-> '''Lord Peter Wimsey''': On the left, from behind downwards. That looks like another of our old friends.
-> '''Harriet Vane''': The left-handed criminal.
-> '''Lord Peter Wimsey''': It's surprising how often you get them in detective fiction. A sort of [[StealthPun sinister]] twist running right through the character.
-->-- ''Busman's Honeymoon'' by Creator/DorothyLSayers

You can determine many things about the way an injury was inflicted by studying the wound. You can determine the kind of weapon used, and what kind of motion it was making.

In TV, you can also tell the handedness of the wielder. For this to be relevant, the killer must turn out to be left-handed (or the killer's right-handedness will rule out a leftie suspect).

This can clear one suspect, and convict another. But, of course, if the determination comes early in the show, you can bet that there will be at least two lefties connected to the case.

In any case, even though [[SuspectIsHatless about a tenth of the human beings on Earth]] are left-handed, being left-handed is good enough to make you the prime suspect. And as any rightie can tell you, sometimes you just use the other hand for killing people.

Even Literature/SherlockHolmes resorted to this at times.

This trope is discredited in real life, though it was once a common tool in detection. Several scholars believe that the search for JackTheRipper was stymied by the ultimately unfounded belief that TheKillerWasLeftHanded.

"Non-secretor" status may be the next generation of this trope. About 10% of people do not have antigens in bodily fluids other than blood, so sweat, saliva, semen, etc., cannot be used for blood type comparison purposes. This particular variation is currently rare on TV, though it is more common in novels.

A subtrope of ASinisterClue.

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!!Examples:
[[foldercontrol]]

[[folder:Anime/Manga]]
* In one ''KindaichiCaseFiles'' story, Kindaichi proves that a victim whose handedness was unknown was left handed based on his use of left-handed scissors, specifically designed to only function efficiently for lefties.
* This trope is used a few times in ''DetectiveConan''.
** Most notably in ''Captured In Her Eyes'', the fourth movie. [[spoiler:But of all the lefties, somehow, it was the psychiatrist who did it! Yes, he's a lefty, but he trained himself to use his right hand. He slipped up though, and dialed the phone with his left.]]
** The second movie, ''The Fourteenth Target'', reverses this by having evidence that the killer was right-handed used to indicate that the left-handed person person who until then was the prime suspect was not in fact the criminal.
* The ''{{CSI}}: Intern at Your Own Risk'' Tokyopop manga has the killer be a Non-secretor.
[[/folder]]

[[folder:{{Film}}]]
* This trope is used to find the mole in ''Svoy sredi chuzhih, chuzhoy sredi svoih''.
* Used in the Sydney Poitier movie ''Film/InTheHeatOfTheNight''.
* It's not a murder, but the doctor in ''Film/{{Gattaca}}'' reveals that he's always known that Vincent isn't who he says he is, by claiming that he used the wrong hand to get his urine samples.
* A variation of this occurs in David Finchers ''Film/{{Zodiac}}'', where the eponymous Zodiac killer is ambidextrous, eg able to write with both his hands, which actually makes it a lot harder for the police to identify him based on his handwriting.
[[/folder]]

[[folder:{{Literature}}]]
* ''Death Watch'' by Creator/JohnDicksonCarr: An early clue establishes the killer was left handed. [[LampshadeHanging Lampshaded]] / [[SubvertedTrope Subverted]]: [[spoiler: It was part of a frame up by the real killer, and the detective has a brief rant about left-handed / right-handed clues.]]
* ''Literature/ToKillAMockingbird'' used this trope in the court-room scene. It made a lot more sense in this case, as the accused's left hand was badly mangled in a childhood accident and he couldn't use it. The actual strategy used was a little more complicated. The accuser said that she was held down and beaten, at the same time; something not possible when the alleged assailant only has one properly functioning arm. The main invocation of the trope, though, comes when the accuser has bruises on her right eye - it would be very hard for the accused to hit her on that side of her face when his functioning arm is on his right. (The accuser said that they were facing each other on the ground.) Unfortunately, despite the fact that this was overwhelming evidence that the accused was innocent, he was still convicted, [[DeliberateValuesDissonance because he was black.]]
* Literature/SherlockHolmes used this exactly once and it was handled somewhat more sensibly than usual: the victim was killed by a blow to the head (delivered with the strong arm) and the footprints indicated that the killer was standing right behind him.
** Inverted in an episode of the ''New Adventures of Franchise/SherlockHolmes'' radio show. In the episode, Holmes and Watson meet a radiologist who [[ChekhovsGun casually describes]] his head assistant as ''adroit'' and his other two assistants as ''gauche'', then chuckles that this is literally true as well as metaphorically. When the radiologist is (naturally) murdered by one of his assistants, Holmes deduces, in his brilliance, that it must have been delivered with someone using their right hand. Thus, only the "adroit" (right-handed) assistant could've killed the radiologist.
*** Messed with in ''Series/StarTrekTheNextGeneration'' while Data and Geordi are doing one of their Holmes and Watson routines. An error in the holodeck messes with Data's deduction - a character is right handed when he's meant to be left.
* Creator/AgathaChristie twice ("The Market Basing Mystery" and "Murder In The Mews") used a variant in which [[spoiler: the ''victim'' is left handed, and committed suicide with a shot to the temple. A friend then [[SuicideNotMurder framed the person they blamed for the suicide for murder]] by moving the gun to the right hand, thereby making the shot look impossible.]]
** In ''Literature/MurderOnTheOrientExpress'', Poirot notices that [[spoiler: the victim's stab wounds indicate that he was stabbed by both a left-handed and a right-handed person.]]
** Played with in ''Towards Zero''. The doctor notes that from the position of the body and the angle of the blow the strike would be very tricky to do right handed - [[LampshadeHanging lampshading]] the trope at the time. [[spoiler:Subverted when it turns out the blow ''was'' struck with the right hand - but it was a backhand blow from a star tennis player.]]
* Inverted in the short story "Literature/TomorrowTown" by Creator/KimNewman: one of the suspects is ruled out ''because'' she's left-handed.
* Used in ''Literature/TheRomanMysteries'' story ''The Slave Girl from Jerusalem''.
* Used extensively in ''The Man in Lower Ten'' by Mary Roberts Rinehart, where the detective is convinced the angle of the stab wound means the killer was left-handed. As it happens, the man the killer framed is also left-handed, though he immediately tries to hide this fact. And later on, a RedHerring is arrested on the strength of his left-handedness, though the mistake is quickly discovered.
-->"Stuart," he said sternly, "there are two very serious things we have learned about you. First, you jab your scarf pins into your cushion with your left hand, which is most reprehensible". . .
* In ''[[MikeShayne The Corpse That Never Was]]'', the killer had written things left-handed [[spoiler:because he'd practiced writing left-handed to frame his father-in-law]].
* Parodied in Creator/MarkTwain's "A Double-Barrelled Detective Story," in which Franchise/SherlockHolmes deduces the following from a matchstick:
-->"This: that the assassin was left-handed. How do I know this? I should not be able to explain to you, gentlemen, how I know it, the signs being so subtle that only long experience and deep study can enable one to detect them. But the signs are here, and they are reinforced by a fact which you must have often noticed in the great detective narratives--that all assassins are left-handed."
* In one ''Literature/LordDarcy'' story, "A Matter of Identity", the detective looks at the location of a wound and reasonably concludes from the location that the killer was either left-handed or had a vicious right-handed backswing. Later investigation proves that deduction wrong: [[spoiler: While the corpse was connected to a crime, his death was an accident, not murder - he fell down the stairs.]]
* In ''The Man in the Queue'', by Creator/JosephineTey, Inspector Grant spends a great deal of time deducing the handedness with which the killing blow was dealt, and then looking for someone who uses that hand, only to find out at the end that [[spoiler:the killer is ambidextrous]].
* In a GideonOliver book, the victim was killed by a left-handed blow. But they don't have any left-handed suspects. There was one false alarm, when Gideon saw someone writing in a mirror and accused them of being left-handed, but then realized that the mirror had flipped it, and the person was actually writing with their right hand. [[spoiler: Revealed later on that the person actually was naturally left-handed, but had had it suppressed as a child so that he would grow up 'normal'. And during the highly emotional murder, had reflexively struck out with their left hand. The clue was the writing posture- according to the book, lefties-forced-to-be-righties often use a left-handed style with their right hand, and in the mirror, the person had been holding the pen like a lefty with his right hand.]]
* Not only do the ''Literature/LordPeterWimsey'' novels use the trope from time to time, but the books openly lampshade it.
* In ''The Fourth Protocol'' by FrederickForsyth, MI5 investigator John Preston is investigating Jan Marais, a South African diplomat he believes to be a [[TheMole Soviet agent]], but the South African authorities have thoroughly investigated his background and found nothing amiss. Preston comes up with a plausible theory on how Marais was replaced by a Soviet agent while a prisoner in World War 2, but is told it's just speculation. Preston shows them a photograph of Jan Marais taken on a cricket pitch as a youth, where he's gripping the ball as a left-handed spin bowler. He says he's had Marais under surveillance for months, and the man is right-handed.
[[/folder]]

[[folder:LiveActionTV]]
* ''{{CSI}}'', ''Series/{{House}}'', ''LawAndOrderSpecialVictimsUnit'', and a Patricia Cornwell novel each did a more unlikely variant, though, where the killer had two separate sets of DNA, so saliva samples did not match blood or semen. (The reasons differed: in one, this was the result of a rare birth defect. In the Law and Order episode, it was the result of a bone marrow transplant.)
** The birth defect version was probably RippedFromTheHeadlines, as there was an episode from a TrueCrime show that covered the same thing.
** In the ''CSI'' episode, the killer's semen DNA did not match the swab in his mouth, but it did point to a sibling. All his brothers and cousins were tested and came back negative.
*** Sounds like they did their homework for a change. A "chimera" has 2 sets of DNA in different cells of his body, with both sets related in the same way brothers/sisters are related. It's basically the opposite of identical twins: instead of 1 zygote splitting in 2, 2 fuse into 1. As long as both are of the same gender this tends not to cause any problems for the child, otherwise the child might be inter-sex. The condition might be far more common then previously thought (most people never have their DNA analyzed, so they'd be completely unaware they have the condition). There is F.I. the real-life case of a woman who gave birth to several children that (according to DNA) weren't hers. The government moved in to investigate and officials were present when she gave birth to yet another child that wasn't hers. Nobody understood what had happened (the mother herself least of all) until they stumbled upon a doctor who knew about chimeras.
* ''{{CSI}}'' also both subverted and plays this one straight. In one episode, Doc Robbins says that the whole TheKillerWasLeftHanded thing is a myth, but ventures a guess as to the handed-ness of the killer anyways. In future episodes, he plays it straight.
* In an episode of ''Series/{{Angel}}'', while attempting to prove she didn't kill someone, Harmony uses the possibly-true fact that vampires tend to always bite on one or the other sides of their victim's neck.
** Makes sense. When kissing, people have a preferred side they tilt their head to, vampires are usually human-based, so it might carry over. However, the first person to tilt his/her head sort of forces the other party to counter-tilt, so in case of a deliberate exposing of the neck...
* "Mr. {{Monk}} and the Garbage Strike" had an inversion of the first Creator/AgathaChristie example above. [[spoiler: The left-handed victim was shot on the right side of the head. To make it look like a suicide, the killer wrapped the victim's left arm in a bandage to make it look broken.]]
* Used inexplicably on ''{{Fortier}}'' where a contract killer uses an expensive Ruger specially modified for left-handed use and discards it at the scene. While it isn't all that rare to discard a "clean" murder weapon, leaving such a distinctive calling card on the scene where it will definitely be found is...odd.
* Almost used in an episode of ''Series/DiagnosisMurder'', where the killer must be right handed. This lets the left handed man off, until it's revealed that everyone is innocent. Since someone must be guilty, it turns out the lefty is ambidextrous.
* ''Series/{{Columbo}}'': The killer in "Death Lends a Hand" was ambidextrous, but seemed to favor his left hand, consistent with the victim's injury. But what really triggered Columbo's suspicion was the ring on the killer's left hand, which matched the shape of a gash on the victim's cheek.
** In the Magazine/{{MAD}} parody "Coldumbo" the lieutenant says "That's odd... the angle with which the killing blow was dealt indicates a [[SuspectIsHatless right-handed killer]], and when you were writing I noticed ''you're'' right-handed, doctor."
* ''TheGoldenGirls'' when the girls were participating in the Maltese Falcon Club, a murder mystery hotel, Dorothy solves one of the cases when the knife prop used to cut to the victim's throat was found on the left side of the victim (the "killer" only had seven seconds to commit the "murder" so it was cut and drop the knife quickly) and one of the suspects wore her watch on her right wrist.
* Played painfully straight on an episode of ''{{NCIS}}'' where the killer being left handed cleared a murdered man's commanding officer and lover, despite the fact they knew that the killer was a woman in the Navy.
* In ''Series/{{Sherlock}}'', the [[Franchise/SherlockHolmes titular detective]] concludes that an apparent suicide was in fact a murder, because the left handed victim had been shot on the right side of his head, something which would be rather awkward to achieve with your left hand.
* Inverted in ''Series/LawAndOrderSVU'' the team is working on a case of a girl who was killed when she spied on a butcher shop with horrendous health and cleanliness issues. The manager who was caught quickly confessed that he killed her, but Olivia reviewed the girls videos and noticed that the manager was left handed, but the one who killed the girl was right handed, and the real killer turned out to be an old lady who owned the butcher shop.
* A variant in ''MurderSheWrote'', "Funeral at Fifty-Mile". The victim was found hanging from the rafters in a barn. Later, when Jessica began investigating, someone placed a noose outside her window to scare her off. The noose that killed the victim was tied by a left-handed man, but the noose outside her window was tied by a right-handed man. This led Jessica to realize that either the killer was ambidextrous or that more than one person was responsible for the murder. [[spoiler: It was the latter.]]
* Ellery says this after surveying the crime scene in the ''Series/ElleryQueen'' episode "The Adventure of the Judas Tree''.
* In the ''{{Series/Castle}}'' episode [[spoiler:Tick, Tick, Tick]], this tips off Castle that the case isn't solved. More justifiably so this time: [[spoiler:killers may be more or less ambidextrous when it comes to bashing people with stuff, but fewer people ''shoot'' ambidextrously]].
* {{Lampshaded}} in an epsiode of ''Series/DueSouth'': An FBI Agent excitedly declares that they now know the blood type of whoever commited the crime. [[DeadpanSnarker Constable Fraser]] points out that this narrows it down to roughly a third of the entire population of Chicago.
[[/folder]]

[[folder:{{Theatre}}]]
* Parodied in ''Theatre/AnimalCrackers'', in which the forgery substituted for the stolen "Beaugard" is determined to be the work of a left-handed painter.
[[/folder]]

[[folder:VideoGames]]
* Happens in case 4 of ''VisualNovel/PhoenixWrightAceAttorney'', since [[spoiler: the gun bears prints from the defendant's right hand, while the photograph shows the killer holding it in his left.]] The trope is then subverted in that Edgeworth actually brings this up in a later case in ''VisualNovel/AceAttorneyInvestigationsMilesEdgeworth''. Upon proving that [[spoiler:the culprit that hit him with a sword did so with their right hand, Kay says that Lance, the person who Edgeworth suspects did it, is left-handed. Edgeworth however says that he simply used his non-dominant hand on purpose to throw suspicion away from him, stating that swinging a simple prop sword is easy, no matter what hand you use]].
* Subverted/Averted in ''{{Fahrenheit}}'', where you can learn during the autopsy that the killer was left handed, from the angle of the knife marks. However, this fact does not appear in the later scene where you must link evidence to connect the killer to Lucas.
[[/folder]]

[[folder:WesternAnimation]]
* In one episode of ''WesternAnimation/{{Fillmore}}'', the hall monitors (yes, it's that kind of show) find that someone has been shredding other students' important papers. From the angle of the shreds, they determine which hand the shredder used to place the papers in. Turns out, [[spoiler: it was ''all'' of the victims, working together to frame a non-existent shredder]].
** Also, the episode where someone was tagging the bathroom stall doors. Fillmore figures it out because [[spoiler: the graffiti on the stalls was written with a marker, with each letter overlapping the letter to the right. This meant the graffiti had to be written from right to left and thus the culprit had to be left-handed]].
* In an episode of ''WesternAnimation/ScoobyDooMysteryIncorporated,'' the gang catch the "Dreamweaver" because he was the only left handed suspect.
* An episode of ''WesternAnimation/{{Ben 10}}'' had a Kangaroo Commando-themed rollercoaster blow up. The actor who played Kangaroo Commando accused his rival of blowing it up, the only evidence being a right handed glove. [[OnlySaneMan Gwen]] points out that K.C's rival is actually left handed, as all his sketches are drawn with the left hand.
[[/folder]]
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