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A whodunit or whodunnit (for "Who [has] done it?") is a complex, plot-driven variety of the detective story in which the audience is given the opportunity to engage in the same process of deduction as the protagonist throughout the investigation of a crime. The reader or viewer is provided with the clues from which the identity of the perpetrator may be deduced before the story provides the revelation itself at its climax. The investigation is usually conducted by an eccentric amateur or semi-professional detective.
The "whodunit" flourished during the so-called "Golden Age" of detective fiction, between 1920 and 1950, when it was the predominant mode of crime writing. Many of the best-known writers of whodunits in this period were British — notably Agatha Christie, Nicholas Blake, G. K. Chesterton, Christianna Brand, Edmund Crispin, Michael Innes, Dorothy L. Sayers, Gladys Mitchell, Josephine Tey. Others — S. S. Van Dine, John Dickson Carr, and Ellery Queen — were American, but imitated the "English" style. Still others, such as Rex Stout, Clayton Rawson, and Earl Derr Biggers, attempted a more "American" style.
Over time, certain conventions and clichés developed which limited surprise on the part of the reader, vis-à-vis details of the plot the identity of the murderer. Several authors excelled, after successfully misleading their readers, in revealing an unlikely suspect as the real villain of the story. They often had a predilection for certain casts of characters and settings, with the secluded English country house at the top of the list.
One reaction to the conventionality of British murder mysteries was American "hard-boiled" crime fiction, epitomized by the writings of Raymond Chandler, Dashiell Hammett, and Mickey Spillane, among others. Though the settings were grittier, the violence more abundant and the style more colloquial, plots were, as often as not, whodunits constructed in much the same way as the "cozier" British mysteries.
Currently popular are live "whodunit" experiences, including game form, where guests at a private party might use cards, a board, or video from a pre-packaged box, to perform the roles of the suspects and detective; and there are a number of murder mystery dinner theaters, where either professional or community theatre performers take on those roles, and present the murder mystery to an audience, usually in conjunction with a meal. Typically before or immediately following the final course, the audience is given a chance to offer their help in solving the mystery.
Journalist Wolfe Kaufman claims that he coined the word "whodunnit" around 1935 while working for Variety magazine,  although an editor of the magazine, Abel Green, attributed it to his predecessor, Sime Silverman. The earliest appearance of the word "whodunit" in Variety occurs in the edition of August 28th, 1934, which refers to "U's Whodunit Universal is shooting 'Recipe for Murder,' Arnold Ridley's play".
Examples of whodunits 
- "The Three Apples" in the One Thousand and One Nights, the earliest archetype for the whodunit murder mystery
- Wilkie Collins's The Moonstone (1868), widely regarded as one of the first true whodunits
- Gaston Leroux's The Mystery of the Yellow Room (1907), a locked-room mystery
- Anna Katharine Green's Initials Only (1911)
- E. C. Bentley's Trent's Last Case (1913)
- Agatha Christie's The Mysterious Affair at Styles (1920) introduces Hercule Poirot.
- A. A. Milne's The Red House Mystery (1922), by the author of the Winnie the Pooh books.
- Agatha Christie's The Murder of Roger Ackroyd (1926), featuring Belgian sleuth Hercule Poirot in one of Christie's best-known works
- Dorothy L. Sayers's Unnatural Death (1927), one of the first Lord Peter Wimsey novels
- S. S. Van Dine's The Greene Murder Case (1928)
- Ronald Knox's The Footsteps at the Lock (1928) — though Knox is better remembered as the author of ten commandments for writing whodunits and for his short story "Solved by Inspection"
- Anthony Berkeley's The Poisoned Chocolates Case (1929) features six different solutions to the murder (and is an expansion of Berkeley's classic short story, "The Avenging Chance")
- Ellery Queen's The Greek Coffin Mystery (1932), regarded by some as the best of his early novels in the Golden Age style
- C. P. Snow's Death Under Sail (1932), his first novel, after which he turned to mainstream fiction; it features unusually complex characters for a mystery of this period
- Rex Stout's The League of Frightened Men (1935), the second Nero Wolfe novel
- John Dickson Carr's The Hollow Man (1935, U.S. title The Three Coffins), usually considered the quintessential locked-room mystery, replete with a tongue-in-cheek philosophical disquisition on the subject by the detective, Dr. Gideon Fell
- Nicholas Blake's Thou Shell of Death (1935), a locked-room mystery
- Josephine Tey's A Shilling for Candles (1936) — which became the basis for Alfred Hitchcock's film Young and Innocent (1937)
- Ethel Lina White's The Wheel Spins (1936) — which was filmed by Hitchcock as The Lady Vanishes (1938) (with a changed ending)
- Clayton Rawson's Death from a Top Hat, a locked-room mystery
- Michael Innes's Lament for a Maker
- Cyril Hare's Tragedy at Law (1942)
- Helen McCloy's Cue for Murder (1942), set in the Broadway district and featuring Dr. Basil Willing
- Christianna Brand's Green for Danger (1944), which was made into a celebrated film in (1946)
- Edmund Crispin's The Moving Toyshop (1946), a Golden Age mystery which also parodies certain conventions of the genre
- Carlo Emilio Gadda's That Awful Mess on Via Merulana (1946) is notable for not telling whodunnit at the end.
- Shear Madness, a long-running play that opened in 1980.
- Wes Craven's Scream is an example of a whodunit film no longer belonging mainly to the crime, but to the horror genre. A more or less logical evolution after several whodunits where the killer keeps reducing the number of suspects by turning them into victims.
Recent additions to the subgenre of the whodunit include the novels of Simon Brett, the Thackery Phin novels of John Sladek, Lawrence Block's The Burglar in the Library (1997) (which is a spoof set in the present in an English-style country house), Kinky Friedman's Road Kill (1997), Ben Elton's Dead Famous (2001), and Gilbert Adair's The Act of Roger Murgatroyd (2006).
An important variation on the whodunit is the inverted detective story (also referred to as a "howcatchem" or "howdunnit") in which the guilty party and the crime are openly revealed to the reader/audience and the story follows the investigator's efforts to find out the truth while the criminal attempts to prevent it. The Columbo TV movie series is the classic example of this kind of detective story (Law & Order: Criminal Intent and The Streets of San Francisco also fits into this genre). This tradition dates back to the inverted detective stories of R Austin Freeman, and reached an apotheosis of sorts in Malice Aforethought written by Francis Iles (a pseudonym of Anthony Berkeley). In the same vein is Iles's Before the Fact (1932), which became the Hitchcock movie Suspicion. Successors of the psychological suspense novel include Patricia Highsmith's This Sweet Sickness (1960), Simon Brett's A Shock to the System (1984), and Stephen Dobyns's The Church of Dead Girls (1997).
Parody and spoof 
In addition to standard humor, parody, spoof, and pastiche have had a long tradition within the field of crime fiction. Examples of pastiche are the Sherlock Holmes stories written by John Dickson Carr, and hundreds of similar works by such authors as E. B. Greenwood. As for parody, the first Sherlock Holmes spoofs appeared shortly after Conan Doyle published his first stories. Similarly, there have been innumerable Agatha Christie send-ups. The idea is to exaggerate and mock the most noticeable features of the original and, by doing so, amuse especially those readers who are also familiar with that original.
One of the earliest parodies of the whodunit genre in general is Englishman E. C. Bentley's (1875–1956) novel Trent's Last Case (1913), which introduced Philip Trent, a detective who gets everything wrong right from the start: assigned to investigate the murder of English millionaire Sigsbee Manderson, who is found shot in the library of his country house, Trent makes his first major mistake when he falls head over heels in love with the main suspect. In the course of his investigation he jumps at the wrong clues, in his reasoning he carefully eliminates the wrong suspects, and finally he arrives at a conclusion concerning the identity of Manderson's murderer which turns out to be completely wrong (though Trent is not presented as a bumbler at all). At the end of the novel, the real perpetrator casually informs him during dinner that he/she has shot Manderson. These are Trent's final words to the murderer:
- '[...] I'm cured. I will never touch a crime-mystery again. The Manderson affair shall be Philip Trent's last case. His high-blown pride at length breaks under him.' Trent's smile suddenly returned. 'I could have borne everything but that last revelation of the impotence of human reason. [...] I have absolutely nothing left to say, except this: you have beaten me. I drink your health in a spirit of self-abasement. And you shall pay for the dinner.'
A more recent example of a spoof, which at the same time shows that the borderline between serious mystery and its parody is necessarily blurred, is U.S. mystery writer Lawrence Block's novel The Burglar in the Library (1997). The burglar of the title is Bernie Rhodenbarr, who has booked a weekend at an English-style country house just to steal a signed, and therefore very valuable, first edition of Chandler's The Big Sleep, which he knows has been sitting there on one of the shelves for more than half a century. Alas, immediately after his arrival a dead body turns up in the library, the room is sealed off, and Rhodenbarr has to track down the murderer before he can enter the library again and start hunting for the precious book.
Murder by Death is Neil Simon's spoof of many of the best-known whodunit sleuths. In the 1976 film, Sam Spade (from The Maltese Falcon) becomes Sam Diamond, Hercule Poirot becomes Milo Perrier, etc. The film makes particular fun of the relationship between each detective and his or her sidekick. The characters are all gathered in a large country house, given meaningless clues, and all of them fail to solve the mystery.
Another example is the Lord Darcy stories by Randall Garrett. Despite their fantasy fiction setting, they are straight whodunits. However, the names of many of the supporting characters are puns, suggesting Garrett's friends, or the lead characters in other detective stories. Often, the personality of the character also reflects this.
The 2001 film Gosford Park paid homage to the classic whodunit premise, while at the same time presenting an original story.
Homicide investigation 
The term whodunit is also used among homicide investigators to describe a case in which the identity of the killer is not quickly apparent. Since most homicides are committed by people with whom the victim is acquainted or related, a whodunit case is usually more difficult to solve.
See also 
- Crime fiction
- Detective fiction for an overview
- Historical whodunnit
- List of crime writers
- Murder mystery
- Mystery fiction
- Mystery film