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Notes -
Mysteries are better once you understand they are not homicide investigations, but manners comedies. The solution is unimportant, and is often shoe-horned in to wrap up the real story, which is uncovering all the dastardly details of rich people's lives. The murder is forever in august company, never a drug addict shivved by feral teenagers. The parade of red herrings are inevitably the dirty laundry of the social elite. While pretending to denounce this poor behavior, the focus of the story reinforces to middle-class mass consumers that their social betters are really worse, but also that this is the aspirational life. Working class people are maudlin cartoons no closer to reality than step-n-fetchit blackface is to real black people.
The murder mystery is a middle-class fantasy that rich people are corrupt in the ways that middle class people look down on in the poor. In reality of course, getting rich, famous or powerful means you don't need to murder people to get what you want, and the crime rates bear this out. The corruption of a class is generally invisible to those below, who generate conspiracy theories to explain the obvious dissonance. One of those conspiracy theories is the murder mystery.
Agreed: they are escapist fantasies, often about lives of rich people and often moralistic in a way to appease genteel middle class tastes. Even more true for later adaptations, such as ITV Poirot and BBC Miss Marple, because they double as nostalgic period shows.
Disagreed: it says anything about what middle class think of rich people in a psychologically or culturally relevant manner. A generic Christie murder mystery includes a cast that includes a few rich people but rest of the cast (suspects) are servants, rich-adjacent or simply middle-class people who hang around for plausible or implausible reasons. I am confident that identity of culprit is statistically random, and thus usually not the rich upper class heir. Christie wrote puzzle pieces which are supposed to surprise the reader with a technically possible but usually unexpected solution.
Rich upper class people are there because by virtue of being rich, one can conjure many superficially plausible reasons for many people to want them dead. None of the reasons interrupt the puzzle with inconvenient social realities unless the author wants so. Their other function is to introduce other interesting plot devices, like an exotic locale, or an exotic murder weapon, or an exotic motive, or both. Sometimes other decidedly non-rich people serve some or all the same functions, such as archeologists or middle-rank military officers who have served in exotic places.
Consider some highlights of Christie's oeuvre. "Why was the victim murdered in the shadow of pyramids? ...he was an archeologist." "Nobody has written a good murder mystery about an airplane yet. Why was the murder victim found dead in an airplane? ...obviously, for starters, he was a rich enough to travel in business class." "Why would a murder victim be killed in an enclosed place, filled only with his family and friends who all had reason to kill him and no distracting possibility that someone else did it?" "...obviously, he was weird and rich, and was killed in his yacht." "Why would my detective receive a summons to solve a murder before it has happened, and I am bored of the setup where it is the murderer who invites the detective to watch his crimes happen? Naturally, the task is given by a weird, quirky millionaire in his last will and testament ... who previously was upstanding guy who solved murders together with my detective and found something weird just before he died!" "How can my quirky detective afford all the quirks and luxurious lifestyle I have written for him? "...he is a crime-solving consultant for the government and rich people." "Why is my detective in an exotic locale in order to board this famous train? ... as I said, he works for the government, on secret military stuff." "Why was the murder weapon an unusual arcane object? ... the victim was a rich antique collector." "Why was he murdered in a closed room together with a group of highly esteemed citizens, yet nobody saw anything? ... he was a rich antique collector and they were all playing bridge. Actually, one of the clues for the detective is to work out everyone's bridge game strategy. My readers love that game."
It all makes much more sense as "rich guy was there to make the plot happen" (not too different from "a wizard did it") than for "let's look at the depravities of the rich people". She also tried her luck in writing some books where "secret spy stuff made the plot happen", but those don't play so well. Considering "depravity", Christie's mysteries are chaste enough to child-friendly. The gruesome murders are abstract enough to be almost cozy. There is surprisingly little depravity, and the little of it's mild. Jealously, vengeance, greed, blackmail about something not too shocking are the common motives for murder. Sex is nearly always presented in very abstracted way: an affair, a romance.
"Social critique" homicide fiction post-date the classic murder mystery fiction. It was pioneered by Raymond Chandler (who was all about showcasing the general depravity of the rich people, and vibes are considered more important than consistent puzzle-like mystery.) Scandinavian noir does the same thing, but with more explicitly left-ward values valence.
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Cute theory, but…no?
Sherlock Holmes: yes to Hound of the Baskervilles, no to Valley of Fear, Study in Scarlet, or Sign of Four. There are too many short stories to cover; plenty of them have neither a high-class victim or perpetrator.
Dupin: yes to Purloined Letter, no to Rue Morgue and the forgettable Marie Roguet.
Poirot: I actually haven’t read any of these, and I don’t want to spoil them, so sure. Maybe they’re shameless class envy.
I think a better explanation would be that mysteries demand drama, and two hoboes shivving it out in an alley isn’t dramatic unless there’s some extra spice. A treasure from India worked just as well as an English inheritance.
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For a murder mystery to be a mystery you need the murderer to be competent enough to hide their crime. I think this excludes drug addicts and the feral teenagers who would probably commit the crime out of impulse, leave tons of evidence at the scene, and already be suspected by the police.
This is hardly an interesting story. For that you need a villain with the foresight to plan the murder, the intelligence and knowledge to do a competent cover-up, and the self-control to stick to the plan. It seems very unlikely that such a person would not be either middle-to-upper-class or in a position of power in the criminal underworld. Especially if the crime requires an accomplice who must necessarily be loyal to the murderer.
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I don't think this is a remotely accurate portrayal of Christie's oeuvre. In the most recent Christie novel I read, for example, the killer turns out not to be a member of the landed gentry, but rathera penniless physician who resorted to blackmail to maintain his standard of living.
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