In murder mysteries, there are two kinds of evidence: evidence which tells the detective what happened, and evidence which can get a practical result from society. The practical result is often a criminal conviction, but it need not be; a wedding being called off, the payment of an insurance policy, or the settling of a will all require similar sorts of evidence.
Of the two, it is the former type of evidence, not the latter type, which is of interest to the reader.
The main distinction between the two types of evidence is not really one of the strength of the evidence, that is, of the level of certainty which it conveys. In fact, one of the common features of murder mysteries is the early presence of highly convincing evidence which will convict an innocent person unless the detective uncovers the truth. No, the distinction is not in certainty. The distinction is, rather, what is required knowledge and understanding is required to apprehend the true meaning of the evidence.
Convenient names for the sorts of evidence of which we are speaking might be complex evidence and simple evidence. Complex evidence requires extensive background knowledge and understanding of human nature. Simple evidence does not; it tells its story plainly. (Using this terminology, we can say that it is common for murder mysteries to, early on, have complex evidence which appears to be simple evidence.)
In order to achieve societal action, such as convicting the murderer in a court of law or getting some other legal effect, one must have simple evidence. However, simple evidence is, in murder mysteries, hard to come by. This is, of course, a selective effect. In the case where the murderer’s fingerprints are on the murder weapon, and the murderer was seen killing the victim by multiple witnesses who know the murderer personally, and the murderer was caught immediately afterwards—these are not the stuff of murder mysteries.
Detective stories have the structure of story-within-a-story: the murder is the interior story while its detection is the outer story. Within the outer story, it is frequently the detective’s main purpose in his investigation to try to uncover simple evidence about the inner story. This makes it curious that his success or failure at achieving this goal is (almost) irrelevant to whether the story is a good story.
An excellent example of this is the Poirot story Five Little Pigs. In it, the daughter of a woman who was hanged for murdering her father, seventeen years ago, comes to Poirot asking him to uncover the truth. She just received a letter from her mother, written immediately prior to her execution but entrusted by lawyers to be delivered on her daughter’s 25th birthday, telling her that her mother was innocent.
Poirot undertakes the investigation and interviews all of the people principally concerned. At the end, he explains how all of the evidence which had pointed to the guilt of the woman’s mother actually pointed to the guilt of someone else. That person speaks alone with Poirot, afterwards, and asks him what he intends to do. Poirot says that he will give his conclusions to the authorities, but that it is unlikely that they will pursue it and very unlikely that they will get a conviction. And that’s fine. It’s a very satisfying ending to the story.
I suspect that the answer (which may be obvious) is that complex evidence is fun, while simple evidence is not fun. It takes brainwork to understand complex evidence, while simple evidence is too easy to be interesting. What matters in a murder mystery is being interesting, not achieving results. Achieving results is, really, the domain of an action story, or possibly a drama. This is why detectives tend to hand their cases off to the police at the end of the story. It’s best if the tedious work happens off-screen.
But there’s an interesting complication to this.
It is not a good story if the detective (and hence the reader) merely finds out what happens without anyone else learning it. Why this is so relates to the detective’s role within the story. As I’ve said before, the detective is Christ figure: the world has been corrupted by the misuse of reason, and the detective enters it in order to restore order to the world through the proper use of reason. So while society need not act, something must be put right. That is, someone beside the detective must learn the truth and be better off for it.
A good example of this is the Sherlock Holmes story The Blue Carbuncle*. It begins with a curious set of coincidences which place the key evidence in front of Sherlock Holmes, and with some investigation he discovers who it was who stole the gemtsone. He invites the man to his room, and he comes. After Holmes confronts him with the evidence, he falls apart and confesses, sobbing. I’ll quote just the last part:
“Get out!” said he.
“What, sir! Oh, Heaven bless you!”
“No more words. Get out!”
And no more words were needed. There was a rush, a clatter upon the stairs, the bang of a door, and the crisp rattle of running footfalls from the street.
“After all, Watson,” said Holmes, reaching up his hand for his clay pipe, “I am not retained by the police to supply their deficiencies. If Horner were in danger it would be another thing; but this fellow will not appear against him, and the case must collapse. I suppose that I am commuting a felony, but it is just possible that I am saving a soul. This fellow will not go wrong again; he is too terribly frightened. Send him to jail now, and you make him a jail-bird for life. Besides, it is the season of forgiveness. Chance has put in our way a most singular and whimsical problem, and its solution is its own reward. If you will have the goodness to touch the bell, Doctor, we will begin another investigation, in which, also a bird will be the chief feature.”
Here there are two aspects to the world which Sherlock Holmes has put right, even though he has produced no evidence for a jury. In the first, the man wrongly accused of the crime will not be convicted of it because the principle witness against him has fled. The second is less certain, but the true criminal may well repent of his crime since he’s seen what evil he’s capable of and still has a chance to make his way in society honestly.
This satisfies the role of the detective as Christ figure. In fact, it even has a curious echo (perhaps intentionally) of the story of Christ and the woman caught in adultery, and how he releases her from the punishment for her crime on the condition that she sins no more. Neither is, strictly speaking, a satisfying story, but they have something else to them—the idea that there is something better than justice. That’s a very tricky notion, because mercy should never be unjust—but at least in the story of the Blue Carbuncle, what was stolen is returned, and so justice is at least mostly satisfied in restitution.
Be that as it may, the primary point under discussion is satisfied. Holmes collects complex evidence which tells him (and thus the reader) the tale, and this is the interesting part. Achieving a practical effect from society is of minor concern.
(I suspect that part of the reason why The Adventure of the Blue Carbuncle ends as it does is that, though it predates Fr. Knox’s decalogue, it violates rule #6 (“no accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right”). The Adventure of the Blue Carbuncle is predicated upon a series of accidents, all of which help the detective. If he achieved a practical societal effect, his reputation would benefit by pure chance. By letting the criminal go, he remains in anonymity and so the accidents which help him produce only an interesting set of circumstances.)
*A carbuncle is a red gemstone, most often a garnet, so a blue carbuncle is something of a contradiction in terms. The story suggests it is a blue diamond, though it could be a blue garnet or even a blue sapphire.