Various Principles of Forensic Science! Famous Principle-Principle of Exchange

The specialized branch of forensic science embraces seven principles, these are Principle of individuality, Principle of Exchange, Principle of Comparison,


Shubham Kumar

1/29/20247 min read

Since forensic science is more or less a branch of science, therefore, just like all other branches, it does have guiding principles. The forensic experts are strictly subjected to follow these principles during the investigation of crime.

In this article we are going to take a glance on the guiding principles of forensic science.

In order to understand the principles of forensic science, first you need to know what exactly forensic science is.

Forensic science is a branch of science that uses the scientific discipline and principles in order to applicate it in the crime scene investigation to collect, analyze, preserve, recognize, individualize and evaluate the physical evidence present on the crime scene, or related to the crime scene, moreover, resolving the other complex issues through the application of natural scientific principles, to seek justice in administration of court.

In forensic science, the term ‘forensic’ is derived form the Latin term ‘forum 'which means ‘to gather people seeking justice ‘. If defined in layman language, the term forum refers to the legal procedure in which the justice is being served on the basis of evidence.

The branch of science which deals with the crime scene investigation, sure has the strictest code of conduct and guiding principles, which is to be followed so as to investigate more accurately and precisely.

A Glance at Principles of Forensic Science

The specialized branch of forensic science embraces seven principles, these are Principle of individuality, Principle of Exchange, Principle of Comparison, Law of Progressive Change, Principle of Analysis, Law of Probability and facts do not lie.

Principle of Individuality

The Principle of Individuality states that "Each and every entity in the universe possess special individual features, regardless the same manufactural unit"

According to this principle, the hair samples, fibers, blood and DNA of the culprit found on the crime scene in unique. These traces are possessed by the culprit only. Similarly, the blood, hair samples, fibers, fingerprints, DNA, etc of victim found on culprit is also unique. These traces are possessed by the victim only. Moreover, the tools used to commit the crime, also posses unique features.

For Example- In a crime of trespassing, the investigators find the fingerprints of 3 persons other that the owner. When these fingerprints are matched with the 15 suspects, 3 of them got matched. This confirms that those 3 are the culprit. Moreover, from the 15 suspects, only 3 can be matched, however 0, 1 and 2 have possibilities, but not more that 3. This is because, fingerprints are the unique feature possessed by an individual.

Th principle of individuality reflets light on the fact that everything in the universe possess unique features, on the basis of which they can be differentiated.

Principle of Exchange

The Principle of Exchange states that "Whenever two entities come in contact, mutual exchange of traces taken place". This principle holds prominent grounds in ensuring the presence of trace evidence in the crime scene.

The French Scientist, Edmond Locard, was the first person to devise the principle or law of exchange. This principle of exchange is also regarded as the Locard's Principle.

According to this principle, when a victimizer (culprit, the one who is committing crime) and his mechanical tools (tools used to commit crime), comes in contact with the victim (on which the crime is being committed), or the area surrounding the victim, all the entities in the scenario Exhange traces with each other. The victimizer and his mechanical tools will leave traces on the victim and surrounding areas upon contact.

Simultaneously, the victimizer and his mechanical tools will pick traces from the victim and surrounding areas. Therefore, the mutual exchange of traces took place between the victimizer, victim and the area surrounding the crime. If forensic experts managed to link the picked up or leaved traces from their original source (victimizer, victim and area surrounding the crime), they will establish the contact and uncover the telltale behind the crime committed.

Example: In the Hit and Run cases, the victim will receive the paint scratch from the culprit's vehicle. Similarly, the culprit will receive the paint scratch from the victim's vehicle.

If a criminal is trespassing through airducts, he will leave its footprints in ducts. Simultaneously, will too some dirt with him too.

Law of Progressive Change

The Law of Progressive Change states that "Everything changes with the passage of time"

This principle conveys the message that, nothing in this world is permanent, immutable and non-variable. Moreover, the rate of change varies tremendously with different objects and under different situations. The law has immense impact on the forensic science.

This principle points out towards the changes that criminals and the crime scene underwent with passage of time.

If the criminal is not caught in time, he will become unrecognizable. However, certain evidence are constant throughout life. Like fingerprints, Blood Group, Antigens, gait analysis, bone marrow, etc. Nevertheless, these evidence are not always founds, therefore, forensic experts rely on DNA profiling.

Furthermore, the crime scene or scene of occurrence, also undergoes rapid changes. The atmospheric condition in the crime scene, the foreign entrance, animal, bacterial growth and living beings, makes extensive changes in short time. The longer the delay in investigation, the fast and sever change will occur in crime scene. After some time, the crime scene may be not even recognizable.

It's not only that the criminal and the crime scene undergo progressive change. The objects involving the crime, also possess the same changing capability just like criminal and crime scene do. For example, the firearm barrels are washed by the hot gases, tempering the lands and grooves. The heat can increase the bore diameter. Metal objects rusts. The shoes wearied during the crime, can be wear and tear.

The criminals undergo progressive change more rapidly than the crime scene does.

Law of Progressive Change calls for the prompts actions in all the aspects of the criminal investigation.

Principle of Comparison

The Principle of Comparison states that "Only likes can be compared”

This principle holds potential grounds in diverting the focus from the unnecessary objects and focusing only on required subjects. Principle of Comparison should be strictly followed to save time and catch the culprit in time. This principle works relative to law of progressive change. Both the principles are focused towards saving time and catching the culprit in time.

Principle of Comparison reflects light on the necessity of providing the similar and corresponding items for comparison with questioned subjects. These items can be samples, specimens, standards and related objects.

For example- In a crime scene, if we found the residues of shots/pellets. A forensic investigator, at glace, tells that the victim was attacked by a shotgun, since pellets are found. Therefore, he ordered to find out the velocity and manufacturing details of the shotgun used. Now so as to investigate the details, it will be futile to send the revolvers, rifles, pistols along with the shotgun. Only different manufactures of shotgun should be send to laboratory.

Principle of Analysis

The Principle of Analysis states that "The analysis can be done no better than the simple analyzed”

Forensic Science is all about the evidence collection, preservation and analysis, to seek justice in administration of court. Before an evidence can reach to analysis, it is first to be properly collected, than preserved and then send for analysis. However, if the collection is not properly collected, it will even render the efforts of experienced analyst.

For example- A criminal while running from the crime scene, accidently clashes with the wall. However, somehow managed to escape. The investigating officers thought that the collision with wall might involve exchange of some powdered painted particle trace evidence on his clothes. He collected some amount of the paint from that wall by the use of penknife and send it as the control sample. When the culprit is caught, the powdered painted particle was extracted from his clothes and match with the control sample. But the samples don't match.

Can you tell why the sample wasn't matched, though they are the paint particle from the same wall. It's because of improper collection and handling of sample.

This principle reflects lights on the necessities on correct sampling and correct packing for useful and proper result.

Principle of Probability:

The Principle of Probability states that "All the identification, definite or indefinite are made consciously or unconsciously on the basis of probability. ”

It determines that the chances of occurrence of a particular way, out of number of ways in which the event takes place of fails to take place with equal facility

Example: In the case of identifying the dead body of a spectacled woman with a nose piercing and a tattoo mark on the right arm, the probability of identification = The probability of spectacles x Probability of nose piercing x Probability of the presence of a tattoo over the right arm.

Facts do not lie

This principle states that "Facts do not lie, unlike, humans, that may be lured or threatened to give false statements."

According to this principle, this circumstantial, physical, biological and digital evidence, related to the crime scene, speaks the truth about the crime scene. Regardless, eyewitness, that can be lured or threatened to give false statements.

For example- A violent murder crime committed in house. During investigation, the investigator found that half of the scene was burned and the burring fuel used by kerosene. The maid of the house claimed that, she was not present when this breezier incident took place. However, during the investigation, the gas droplets of kerosene and some carbon compounds was found from her hair. This evidence claims that she was present when the murder committed.

By applying all these principles with much perfection and precision, the resolution of the crime becomes easier with more accuracy, authenticity, and accountability.


  • "Forensic DNA Typing" by John M. Butler

  • "The Forensic Casebook: The Science of Crime Scene Investigation" by Ngaire E. Genge

  • "Forensic Science: A Very Short Introduction" by Jim Fraser

  • "Practical Crime Scene Processing and Investigation" by Ross M. Gardner