History Main / LocardsTheory

''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.''

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[[folder: RealLife]]

In works that portray FunctionalMagic, Locard's Theory is a subset of the magical principle known as the LawOfContagion.

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* That a man cannot stay somewhere without leaving some kind of trace is a mantra of Inspector Lunge in ''Anime/{{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 ''Manga/AstroBoy''-inspired ''Manga/{{Pluto}}''. The fact that no trace evidence can be found at any of the murder scenes leads the investigators to conclude that the SerialKiller they're looking for is a robot.

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* Mentioned by name at least twice in ''Series/CSIMiami''.
** And once by the ''villain'' in ''Series/{{CSI}}''.

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[[folder: VideoGames]]
* In ''VideoGame/NeverwinterNights2'', your investigation into the Ember massacre partly involves a magical version of this.
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* Used in a form in the Literature/{{Discworld}} novel ''Discworld/FeetOfClay''. Golems don't have fingerprints, but they do have traces of where they usually hang out.

In works that portray FunctionalMagic, Locard's Theory is a subset of the magical principle known as the [[http://www.themystica.com/mystica/articles/l/law_of_contagion.html Law of Contagion.]]

* 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 (and other people) in a small way as they pass through life.

* 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 laws of physics dictate that any time two object make contact, trace materials are exchanged. Therefore, a killer always leaves a calling card.

* ''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.

* In several of HarryTurtledove's works, magic runs on two key laws: the Law of Contagion (this) and the Law of Similarity.

* Mentioned by name at least twice in ''CSIMiami''.
** And once by the ''villain'' in ''{{CSI}}''.

In works that portray FunctionalMagic, Locard's Theory is a subset of the magical principle known as the [[http://www.themystica.com/mystica/articles/l/law_of_contagion.html Law of Contagion]].

** [[FridgeLogic 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.

* Central to ''{{Gattaca}}''.

* 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, we all influence numerous things in a small way as we pass through life.

* 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 ''some''thing 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