The Killer Was Left-Handed

"Lord Peter Wimsey, Gentleman Detective: "On the left, from behind downwards. That looks like another of our old friends"

Harriet Vale, Mystery Writer Detective: "The left-handed criminal"

Lord Peter Wimsey, Gentleman Detective: "It's surprising how often you get them in Detective Fiction. A sort of sinister twist running right through the character.""

- Busman's Honeymoon, by Dorothy L. Sayers.

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 two lefties connected to the case.

In any case, even though 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 Sherlock Holmes 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 Jack the Ripper was stymied by the ultimately unfounded belief that The Killer Was Left-Handed.

"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.

Compare A Sinister Clue.

Anime/Manga

 * In one Kindaichi Case Files 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 Detective Conan.
 * Most notably in Captured In Her Eyes, the fourth movie.
 * The CSI: Intern at Your Own Risk Tokyopop manga has the killer be a Non-secretor.

Film

 * This trope is used to find the mole in Svoy sredi chuzhih, chuzhoy sredi svoih.
 * Used in the Sydney Poitier movie In the Heat of the Night.
 * It's not a murder, but the doctor in 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 give his urine samples.

Literature
""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". .."
 * Death Watch by John Dickson Carr: An early clue establishes the killer was left handed. Lampshaded / Subverted:
 * To Kill a Mockingbird 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, if circumstantial, evidence that the accused was innocent, he was still convicted because he was black.
 * Sherlock Holmes 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 Sherlock Holmes radio show. In the episode, Holmes and Watson meet a radiologist who 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 Star Trek: The Next Generation 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.
 * Agatha Christie twice ("The Market Basing Mystery" and "Murder In The Mews") used a variant in which
 * And in Murder on the Orient Express, Poirot notices that
 * 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 - lampshading the trope at the time.
 * Inverted in the short story "Tomorrow Town" by Kim Newman: one of the suspects is ruled out because she's left-handed.
 * Used in The Roman Mysteries 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 Red Herring is arrested on the strength of his left-handedness, though the mistake is quickly discovered.

""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 The Corpse That Never Was, the killer had written things left-handed.
 * Parodied in Mark Twain's "A Double-Barrelled Detective Story," in which Sherlock Holmes deduces the following from a matchstick:


 * In one Lord Darcy story, 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:
 * In The Man in the Queue, by Josephine Tey, 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.
 * In a Gideon Oliver 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.
 * Not only do the Lord Peter Wimsey novels use the trope from time to time, but the books openly lampshade it.

Live Action TV

 * CSI, House, Law and Order Special Victims Unit, 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 Ripped from the Headlines, as this troper recalls an episode from a True Crime 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 The Killer Was Left-Handed 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 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 Agatha Christie example above.
 * 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 Diagnosis Murder, 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.
 * An episode of Columbo had a slight subversion - the killer was ambidextrous, but used his left hand in the killing.
 * In the Mad Magazine parody "Coldumbo" the lieutenant says "That's odd... the angle with which the killing blow was dealt indicates a right-handed killer, and when you were writing I noticed you're right-handed, doctor."
 * The Golden Girls 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 Sherlock, the 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.

Video Games

 * Happens in case 4 of Phoenix Wright: Ace Attorney.
 * Not exactly. No-one states that the killer has to be left-handed, it's just that
 * Thus, it is a Justified Trope.
 * I don't think people get what this trope is about...It's about moments when someone suspects a person because of the fact that the weapon was held in a certain hand, even though in reality no one brings up the fact that the killer could have simply used their opposite hand anyway. In Ace Attorney, when ever the fact about someone's dominant hand contradicting a weapon comes up, either said weapon is a gun or there's prove as too how the gun was held contradicting how the suspect apparently held the gun. In the former, it makes sense in that firing a gun with your none-dominant hand is very difficult so it's hard too believe someone would just swap hands because they would probably be unable too pull this off, and in the later you have proof as too the fact that the person held the weapon in a certain hand so there's no way they could just use this excuse.
 * On that note, the trope is subverted in that Edgeworth actually BRINGS THIS UP in case 5:3. Upon proving that.
 * Subverted/Averted in Fahrenheit (2005 video game), 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.

Western Animation

 * In one episode of 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,.
 * Also, the episode where someone was tagging the bathroom stall doors. Fillmore figures it out because.