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ANSWERS: 3
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Locard's rule in essence is that any contact between two physical bodies will result in an exchange of material, leadin to physical clues that can be used as evidence at crime scenes. These may be as obvious as footprints or as small as a miniscule cotton fibre. Since inanimate objects cannot lie in court, they can be relied upon as being present at the scene. Whether they were planted there on purpose to divert suspicion to someone else is another matter entirely, but even the act of placing them there would leave a physical trace.
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Locard’s Exchange Principle: "Wherever he steps, whatever he touches, whatever he leaves, even unconsciously, will serve as a silent witness against him. Not only his fingerprints or his footprints, but his hair, the fibers from his clothes, the glass he breaks, the tool mark he leaves, the paint he scratches, the blood or semen he deposits or collects. All of these and more, bear mute witness against him. This is evidence that does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. Physical evidence cannot be wrong, it cannot perjure itself, it cannot be wholly absent. Only human failure to find it, study and understand it, can diminish its value."
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Locards exchange theory is also especially important when considering issue of contamination of evidence. There have been several high profile cases where the defence will discredit evidence citing contamination (Hanratty, Barry George and others). The issue of contamination is a primary concern at a crime scene as SOCOS are all too aware, it is not just the offender who may leave behind or take away traces with them. (Hence the protective clothing, the need for two different officers to deal with suspect and victim, the separation of evidence at the lab,evidence relating to the suspect and victim is never examined at the same time or in the same lab/bench area)
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