance to all your characters.

Readers still want to be fooled, and we still want a detective who can close cases and, in doing so, restore some order to a chaotic world. But more and more, contemporary detective fiction is driven by nuance. We want to feel something for the people caught up in the chaos, and not just admire the mechanism that resolves it.

David Griffin Brown is an award-winning short fiction writer and co-author of Immersion and Emotion: The Two Pillars of Storytelling, Story Skeleton: The Classics, and Fake Query Letters by Dead Authors. He holds a BA in anthropology from UVic and an MFA in creative writing from UBC, and his work has appeared in literary magazines such as the Malahat Review, White Wall Review, and Grain. He has published three novels: When the Sky Breaks, We've Come for Your Eggs, and No Country for Old Dragons. David founded Darling Axe Editing in 2018, and as part of his Book Broker interview series, he has compiled querying advice from over 100 literary agents. As an editor, he pays special attention to structure, relationship arcs, and voice. David lives in Victoria, Canada, on the traditional territory of the Songhees and Kosapsum Nations.