As long as the criminal remains upon two legs so long must there be some indentation, some abrasion, some trifling displacement which can be detected by the scientific researcher.
~ Sherlock Holmes, The Adventure of Black PeterForensic science has changed the way crime investigations are handled. By examining and analyzing the physical evidence and reconstructing the circumstances of the crime, forensic investigators are able to come up with scientific information that they can present in court. A person who is responsible for one of the most important principles in forensic science is Edmond Locard. He came up with the Locard’s exchange principle or Locard’s theory which states that “Any action of an individual, and obviously, the violent action constituting the crime, cannot occur without leaving a trace.”
A devout viewer of crime investigative series on television will be able to understand the importance of this principle. Haven’t we all observed how the investigator goes to the site of a grisly murder and examines the crime scene, to check for blood stains, footprints or fingerprints, murder weapons and even the slightest of traces of blood in the nails? This is known as trace evidence, and according to Locard’s principle whenever a crime is committed, trace evidence no matter how small or less, will always be present.
Understanding Locard’s Exchange Principle
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.”
Development of Locard’s Exchange Principle
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.
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.
Is it Possible to Get Away With “A Perfect Crime”
According to Locard’s “every contact leaves a trace” theory, it is impossible to commit a crime without leaving behind fragmentary (or trace) evidence. It is the investigators job to collect the physical evidence and carefully examine it, so as to create systematic documentation. The documentation of the physical evidence along with the statements of the witness, suspects, and victims help in crime reconstruction.
According to criminalist Dr. Richard Saferstein, The physical evidence left behind at the crime scene plays a crucial role in reconstructing the events that took place surrounding the crime. He further states that, the collection and documentation of physical evidence is the foundation of a reconstruction.
The trace evidence can be divided into:
- Physical (clothing, glass fragments, paint chips etc)
- Biological (DNA, fingerpints, hair)
- Natural evidence (soil, pollen, seeds and plants)
The Drawbacks of Locard’s Theory
One of the greatest drawbacks of Locard’s exchange theory lies in evidence dynamics. This refers to the alteration of physical evidence before it has been examined by investigators. There are many factors that can lead to the tampering and destruction of evidence.
- Staging (manipulation of objects in crime scene) by the offender
- Secondary transfer of evidence
- Actions of the victim before the crime
- Witness actions
- Natural factors like animal or insect activity, weather, decomposition.
- Fire suppression efforts
- Actions of police, scene technicians and medical personnel.
These factors can lead to the removal or obliteration of the evidence. They can often mislead the investigators and cause problems with crime reconstruction. Misinterpretations or misleading evidence can lead to inaccurate crime reconstruction. To avoid this, the investigator needs to make sure that the crime scene investigation and reconstruction is carried out with care.
Locard"s Exchange Principle in Forensic Science
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