How Locard's Exchange Principle Works

After two 10-year-old girls were murdered by Ian Huntley in Soham, England, the burnt remains of their Manchester United shirts were found along with Huntley's hair. How does Locard's exchange principle come into a crime scene? See more police pictures.
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You've seen it on every crime drama on television: the gruff investigator breezes through the yellow police tape after flashing his badge, moving toward the scene of the grisly murder. He crouches down before the body; a few flashes coming from behind let us know photographs are being taken for documentation. While glancing over the body, carefully lifting pieces of clothing, nothing seems out of the ordinary -- just bruises and blood stains. But after a moment, he locks on the tiniest smudge near the victim's neck. In the following scenes, we'll see the investigator huddled over microscopes, computer screens and detailed documents, comparing samples and looking for matches.

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When a crime is committed, police officers and investigators are left with mere fragments of a large and complex puzzle. In order to solve this puzzle, a networ­k of trained specialists need to take into account several factors. These factors include time of death, location, temperature, trace evidence and any other clues that investigators are able to collect.

­Television shows like "CSI: Crime Scene Investigation" make forensic science look driven by modern technology, with high power microscopes and computers doing lots of the work. However, the Chinese had successfully used fingerprinting to identify the origins of documents and clay sculptures as far back as the eighth century and could d­istinguish drowning from strangulation by the 13th century. But it wasn't until the late 19th century that scientists began seriously considering the importance of evidence at the scene of a crime, and rigorous standards for studying and analyzing criminal traces rapidly progressed during this era.

However, modern forensic science didn't just begin when microscopes became more powerful. Many ideas and philosophies about the nature of crime moved the study forward, and one of the most influential ideas in forensic science history is known as Locard's exchange principle.

­What exactly is Locard's exchange principle? What does it have to do with forensic science? And who was Locard, the man behind the principle, anyway? Read the next page to find out how the simple yet groundbreaking idea behind Locard's exchange principle changed the way we fight crime.

Who was Edmond Locard?

Fictional detective Sherlock Holmes examines the ground for trace evidence, something Locard would've admired.
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In 1887 -- when Sir Arthur Conan Doyle published "A Study in Scarlet," the first story featuring iconic English detective Sherlock Holmes -- scientists were attempting to separate fact from fantasy at the crime scene. Despite the fictional world of Dr. Holmes, Doyle's stories were a major influence on forensic science and, as we'll see, Edmond Locard himself. Previously, evidence took a backseat to witness testimonies, the latter of which could often be dubious. In England, for instance, superstition, squeamishness and emotional respect toward a dead victim prevented investigators from performing invasive procedures like incisions, thereby limiting the amount of data they could collect.

By the turn of the century, however, rapid advances in areas of study such as microscopy and anatomy strongly introduced science into the process of criminal investigation. The necessity to pay strict attention to the physical details at a crime scene and meticulously record observations became habit. 


Alphonse Bertillon, a French criminal investigator, developed one of the earliest systems of documenting personal evidence on criminals in the late 19th century. Called Bertillonage, the procedure was a relatively simple way of recording physical measurements onto identification cards and then filing them in order along with photographs of the individual. Although basic when compared to fingerprinting and today's computer systems, Bertillonage was an effective way of keeping precise information on criminals and acknowledging the importance of physical evidence.

An early method of measuring the feet of criminals, which was part of the Bertillon method used by the police force in Paris.
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One of the most important figures in the history of forensic science was a student of Bertillon, Edmond Locard, who would carry many of his teacher's influences with him. Locard worked as a medical examiner during World War I and was able to identify causes and locations of death by looking at stains or dirt left on soldier's uniforms, and in 1910, he opened the world's first crime investigation lab in Lyons, France. Like Doyle's Holmes, he was somewhat of an Everyman, and he worked with great faith in analytical thought, objectivity, logic and scientific fact. 

Locard also wrote a highly influential seven-volume work on forensic science, titled "Traité de criminalistique," and in it and his other works as a forensic scientist, he developed what would become known as Locard's exchange principle. In its simplest form, the principle is known by the phrase "with contact between two items, there will be an exchange." 

Sounds easy enough, but how does it relate to a crime scene? To learn what Locard's exchange principle means, read the next page.


Locard's Exchange Principle

A scientist at Preston Forensic Science Laboratory removes a hair from a hat left at the scene of a shooting.
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Although Locard's exchange principle is generally understood as the phrase "with contact between two items, there will be an exchange," Edmond Locard never actually wrote down those words in the vast amount of material he produced, nor did he mention anything concerning a principle. Locard, however, did write the following:

"It is impossible for a criminal to act, especially considering the intensity of a crime, without leaving traces of this presence."


In other words, Locard believed that no matter where a criminal goes or what a criminal does, he will leave something at the scene of the crime. At the same time, he will also take something back with him. A criminal can leave all sorts of evidence, including fingerprints, footprints, hair, skin, blood, bodily fluids, pieces of clothing and more. By coming into contact with things at a crime scene, a criminal also takes part of that scene with him, whether it's dirt, hair or any other type of trace evidence.

Dr. Locard tested out this principle during many of his investigations. In 1912, for instance, a Frenchwoman named Marie Latelle was found dead in her parents' home. Her boyfriend at the time, Emile Gourbin, was questioned by police, but he claimed he had been playing cards with some friends the night of the murder. After the friends were questioned, Gourbin appeared to be telling the truth.

When Locard looked at the corpse, however, he was led to believe otherwise. He first examined Latelle's body and found clear evidence that she was strangled to death. He then scraped underneath Gourbin's fingernails for skin cell samples and later viewed the results underneath a microscope. Very soon, Locard noticed a pink dust among the samples, which he figured to be ladies makeup.

Although makeup was popular around the time of the murder, it was by no means mass produced, and this was reason enough for Locard to search a little further. He eventually located a chemist who developed a custom powder for Latelle, and a match was made. Gourbin confessed the murder -- he had tricked his friends into believing his alibi by setting the clock in the game room ahead. Locard's exchange principle had worked.

To learn more about the world of crime, forensic science and how to catch the bad guys, follow the links on the next page.

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  • Bisbing, Richard. "Locard exchange." Modern Microscopy Journal. Jan. 29, 2004.
  • Chisum, W.J. Turvey, Brent. "Evidence dynamics: Locard's exchange principle and crime reconstruction." Journal of Behavioral Profiling. Vol. 1, no. 1. January 2000.
  • Wagner, E.J. "The Science of Sherlock Holmes: from Baskerville Hall to the Valley of Fear, the real forensics behind the great detective's greatest cases." New Jersey: John Wiley & Sons, 2006.