Locard's Theory

2 Following

• 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.
• Naoki Urasawa:
  • Monster (1994): That a man cannot stay somewhere without leaving some kind of trace is a mantra of Inspector Lunge. 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.
  • 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.

• Gattaca: The reason the era's investigators are called "Hoovers" — not just for J. Edgar Hoover, but because they spend a lot of time simply vacuuming crime scenes for genetic evidence. They don't even bother with dusting for fingerprint evidence because fake fingertips like Vincent's are so easy to make.

By Author:

• Ray Bradbury: In "The Fruit at the Bottom of the Bowl", the main character takes 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.
• Robert Fulghum: Mentioned by name in All I Really Need to Know I Learned in Kindergarten. He expands it into Fulghum's Exchange Principle, where everybody will influence the world around them in at least some small way as they pass through life.
• Harry Turtledove: In several of his works, magic runs on two key laws: the Law of Contagion (this) and the Law of Similarity.

By Work:

• A Certain Magical Index: Idol Theory is a form of The Law of Similarity, which states that an "idol" or a duplicate that employs the symbolism of a greater object or deity will always receive a part of the power of that greater object. It also means that whatever affects the greater object will also affect the lesser object. Sorcerers also hypothesize that the reverse may also be true. Touma compares this to how solar panels receive energy from the sun; there are several solar panels around the world which receive a part of the limitless energy provided by the sun—although it's not even a fraction of the original, the panels still gain energy from it.
• Deep Six (1984): The US President and all his staff get kidnapped right off the Presidential yacht. Poof, gone, vanished, leaving only the empty boat behind. An exceptionally bright investigator essentially uses Locard's Theory to crack the case: he knows the President has been on board, yet even the best forensic technicians in the business can't find any trace of him aboard the abandoned boat. Therefore, the boat they found wasn't the Presidential yacht, but rather an exact duplicate.
• Feet of Clay: Golems don't have fingerprints, but they do have traces of where they usually hang out.
• The Monster Baru Cormorant: Discussed, with regards to secret organizations. The Cryptarch’s Qualm:
  Your power is secret, and in secret it is total. But to use your power you must touch the world. To touch you must be touched, to be touched is to be seen, to be seen is to be known. To be known is to perish. Act subtly, lest you diminish.
• Rivers of London: All magical workings leave vestiga that another magic user can sense. In one of the comics, Peter describes checking for vestiga at a possibly-magic crime scene as the magical equivalent of Locard.
• Sherlock Holmes: Sherlock's famous search-everywhere method is based on this principle. As he eloquently puts it, no crime has ever been committed by a flying, intangible creature. As long as the perp can stand on two legs and interact with the world, there will be marks of their presence left. Conversely, one carries hints of the places one has been (dust), the food one has eaten (crumbs), and the activities one has done (ink stains). One of the easiest ways to piss him off is for police to tromp around a crime scene and obliterate the murderer's footprints.
• Traces: In Framed!, a slight contact from Ed Hoffman when he was poisoning Ms. Kee left her with a flake of white paint, two blue denim fibres, six fine particles of magnesite, a smear of wax, and a deposit of sawdust on her right side.

• Bones: "Bugs and slime guy" Hodgens regularly demonstrates this with his analysis of embedded particulates.
• Death in Paradise: In "Switcharoo", DI Neville Parker excitedly explains the basics of Locard's principle, saying that it is equally impossible for a criminal not to leave some trace at the crime scene, or to leave the crime scene without carrying some trace away with them. Not only does Parker find a trace (a fake hair from a cheap wig), but its presence solves the greater Locked Room Mystery: the murderer killed the victim in one room of the hotel, then moved the body to a different, identical room, which all the witnesses and detectives mistook for the actual murder scene.
• Dragnet: Sgt. Joe Friday is 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
• Murdoch Mysteries: Early in the first season episode "Bad Medicine", Constable Crabtree speaks a bit despairingly of the culprit's escape from a murder scene without a trace. Murdoch reassures him thusly:
• Sister Boniface Mysteries: In the Season One finale, "Crimes and Miss Demeanors", Locard himself appears in the obligatory Imagine Spot, and admonishes her "I never called it a principle, you know, or used the word 'exchange'."
• Star Trek: Voyager: In one episode, 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.
  The laws of physics dictate that any time two objects make contact, trace materials are exchanged. Therefore, a killer always leaves a calling card.

• Neverwinter Nights 2: Your investigation into the Ember massacre partly involves a magical version of this.
• Space Station 14: Detectives have a forensic scanner, which reveals the interaction history of an object by listing all the trace substances found on it, allowing fingerprints and DNA sequences from blood traces to be compared to the crew manifest to deduce a perp. Gloves can obscure fingerprints (provided they aren't fingerless), but fibers will be left behind, which can narrow the search to a particular department where those gloves are common. Items can be cleaned with soap or a damp rag to remove fingerprints and DNA, but even these will still leave distinct traces of their own ("slippery residue" and synthetic white fibers), confirming that the crime scene was tampered with.

• 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...)
• Recent research indicates that each person has a microbial cloud that is unique to them. Traces of the cloud are left on anyone or anything the person comes into contact with - and anywhere the person goes - and vice versa.