Locard's Exchange Principle

On a live system, changes will occur simply due to the passage of time, as processes work, as data is saved and deleted, as network connections time out or are created, and so on. Some changes happen when the system just sits there and runs. Changes also occur as the investigator
runs programs on the system to collect information, volatile or otherwise. Running a program causes information to be loaded into physical memory, and in doing so, physical memory used by other, already running processes may be written to the page file. As the investigator collects information and sends it off the system, new network connections will be created. All these changes can be collectively explained by Locard’s Exchange Principle.

In the early 20th century, Dr. Edmond Locard’s work in the area of forensic science
and crime scene reconstruction became known as Locard’s Exchange Principle.
This principle states, in essence, that when two objects come into contact, material is
exchanged or transferred between them. If you watch the popular CSI crime show
on TV, you’ll invariably hear one of the crime scene investigators refer to possible
transfer.This usually occurs after a car hits something or when an investigator examines
a body and locates material that seems out of place.

Edmond Locard (1877–1966) 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 (1914–1918), 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.

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