When investigating a mystery or a crime, a detective has to go through a lot of steps. The detective must investigate the crime scene to figure out what happened and how it was done, and suspects are identified based on why they want to do this.
Sometimes the suspect's motive is enough to incriminate him, even if no concrete evidence is presented or it doesn't add up. Quite possibly, an Amateur Sleuth might have gone on a wild guessing spree and thought it made sense, but had only jumped to conclusions and ended up making baseless accusations. The sleuth's conclusions may have not necessarily been led by Insane Troll Logic, but he forgot the big detail: you need actual solid evidence and other proof before going out and pointing fingers. In fiction, this is not always the case, and one can bypass all methods of proving who the perpetrator was if a given suspect's motive is reason enough.
Often this leads to trouble when the suspect is innocent and protests that his motive isn't proof. A repeat offender may state that it's Not Me This Time. May lead to a Clear My Name. Sometimes a prosecutor knows the suspect is innocent but will use this fallacy to his advantage to frame the suspect. The simple (but not easy) solution is to actually bring evidence to light, quite possibly to realize that the prosecutor's accusations were wrong.
Expect this trope to come out a lot as a Red Herring Twist in Police Procedural shows—especially if there was an Asshole Victim. If the victim was really unpopular, this trope could evolve into Everybody Did It, because Everyone Is a Suspect.
In a real legal situation, motive is one of the least important factors regarding prosecution of a guilty party. It's not required that the reason the defendant had committed the crime be shown, only if he acted with criminal intent. In fiction, motive is the factor that makes a good cohesive story, which is why this trope is able to persist.
This trope is also popular among fictional and real-life Conspiracy Theorists, who come to the conclusion that if a certain group or government would benefit from a major tragedy, they must be the ones who caused it.
Compare Not Proven, where a lack of evidence allows a guilty party to go free; Conviction by Contradiction, when a flaw in the alibi implicates the suspect, and Conviction by Counterfactual Clue, when the evidence proving guilt is totally wrong in the first place. See also Occam's Razor, which states the simpler the solution the less likely it is to be wrong.
- Played straight and inverted in Lois Lane #39, Superman and Lois have finally tied the knot, only for Lois to fly into a rage when she finds portraits, mostly of Superman's old girlfriends. When the portraits turn up painted on, Superman thinks Lois did it. Lois points out that one of the messed-up portraits was Jimmy Olsen's, and while she might be jealous of her husband's old girlfriends, she would have no reason to mess up a picture of Jimmy. Superman agrees.
- Defied in Hoodwinked!. Red Puckett tells her side of the story. In her story, she thinks the Wolf is up to something bad involving a rash of recipe thefts because he's following her around, asking her personal questions, and she finds that creepy. Then Flippers tells Red they don't arrest people for being creepy. At the same time, the Wolf is a investigative freelance journalist who believes that Red and her Granny were behind the thefts on a misinformed tip from one of his informants. Both of them are innocent regarding the thefts and are subsequently released after their alibis are confirmed so that Flippers can identify the real culprit.
- 12 Angry Men uses this trope as one of the main thrusts behind the drive to convict the defendant. The defendant had yelled "I'LL KILL YOU!" at the victim shortly before the victim had died, and this was taken as "evidence". Juror #3, the one most adamant for a conviction, argued that no one says something like that unless they truly mean it, and during the course of the film, Juror #8 gets #3 so angry that he lunges at #8, screaming "I'll kill him! I'LL KILL HIM!" before #8, remaining calm and cool, throws this line of reasoning right back in his face: "You don't really mean you'll kill me, do you?"
- The Fugitive: The fact that Richard Kimble would have gained something from killing his wife (namely, money—even though he was already pretty well off financially as one of the top ranked surgeons in Chicago) is one of the many reasons why the Chicago Police Department arrest him and don't give a second thought to continue the investigation. Needless to say, Samuel Gerard is...pretty quick to make note of how stupid this is the first chance he gets to talk to the original investigating detectives. And turns out, she did die for money—just not hers. And there's the fact that the "one-armed man" was an ex-Chicago cop, raising the possibility the Chicago Police covered for one of their own...
- Similarly, in the Distaff Counterpart Double Jeopardy, the prosecution harps on the fact that Libby was the beneficiary of her husband's hefty insurance policy. Common sense fails to dictate to them or the jury that as his wife, she would naturally be this, and that as a wealthy couple, the payout would be larger than average.
- In The Shawshank Redemption, Andy Dufresne landed himself in jail for being suspected of the murder of his wife simply because he told her that he would see her in hell before he would see her in Reno. He also got sentenced because his shock at the events going on (and thus lack of expression) had the judge and jury believe he was a "stone-cold killer".
- There was also a preponderance of circumstantial evidence: she and her lover were murdered the same night as their argument, there was evidence placing him at the scene (and he was there, and had actually considered at least scaring them with a gun, before throwing it away in disgust with himself), he owned a gun which he couldn't produce for comparison and was never found... None of this is conclusive, but taken together, it's not implausible that a jury would convict him.
- In the live action movie of Phoenix Wright: Ace Attorney, this shows up in the final trial, when Phoenix accuses Manfred von Karma of murdering Gregory Edgeworth. The killer insists that without a motive, Wright has no grounds to accuse him of doing it. Played with in that Phoenix has provided good evidence that his suspicions are correct, and the insistence of a motive was the killer clutching at straws, since he figured no one could prove the reason he did it.
- Played with in The Postman Always Rings Twice. Given Cora's husband had taken out a large life insurance policy the day before he died, Nick and Cora have a pretty big, obvious motive for killing him. The District Attorney puts a lot of emphasis on this point when talking to them, but it's all just a ploy. He wants to make them think their motive alone is enough to convict them so that they'll turn on each other and provide him with a confession. In truth, he admits that he doesn't remotely have enough evidence to convict them, motive or no motive.
- Where the Sidewalk Ends: Once the police realize Triggs had a motive to kill Ken they're more than happy to pin the murder on him, despite evidence backing this claim being circumstantial at best. The actual killer is horrified as he was trying to frame someone else entirely.
- Jack Blank has an enemy in the Well-Intentioned Extremist Jonas Smart. Ever since the first invasion of the Rüstov, Smart had sworn to protect the people of the Imagine Nation by implementing a lot of brutal practices, including Sinister Surveillance of the population and perpetuating Fantastic Racism against Mechas, since The Great Collaborator, the one who betrayed the Imagine Nation by opening it up to the Rüstov, was a Mecha named Silico. Jack and his new friend Jazen, also a Mecha, come to the conclusion that Smart had orchestrated the whole Great Collaborator thing and pinned it on Silico to gain emergency wartime powers and keep them secure. When they bring this up to Smart, he's downright disgusted that they believe that of him because he would never even think of siding with the Rustov, and he is quick to point out that they have absolutely no proof supporting their accusations. In short, Jack and Jazen were dead wrong.
- Exploited during the Summation Gathering in Agatha Christie's A Murder Is Announced when Inspector Craddock accuses Edmund of committing the murder, solely on the basis that he supposedly was in a position to inherit. Both of them are faking the scene, as a ruse to trap the true killer.
- This trope is the basis of Tyrion's trial in A Storm of Swords. Many witnesses claimed they heard him threaten Cersei and Joffrey, and that convinces the entire realm that he was guilty of murder. Though Tyrion had little love for his nephew, and was harsh toward him, he is completely innocent.
- The same trope shows up at the very climax of the book. In the first book, the mystery of Jon Arryn's death was the Driving Question. At the end of that book, Ned Stark finds out that Cersei Lannister had powerful motives for wanting him dead and the ensuing fiasco leads to a Civil War. Turns out, that Cersei was framed at that time by Littlefinger, who later killed Joffrey and framed Tyrion for it. Being a Magnificent Bastard, he used this to ensure that Poor Communication Kills.
- Averted in Without Remorse. Ryan and Emmett eventually realize that Kelly, who is a retired SEAL who has the skills to kill all the drug dealers in Baltimore, also has an excellent motive to murder all the drug dealers in Baltimore, and from that conclude that he is 'the Invisible Man', who has been killing all the drug dealers in Baltimore. But in the next paragraph they openly admit that they have no real evidence, just a very convincing motive, so they have to keep investigating until they find some.
- Sage Adair's friends and associates are constantly being falsely arrested on these grounds.
- The fifth Fear Street book actually inverts this. The protagonist's goal is to prove her half-brother innocent of murder, and refuses to go to the police without actual proof that the victim's husband was the killer. However, it never seems to occur to her that the evidence she did uncover ( an affair, and plans to flee the country) would have at minimum hurt his credibility as a witness, and thus helped her brother's case. Something of a Justified Trope as she's a teenager, not a lawyer.
- Psych more or less runs on this trope. The main character is usually hired by the police because of his ability to piece together things when there's little physical evidence. Most characters assume/are lead to believe its psychic powers. He'll get his suspect, rattle off his list of mostly, if not entirely, circumstantial evidence and usually get a confession out out of the perp. Lampshaded at least once when the police protest they'll actually need more than Shawn's "psychic" hunches to move forward in a case.
- Motive inverts this trope since the detectives usually don't discover the motive for the crime till the end of the episode and by that time they have a fair bit of evidence to convict the perpetrator. The show's gimmick is that we know from the start who the killer is but the reasons for the crime are hidden until the very end.
- This has been applied so many times on the CSI: Crime Scene Investigation and the Law & Order franchises (as mentioned above, usually as a Red Herring Twist) that it would be easier to number the times it wasn't used. The suspects flying into a rage or shutting up (usually because a lawyer tells them to) when the motive is brought up (and in typical fashion of these shows, the interrogator is being rather cynical and smart-mouthed about it) doesn't helps them in any way. And sometimes, even if innocent, by the time the investigators have conclusive evidence the damage has been done.
- A lack of an alibi doesn't help either. Motive or not, most TV cops seem to think a suspect claiming he/she was "home alone or home alone with my wife/husband/sister/brother, etc" is tantamount to a confession, since there really isn't a way someone can prove that and said spouse/sibling could be lying. As well as admitting that they were anywhere near the victim shortly before their death. Nevermind that a guilty person would be far more likely to lie about something like this. One especially bad example has Benson and Stabler showing up at the home of a man who had dinner with their murder victim and proceeding to accuse him of being both her lover and her killer—with absolutely zero evidence to support either accusation.
- In the Star Trek: Deep Space Nine episode "Rules of Engagement" Worf undergoes a hearing on whether he'll be extradited to the Klingon Empire for blowing up one of their transports after it decloaked in front of the Defiant during a firefight. Under Federation law this is an open-and-shut case, and Worf is clearly innocent of murder. Under Klingon law, the motive of the killer is what is important (as in, did you kill out of duty or bloodlust?), and their lawyer somehow convinces the Starfleet admiral in charge of the proceeding to let him go ahead with that line of questioning.
- May be a subversion though. Worf gets off because the Klingons had actually set the whole thing up with an empty transport and used a fake manifest of passengers killed in an earlier accident. Afterwards, Worf admits that he's not actually sure if he did it from duty or bloodlust, and Sisko dresses him down for even firing on an unidentified target when he knew there might be civilian ships in the area.
- In the Bones episode where Booth and Brennan describe their first case, they're interrogating the suspect, against whom they had plenty of damning evidence including the victim's inner ear bones in his trunk lock. The lawyer calmly says that, since they don't have any motive, he and his client will be leaving, and Booth seems to agree. They aren't able to really nail the guy until Brennan realizes he was a drug user, which clearly means the victim saw him getting high, which clearly meant he had to kill her. He was an important politician, so they did need a strong case, but everyone acted like it was the motive that was important, not the evidence.
- In one quest in Arcanum: Of Steamworks & Magick Obscura, the elf wizard Wrath is found dead within his home, and his apprentice Jormund is instantly placed under house arrest as a suspect. Jormund had entered into a lifelong contract with Wrath, who would outlive him by several centuries, and is believed to have killed his mentor in order to bring the contract to an end. Plus, he's the only dwarf living in a community of elves, and elves generally consider themselves to be above the act of murder. The real killer is Wrath himself, who committed suicide in a bid to invert Murder the Hypotenuse and frame his love rival for his own death.
- Zigzagged in the Ace Attorney franchise.
- The prosecutors often don't provide a motive for why the defendant would've committed the murder, instead just using the evidence and testimony gathered by the police investigation. Sometimes, when the real murderer is caught, you don't learn their motive until after they're arrested and you're waiting for the judge to give the Not Guilty verdict. At other times, you have to provide a motive (and evidence that supports said motive) for the person that the defense attorney is accusing of being the real criminal.
- In the sixth entry to the series, Phoenix Wright: Ace Attorney Spirit of Justice, the killer of the fourth case says "I had no motive for killing the victim!" Defense attorney Apollo Justice mentally notes that it's a desperate last gasp, since all of the evidence is pointing in their direction; the killer is just desperately trying to stall.
- One episode of Rugrats has Suzie blame Angelica for stealing her brand-new tricycle. Angelica was completely innocent Suzie's trike was under her porch, Angelica's trike was her own and Angelica's red hands (which Suzie thought was from opening her garage's painted doors) were actually from her finger painting an apology letter.
- Star Wars: The Clone Wars:
- An early episode in the first season involved one clone attempting to frame another for being a traitor. The framed clone was blamed because he looted the fingers of destroyed battle droids from the battlefield and strung them together to make little bracelets, which was explicitly forbidden. The real traitor was found out immediately after he blamed the other because he knew that the Jedi were off the base, and no one told him that, exposing him as the one who bugged the command center and gave strategic secrets to the Separatists.
- "Senate Murders". Senator Onoconda Farr dies and is found by the inspector to have been poisoned. Bail Organa and Padmé Amidala investigate against the inspector's advice, and at one point they accuse the Kaminoan senator of being responsible because Ono was one of the opponents of a bill to create more clone soldiers. Though she's not the nicest senator and did benefit from the death, she vehemently denies the murder. After finding out the poison was Rodian-specific, and that one of the other senators who shared the drink was Rodian like Ono but didn't get poisoned, she was identified as the real culprit.
- Ahsoka Tano is accused this way in the final arc of the fifth season. Ahsoka starts out investigating a crime involving sabotage at the Jedi Temple and starts interrogating potential suspects. Ahsoka is accused of committing the crimes herself even though the evidence is dodgy: while interrogating a witness, the record shows her Force choking the witness when she was only scrambling to help the woman being Force-choked remotely; the audio was suspiciously cut off. Also, Ahsoka is reinforced as guilty when she is found in possession of the same bombs used in the sabotage, when she was actually going to search the warehouse for clues to their actual owner. She quickly grows disillusioned as she protests that she's being framed and her prosecutors are too ready to convict her just for being in the wrong place at the wrong time.
- The Legend of Korra
- Councilman Tarrlok uses the logic that "the terrorist Equalists are angry non-benders, therefore all these non-benders are Equalists". The crowd he refers to is made up of angry non-benders, but clearly aren't terrorists and non-violently protest Tarrlok's false accusation as unfair. Tarrlok uses their protests to reinforce this trope on them.
- Unalaq accuses his brother Tonraq of conspiring to assassinate him. The judge states that there was a meeting where plans of a civil war was discussed, the meeting took place at Tonraq's home, and Tonraq is chief, and therefore he must be guilty. However, Korra knows her father is innocent because she was a direct witness to the event and Tonraq was not only absent from the assassination attempt, but had confessed sincerely to Korra that he had no intention of murdering his brother. Unalaq knew all along that Tonraq was innocent and that Varrick was behind it, but he exploited this trope to have his brother found guilty and taken out of his way.
- Northern Water Tribe members are suspected for bombing the Southern Water Tribe cultural center during a peaceful protest made by Southerners. Mako knows better, witnessing that the perpetrator was a firebender and identifying who he is as well as his affiliation with a bending triad. Other cops on the police force tell Mako to quit because it was clear that the Northerners did it. This poor judgement is portrayed as laziness on their part.
- The Zeta Project. A spine-enhancing backpack was stolen from a disability rehabilitation center. The young suspect was seen at the scene of the crime just before it was stolen, and is believed guilty despite his testimony that he wasn't there, and that because the backpack is missing he is now confined to a wheelchair; the police only believe that his knowledge of the stolen object and motive to take it convicts him. The truth was that Zeta had projected a hologram of the suspect to hide himself while attempting to search for data in the computer of the backpack's inventor and had witnessed the real thief steal the backpack; a witness who saw Zeta but missed the thief believed it was the kid he was disguised as.