Locard's Exchange Principle
Collecting and Preserving Trace Evidence at a Crime Scene
Trace evidence is material found at a crime scene or accident scene in small (maybe almost invisible) but measurable amounts. Trace evidence is critically important as it can definitively link an individual or object to the crime or accident scene.
Trace Evidence Can Help Solve the Case
No matter how much someone tries to clean up a crime scene, something is generally left behind. It may not always be detected, or even be visible to the human eye, but it's almost impossible to take any kind of violent action without shedding something. This principle, called Locard’s Exchange Principle, has become one of the motivating factors in the development of forensic science.
Although trace evidence on its own is often not enough to make a case, it could very well corroborate other evidence or even prompt a confession which could help to solve the case.
What is Locard’s Exchange Principle?
Trace evidence is based on Locard’s Exchange Principle which contends that every contact, no matter how slight, will leave a trace. The trace is normally caused by objects or substances contacting one another, and leaving a minute sample on the contact surfaces.
The main point is that some apparently foreign object or piece of material has been brought to a crime scene and tracing its origin can assist in an arrest and conviction. Similarly, finding some trace from the victim or crime scene on a suspect can also have a strong impact on a case.
Who was Edmond Locard?
Edmond Locard was the director of the very first crime laboratory in existence, located in Lyon, France. Locard's techniques proved useful to the French Secret Service during World War I (1914–1918), when he was able to determine where soldiers and prisoners had died by examining the stains on their uniforms.
The Locard’s Exchange Principle, also known as Locard's Theory simply says that every contact leaves a trace. In his own words: “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.”
Collection of Trace Evidence from a Crime Scene
Trace evidence is any type of material left at – or taken from – a crime scene, or the result of contact between two surfaces, like shoes and the carpet, or fibers from where someone sat on a sofa.
After a crime has been committed the crime scene must be carefully preserved so that the police or the people first on the crime scene don’t contaminate it, because they also leave trace evidence. They need to keep any contact with the crime scene to a minimum.
Once the crime scene is sealed, the trace evidence needs to be collected. Each item is put into a sterilized container (contrary to popular belief, plastic bag are almost never used - actually a paper bag or envelope is used which will prevent further decomposition). This is labeled for later analysis at the laboratory.
Examples of Trace Evidence
Examples of typical trace evidence in criminal cases include:
- fingerprints, hairs, fibers, paper
- glass, paint chips, soils, metal
- botanical materials, gunshot residue
For such trace evidence to be useful, it must be compared to similar items from suspects, but particular care is necessary to ensure a thorough analysis.
Sources:
Crimelibrary
Michigan State Police
South Wales Police
Discovery Chanel
Edmund Locard -Director if the First Crime Lab.
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Hair under a microscope.
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Trace evidence: fibers can be compared.
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Paint chip left at a crime scene can be compared.
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