Locard's Exchange Principle

Locard's Exchange Principle is an important part of forensic science investigation. According to this theory, "Every contact leaves a trace". 

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.

To understand Locard’s theory it is important to understand how important trace evidence is to forensic investigation. When a crime is committed, the police and investigators are left with fragmented pieces of a puzzle. The forensic team helps in reconstructing the puzzle with the help of trace evidence which refers to evidence left behind by the criminal on the crime scene. This can be anything from hair, fibers, pieces of clothing, blood, fingerprints, etc.

According to noted forensic scientist 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 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.”

Before the 19th century, most of the time in court and during investigation, witness testimonies were favored as opposed to evidence. Most investigators refused to perform invasive procedures on the dead body due to emotional respect, squeamishness and sometimes even superstitions. With the progression of scientific studies in anatomy and microscopy, science was integrated into criminal investigations.

The first known use of science and logic for crime investigation was done by Alphonse Bertillon in the late 1800s.

He developed Bertillonage, a simple procedure which involved recording a series of body measurements and noting other physical characteristics. This information was then placed on a single identification card in a police file. Prior to the use of fingerprint recognition, this was the most used system of tracking and identifying criminals.

Apart from body measurement, Bertillon was also responsible for development of photography for criminal identification.

It was Bertillon’s contribution to the forensic sciences that influenced Dr. Edmond Locard, the vanguard of forensic science. He was the director of the first crime laboratory, in Lyon, France and worked as a medical examiner during World War I.

Like Alphonse Bertillon, Locard too advocated the use of scientific studies for criminal investigation. According to him, “Il est impossible au malfaiteur d’agir avec l’intensité que suppose l’action criminelle sans laisser des traces de son passage.” which translates to “It is impossible for a criminal to act, especially considering the intensity of a crime, without leaving traces of his presence.” He tested this principle of exchange during many of his investigations.

In 1930, Locard published three papers in the American Journal of Police Science which are now known as Locard’s Theory of Exchange

Other significant contributions by Locard involves dactylography, which is an area of study that deals with fingerprints. He developed poroscopy, which is the study of fingerprint pores and the impressions produced by these pores.

Application of Locard’s Exchange Principle

One of the best ways to demonstrate how Locard ‘s theory is applied, we take an instance of an investigation done by Locard himself. In 1912, while investigating the death of a Frenchwoman named Marie Latelle, the police questioned her boyfriend Emile Gourbin. He claimed he was playing cards with his friends, and when questioned, the friends attested to this fact. When the corpse was examined by Locard he found evidence of death by strangulation.

He scraped underneath the boyfriend’s fingernails to look for skin cell samples. On close examination under a microscope, Locard found a pink dust in the cell samples. He figured this was makeup and although popular, makeup was not very widely manufactured. After searching further he found a chemist who made custom-made powder for the deceased woman and a match was made. With all evidence against him, Gourbain confessed of murder and tricking his friends into believing his alibi.

Courtesy - sciencestruck.com