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Locard's Theory
Every contact leaves a trace. Every time one object comes in contact with another object, it takes something from that object or leaves something behind.

Principle which underwrites modern forensic science. The reason that criminalists look for fingerprints, DNA, soil samples, fibers, and all the evidence that falls into the category of "trace".

In real forensics, this is a very general principle from which we learn nothing more than that it makes sense to look for trace evidence.

In TV, it is always literally true. Every crime scene has an abundance of fibers, soil, DNA, bodily fluids, and fingerprintable air.

This representation of the Locard principle is one of the reasons for The CSI Effect.

In works that portray Functional Magic, Locard's Theory is a subset of the magical principle known as the Law of Contagion.

Examples:

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    Anime 
  • That a man cannot stay somewhere without leaving some kind of trace is a mantra of Inspector Lunge in Monster. Of course when it comes to Johan, it's not that simple.
    • In fact, this is part of what gets him to trust the doctor's story about Johan. He says something earlier about how "nothing short of a demon" could go out of an area without leaving a trace. Johan apparently was in the area, but his room is so totally clean that he is in fact the "demon" of the story.
    • Naoki Urasawa uses this trope in another of his suspense manga, the Astro Boy-inspired Pluto. The fact that no trace evidence can be found at any of the murder scenes leads the investigators to conclude that the Serial Killer they're looking for is a robot.
  • Put to good use in Death Note; the reason it's so difficult to track down Kira is that the Death Note kills without leaving any physical traces, so L had to look for a culprit based on why the killings occurred the way they did. On the other hand, L was able to use forensic evidence against Misa because of the traces she left on the Second Kira tape, even though she had arranged for someone else to get their fingerprints on it.

    Film 

    Literature 
  • Mentioned numerous times in Jeffrey Deaver's Lincoln Rhyme novels.
  • Robert Fulghum mentions this by name in one of his All I Really Need To Know I Learned In Kindergarden books, then expands it in a philosophical direction. Thus, everyone influences numerous things in a small way as they pass through life.
  • The Fruit at the Bottom of the Bowl has the main character take the idea of this to extremes-after he kills a man, he cleans the entire house meticulously from top to, well, the fruit at the bottom of the bowl.
  • Used in a form in the Discworld novel Feet of Clay. Golems don't have fingerprints, but they do have traces of where they usually hang out.
  • In several of Harry Turtledove's works, magic runs on two key laws: the Law of Contagion (this) and the Law of Similarity.

    Live Action TV 
  • Possibly the earliest TV instance of this is an episode of Dragnet where Sgt. Joe Friday was accused of shooting a suspect unprovoked. His dogged insistence on "he must have left something behind" caused investigators to eventually discover the bullet the suspect had fired. They realized that a line under a shelf was not a carpenter's pencil mark, but a trace from the bullet. It had pushed the shelf up just enough to hide the bullet hole
    • The question of why the suspect's gun would appear unfired is left as an exercise to the viewer. Regardless of whether it was a revolver or automatic an unfired round would still be present, in the form of a spent shell casing lying somewhere or inside the revolver. In any case, drawing a weapon against a police officer is a death sentence most of the time, whether the cop fired first is irrelevant.
  • Mentioned by name at least twice in CSI: Miami.
  • In one episode of Star Trek: Voyager there was NO TRACE at all on the crime scene. Not a single molecule out of place, handily detected by their Everything Sensor. The murderer was a hologram.

     Real Life 
  • Subversion: The 2002 murder of Danielle van Dam. David Westerfield, the neighbor who was indicted for her murder, had a lawyer who used Locard's Theory as his entire case. It didn't work, and now Westerfield's in San Quentin. (Granted, the guy probably wasn't trying very hard to build a case, given Westerfield's attitude...)

     Video Games 
  • In Neverwinter Nights 2, your investigation into the Ember massacre partly involves a magical version of this.

The Killer Was Left-HandedForensic PhlebotinumOmniscient Database
Literary Agent HypothesisLaws and FormulasLord British Postulate

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