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Locard's Theory of Exchange - "Every Criminal Leaves a Trace"

Updated on October 10, 2015

"Crime Scene" by JRLibby

"Bloody hand print from a crime scene".
"Bloody hand print from a crime scene". | Source

Edmond Locard at Work

The father of crime scene investigation.
The father of crime scene investigation. | Source

Lecture on Locard's Theory of Exchange - Trace Evidence

Edmond Locard's Theory of "Trace Evidence"

The tired, grizzled detective arrives at the bloody crime scene. He leans over the body, being careful not to touch or disturb anything. Why? Because he does not want to tarnish or contaminate any 'trace evidence' the killer left behind.

We have all seen this sort of scene on CSI or read it in the latest Patricia Cornwell book. We all know that every significant crime scene must be examined for fingerprints, footprint impressions, tyre marks, DNA, hair follicles, and any other minute atom of 'trace evidence'. We all know that in court, the trace evidence (the forensics) will often secure a guilty verdict.

Without the work of Edmond Locard (13 December 1877 – 4 May 1966) the science of crime scene examination would not be as advanced as it is today and we would not be watching shows like CSI.

Locand was truly the father of crime scene investigative science. His ground-breaking "theory of exchange" underpins the theory and practice of every modern crime scene investigation.

Professor Edmond Locard


Introduction to crime scene investigation

Forensics For Dummies
Forensics For Dummies
A great resource if you want a starting point to pursue a career in forensics. A really good introduction if you just want to understand a bit more about crime scene investigation and get more out of the crime thriller you read!

Locard's Theory - the Principle of Exchange

Professor Edmond Locard's famous theory of exchange can be summed up as as "every criminal leaves a trace".

Locard said, in his 1934 publication "La police et les methodes sceientifiques":

"Any action of an individual, and obviously, the violent actions of a crime, cannot occur without leaving a trace."

In other words, there is always going to be physical evidence left behind at a crime scene by the criminal. The criminal will also take away physical evidence from the crime scene.

Paul L. Kirk in his 1953 book, Crime investigation: physical evidence and the police laboratory, explained Locard's theory of exchange:

"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."

Chilling words indeed.

Dr Edmond Locard in 1966


Image of Sherlock Holmes

Sherlock Holmes in a 1904 illustration by Sidney Paget.
Sherlock Holmes in a 1904 illustration by Sidney Paget. | Source

Edmond Locard - The Sherlock Holmes of France

Pioneer of forensic science, Dr. Edmond Locard became known as the 'Sherlock Holmes of France'.

It's hard to know if life imitated art or vice versa as Locard and Sir Arthur Conan Doyle's Sherlock Holmes were contemporaneous. Locard was 10 years old when Holmes first appeared in 1887 in "A Study in Scarlet". We know that Locard was a big fan of Holmes as he used to recommend reading Holmes to his students.

Sherlock Holmes first appears working in a hospital chemical laboratory. Holmes is portrayed as a scientist, a medical man, and the greatest detective of his age. In reality, the character of Holmes was a major influence on the art of criminal investigation and detection almost from his inception. (Holmes appeared in four novels and 56 short stories up to 1927.)

It is not so surprising that the careers of Locard and Holmes bear some remarkable similarities.

Crime Scene Investigation

An early method of measuring the feet of criminals, which was part of the Bertillon method used by the police force in Paris.
An early method of measuring the feet of criminals, which was part of the Bertillon method used by the police force in Paris. | Source

Dr Locard's Career - up to 1910

Edmond Locard initially became a medical doctor, getting his PhD in medicine in 1902.

He then worked as the assistant to Dr Alexandre Lacassagne, the famous criminologist and professor of forensic medicine at Lyon University.

Locard became interested in law and passed his bar examination in 1907.

In 1908, after becoming a lawyer, Locard took a tour of crime laboratories across the US and Europe. He wanted to meet other experts in his field in preparation for his next ambitious step. As part of this tour he spent time in Paris working alongside anthropologist Alphonse Bertillon, famous for his anthropometric system of identifying criminals.

Bertillon (pictured measuring feet), created a system where physical measurements were used to identify criminals. He recorded this information on a system of cards which were stored along with photographs - the Bertillon System.

In today's world of electronic storage it seems old fashioned, but in early last century France it was revolutionary and was quickly adopted as standard practice. You could take the cards, sift through them and quickly come to know all of the physical measurements and identifying traits of a suspect. You could compare this to how a crime was performed. For example, a suspect's height and arm length could reveal if they were the person who wielded the knife. Bertillon is actually referred to in one few the Sherlock Homes novels as the "highest expert in Europe". However, ultimately Bertillon's measurements became unreliable as peoples physical measurements change as they age.

Locard used all of this experience to fulfill his ambition of forming the first ever police laboratory in his native Lyon in 1910.

Modern Analysis of Crime Scene Information

Digital Analysis of Fingerprints
Digital Analysis of Fingerprints | Source

Edmond Locard - Police Laboratory in Paris

Locard's police laboratory was first set up in the attic of the Lyon Criminal Courts building on an 'unofficial' basis. In his cramped attic he had a basic collection of chemicals, a microscope, a spectroscope and a Bunsen burner. Within two years he gained world-wide fame in his field and official recognition for his police laboratory.

Locard's War Record

During the first World War Locard was a medical examiner with the French Secret Service. His work involved trying to identify the cause and location death by examining the stains and damage of soldiers' and prisoners' uniforms.

Just like his fictional hero Holmes, Locard was fascinated by codes and cryptology. Locard applied his fascination to breaking the German cipher (along with other code-breakers) in September 1914. This prevented an early French defeat.

Locard, analyzing fingerprints


FBI Crime Scene Investigation Handbook

Locard and Princess Anastasia

A quirky episode in Locards career was in 1920 when Locard heard about a woman claiming to be the Russian Princess Anastasia Romanov. This sort of controversy continued throughout the 19th century with many such claims.

It was public knowledge that Anastasia had been taken along with the other members her family (including Tsar Nicolas II) to a building in Ekaterinburg, where the Bolsheviks murdered them. The bodies were disposed of by dumping them into a mineshaft and throwing acid on them. Locard examined photos of the pretender's physical features and found them to be too different from Anastasia's features. Ultimately, when DNA analysis was performed long after Locard's death, he was shown to be right.

Locard in a Mug Shot!

Anthropometric  photograph of Edmond Locard from the Lyon Police Archives.
Anthropometric photograph of Edmond Locard from the Lyon Police Archives. | Source

Locard's Theory Quiz

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Locard's Achievements

The scope of Locard's contribution to forensic science and criminal justice is remarkable. His personal contribution to those around him was also noteworthy. As a man he was generous, often attributing his success to his mentors, Bertillon and Laccasagne. He loved music and theatre and regularly published reviews for the local Lyon paper.

He published over 40 works during his distinguished career.

Locard is renowned for his work developing 12-point fingerprint identification which is still used today. Locard's theory was that if two sets of fingerprints had 12 specific matching points, they could be said to belong to the same individual. "To write the history of identification," said Locard, "is to write the history of criminology." (Locard actually experimented on his own fingerprints, trying to burn them with off hot oil and irons to see if identification could be prevented.)

Locard is also celebrated for:

  • enhancing handwriting analysis - thus greatly improving the detection of forgery among other crimes;
  • systematizing the analysis of dust in the clothes of suspects (a specialty of Sherlock Holmes too);
  • inventing a better method of analyzing blood stains; and
  • inventing poroscopy, which uses the pores in the papillary ridges of fingerprints to identify suspects.

In one dust experiment, Locard accurately detected the occupations of 92 out of 100 people simply by careful analyzing dust taken from their eyebrows.

Locard became an extremely popular lecturer a the Lyon University in the Department of Criminalistics of the Faculty of Law. He continued to perform research, teach and work on high profile police cases until his retirement in 1951. He never ceased his research activities and his death in 1966 was a grave loss to the criminological community. Watch the video for an example of one of Locard's cases.

It would be impossible to quantify how many murderers, rapists, forgers and other criminals Locard's work, directing and indirectly, has helped to convict.

The Original Sherlock Holmes - Edmond Locard

Thanks for your visit!

Thanks for stopping by. If you like my work please leave a comment, vote and leave feedback. Please also feel free to share via Facebook and Twitter! If you would like to see more of my work please click on the links nearby. Thanks again for your visit - Mel.


  • How stuff works "Who was Edmond Locard?":
  • Wikipedia
  • Forensic Science Central:
  • Edmond Locard - Forensic Chemistry - Experts in the field:
  • Laccasagne, Alexandre:
  • Visible Proofs - Forensic Views of the Body:
  • Forensic Outreach:
  • Edmond Locard, EJDissectingRoom:
  • Dr Edmond Locard and Trace Evidence Analysis in Criminalistics in the early 1900: How Forensic Sciences Revolve Around Trace Evidence:
  • Locard's vision: 100 years of crime labs:


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