Every contact leaves a trace. The importance of Locard's Exchange Principle.
We have all watched TV forensic pathologists pause whilst photographing the body, grab a pair of tweezers and bag a piece of trace evidence from under the fingernails or inside a wound. In the storyline, this fibre or sliver of glass proves to be crucial damning evidence against the killer. So how important is trace evidence and is it true every contact leaves a trace?
Locard’s Exchange Principle is named after Edmond Locard, known as the ‘Sherlock Holmes of France’ he was one of the founding fathers of forensic science. Locard opened the world’s first police scientific laboratory in 1910 in Lyon, France, where evidence from crime scenes was scientifically examined in a few small attic rooms.
Within his seven volume ‘Traite de criminalistique’ Locard proposed an overarching principle, he wrote:
“Any action of an individual, and obviously, the violent actions of a crime, cannot occur without leaving a trace.”
In other words every contact leaves a trace, whenever two items come into contact with each other an exchange of material takes place. This is the cornerstone of modern forensic science practice. No matter where the criminal goes or what they do they will leave something at the scene and take something of the scene back with them. These small quantities of chemical or physical evidence found at a crime scene, on a suspect or on an individual can include hair, fibres, paint, glass, gunshot residue and explosives.
Locard made extensive use of this principle during his many investigations. In 1912, for example, Marie Latelle was found dead at her parents’ home. Through examination of her body, Locard found evidence that Marie had died by strangulation, the main suspect was her boyfriend Emile Gourbin. When questioned by police it became clear Gourbin had an alibi, playing cards with some friends. So who killed Marie?
Locard took samples from under Gourbin’s fingernails looking for skin cells under a microscope. Whilst examining these samples, he noticed a pink powder he realised was probably makeup. By investigating local chemists he found one that developed a custom powder for Marie, this matched the powder under Gourbin’s nails. Faced with the evidence Gourbin confessed. In true Agatha Christie style he had faked his alibi by setting the clock forward, thus tricking his friends and throwing the police off the scent.
As Helen Green mentions in our podcast episode Crime Scene Science, only the murderer and victim know what actually happened, the criminal is unlikely to tell us and the body can only be a ‘silent witness’. Therefore, the collection and documentation of trace evidence is vital for piecing together the final hours in order to reconstruct the crime.
The importance of trace evidence and Locard’s Exchange Principle is probably best described by Paul L. Kirk:
“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 fibres 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.”