Locard's Exchange Principle
Locard’s exchange principle (Forensic Science)
Inspired by Sir Arthur Conan Doyle’s stories of the master detective Sherlock Holmes, as well as by the work of French biometrics researcher and law-enforcement officer Alphonse Bertillon and that of Austrian educator Hans Gross, who is credited with establishing the field of criminalistics, Edmond Locard (1877-1966) established the first modern scientific laboratory for the investigation of crime in Lyon, France, in 1910. Locard published numerous books on forensic techniques, including the six-volume magnum opus Traité de criminalistique (1931-1936; treatise on criminalistics). In essays with titles such as “Dust and Its Analysis” (Police Journal, 1928) and “The Analysis of Dust Traces” (American Journal of police Science, 1930), Locard described his experimental work leading to the idea forensic scientists have come to refer to as Locard’s exchange principle or Locard’s theory.
This principle, which ties together the diverse investigations of the forensic sciences, has been called the basis of criminalistics. Variously formulated, it is expressed most succinctly as “Every contact leaves a trace.” This applies broadly to interactions ranging from those that leave fingerprints to those that make the markings on a bullet, but it is of particular relevance to what is known as trace evidence. Locard called the microscopic particles that adhere to people’s clothing and bodies “mute witnesses,...
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Further Reading (Forensic Science)
Bisbing, Richard E. “Fractured Patterns: Microscopical Investigation of Real Physical Evidence.” Modern Microscopy Journal, January 29, 2004.
Chisum, W. Jerry, and Brent E. Turvey. “Evidence Dynamics: Locard’s Exchange Principle and Crime Reconstruction.” Journal of Behavioral Profiling 1 (January, 2000).
Houck, Max M., ed. Mute Witnesses: Trace Evidence Analysis. San Diego, Calif.: Academic Press, 2001.
_______. Trace Evidence Analysis: More Cases in Mute Witnesses. Burlington, Mass.: Elsevier Academic Press, 2004.
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Locard's Exchange Principle (World of Forensic Science)
Edmond Locard (1877966) studied law at the Institute of Legal Medicine and worked subsequently as an assistant to the forensic pioneer Alexandre Lacassagne prior to directing the forensic laboratory in Lyon, France. Locard's techniques proved useful to the French Secret Service during World War I (1914918), when Locard was able to determine where soldiers and prisoners had died by examining the stains on their uniforms.
Like Hans Gross and Alphonse Bertillon before him, Locard advocated the application of scientific methods and logic to criminal investigation and identification. Locard's work formed the basis for what is widely regarded as a cornerstone of the forensic sciences, Locard's Exchange Principle, which states that with contact between two items, there will be an exchange. It was Locard's assertion that when any person comes into contact with an object or another person, a cross-transfer of physical evidence occurs. By recognizing, documenting, and examining the nature and extent of this evidentiary exchange, Locard observed that criminals could be associated with particular locations, items of evidence, and victims. The detection of the exchanged materials is interpreted to mean that the two objects were in contact. This is the cause and effect principle reversed; the effect is observed and the cause is concluded.
Crime reconstruction involves examining the available physical evidence, those materials left at or removed from the scene, victim, or offender, for example hairs, fibers, and soil, as well as fingerprints, footprints, genetic markers (DNA), or handwriting. These forensically established contacts are then considered in light of available and reliable witness, the victim, and a suspect's statements. From this, theories regarding the circumstances of the crime can be generated and falsified by logically applying the information of the established facts of the case.
Locard's publications make no mention of an "exchange principle," although he did make the observation "Il est impossible au malfaiteur d'agir avec l'intensité que suppose l'action criminelle sans laisser des traces de son passage." (It is impossible for a criminal to act, especially considering the intensity of a crime, without leaving traces of this presence.). The term "principle of exchange" first appears in Police and Crime-Detection, in 1940, and was adapted from Locard's observations.
SEE ALSO Criminal profiling; DNA; Fingerprint; Forensic science; Handwriting analysis.