Following Trace Evidence To The 1800s: Locard's Exchange Principle

Anyone who loves true crime is familiar with, “a thread matching the victim’s clothing was found…” or “a shoeprint was traced to a shoe in the suspect’s closet…” which helps catch the killer and put him or her away. Sometimes it is soil, plant matter, shards of glass, hair (human or animal), and is as minuscule as DNA or as evident as a large bloodstain. The term is “trace evidence,” also known as “fragmentary evidence.” In forensic science, it is Locard’s Exchange Principle. Common in today’s investigative work, it was pioneered in the late 1800s by one of Criminal Justice’s most influential figureheads.
Dr. Edmond Locard
 (1877 – 1966) 
Dr. Edmond Locard (1877 – 1966) studied both law and medicine in the esteemed city of Lyon, France. He became a student and later Assistant to Alexandre Lacassagne, a pioneering criminologist, where he stayed until 1910. Dr. Locard was also a medical examiner with the French Secret Service during WWI. Lyon’s police department finally agreed to donate two of its attic rooms to Dr. Locard, along with two officers to assist: the first forensic laboratory. But in 1910, it was called a “police laboratory.”
Dr. Locard’s work went far beyond his Exchange Principal. He penned a seven-volume work, Traité de Criminalistique (“Criminal Traits”). Famous crime fiction author Georges Simenon attended Dr. Locard’s lectures and used the information in his books, some of which were adapted to the screen. Dr. Locard was posthumously nominated to the French Forensic Science Hall of Fame of the Association Québécoise de Criminalistique. Today, he remains “The Father of Modern Forensic Science” in part because his Exchange Principal is the cornerstone of forensic work.
He would work with another pioneer, Alphonse Bertillon, who based criminal typing by body measurements. One of Edmond Locard ‘s contributions to criminology was the study of fingerprints: dactylography. “Locard believed that if twelve points of comparison could be found between two fingerprints, then that would be enough for a positive identification. This was adopted as a preferred means of identification over Bertillon’s method of anthropometry.”
But crime show junkies will most likely recognize “Locard’s Exchange Principle.” In layman’s terms, we can't walk into an area without leaving a part of ourselves and exiting the space without taking something with us. As Dr. Locard explained:

“It is impossible for a criminal to act, especially considering the intensity of a crime, without leaving traces of this presence.”
(Crime Library)

At the time, this theory seemed unusual and near impossible. Today, it is a fact. And law enforcement agencies maintain databases to utilize the method. As an example, the FBI maintains the National Automotive Paint File
Trace Evidence Samples: Hairs
database, where over 45,000 automotive paint samples from manufactures date back to the 1930s. And Sherwin-Williams department, Automotive Finishes, keeps a database that assists in “identifying year, make, and model based on color availability.” The National Institute of Justice has its databases. All used to match paint trace evidence (FBI).
In the last 20 years, Locard’s Principal has expanded to the field of what criminologists are now calling “Digital Forensics.” Originally called “Computer Forensics,” the term has had to develop due to electronic changes: handheld devices, Internet, fax machines, GPS, black boxes on land (i.e., trucks). “Delete” does not occur just because a button is pressed. The majority of files can be retrieved using the right set of tools and skills. What we put out there can be retrieved. 
So true crime lovers can follow “trace evidence” back to the early 1900s. It will come back to Dr. Edmond Locard and his “unusual” theory. Mystery solved. 


Association Québécoise de Criminalistique. "Liste des intronises au Pantheon francophone de la criminalistique.” “Sur les traces d’Edmond Locard.”

Bowen, R. & Schneider, J. (October 1, 2007). Forensic Databases: Paint, Shoe, Prints, and Beyond. National Institute of Justice. NIJ Journal Issue No. 258.

Daniel, L.E., Daniel L. (2012) “Digital Forensics for Legal Professionals.” Digital Forensics. Elsevier: The Netherlands.

Geneanet: Alexandre Arnould Edmond.

The Crime Library. Crime Museum Online. Retrieved from:

Photos: Dr. Locard (Wiki photos) Hairs ( both labeled free usage rights