The laws and principles of all the natural sciences has formed the base of Forensics. In fact, the application all the sciences to analyze the evidence to answer crime mysteries has helped develop several laws and principles of Forensic science.

The term “forensics” has been derived from latin word “forensis” which mean ‘the forum’. Forensic science is a scientific discipline that uses the laws and principles of forensic science for the purpose of administration to terminate doubtful questions in the court of law. These principles of forensic science has a direct impact on criminal proceedings from the scratch of investigation till the accused is convicted in court.

There are 7 basic principles of forensic science which include

  1. Law of Individuality
  2. Law of Progressive change
  3. Principle of Comparison
  4. Principle of Analysis
  5. Locard’s principle of Exchange
  6. Law of Probability
  7. Law of Circumstantial facts.

These 7 principles of forensic science are essential in crime scene investigation to link a suspect with the crime scene as well as victim to the crime scene and suspect to assist in building a strong case against an accused and conviction.

Here are the 7 principles of Forensic science

Law of Individuality

This law states that, “Every object whether natural or man-made has a unique quality or characteristic in it which is not duplicated in any other object.”

The law of Individuality has been verified in different fields and the most common among them is the fingerprints. Millions and Billions of fingerprints have been studied and yet not a single fingerprint has matched with other whether it is of twins or two fingers of same person.

Any two objects such as grains of sand, salt, seeds, twins or man-made objects such as currency notes, coins, laptop, suits, typewriter, etc. they may seem similar to each other yet a unique characteristic is always present between them.

Law of Progressive Change

This law states that, “Everything changes with the passage of time.

In simple words, nothing is permanent and the rate of change varies on different objects. This law has a significant effect on crime scene investigations because of the changes occuring in the crime scene and criminal with the passage of time. The criminal, evidence, objects involved may become unrecognizable.

If the crime scene is not secured in time, a change in weather, presence of animals/humans, etc. changes rapidly. For example, road accident on a busy highway may be unrecognizable or lose all necessary evidence if not secured on time.

The objects such as bullet fragments may develop rust, shoes may develop wear and tear marks, wooden objects may destroy due to presence of termite, knife may develop additional patterns, etc. Therefore, quick action is required in all segments of criminal investigations.

Principle of Comparison

This law states that, “Only the likes can be compared.

It highlights the need to provide like samples and specimens for comparison with the questioned evidences.

In a murder case, the expert is of the opinion that the person was killed by stabbing with a sharp piece of rod. It will be unnecessary to send a knife for comparison.

If a bullet recovered from the body is fired from the shotgun, it will be useless to send a pistol for comparison.

Principle of Analysis

This principle states that, “There can be no better analysis than the sample analysed.”

This principle highlights the need for correct sampling and packaging of evidences to avoid tampering & destruction and for effective analysis and results. Incorrect sampling may mislead the investigation.

In a rape case, the investigating officer collected the clothes of the victim which had semen and blood stains on it. When the clothes were sent for examination of semen and determination of blood group from them. The semen gave AB blood group, where as the victim has A blood group while the accused has B. Why the results were wrong?

Locard’s principle of Exchange

This principle states that, “Whenever two entities come in contact with each other, they exchange the traces between them.”

This principle was stated by French scientist known as Edmond Locard, and therefore it is known as Locard’s principle.

According to Locard, when a criminal or a murder weapon or any of his object comes in contact with the victim or the surrounding, they leaves some traces in that area whereas also picks up the traces from the places or person they have been in contact with. If those traces are identified by the expert, and track it to its original source, the criminal can be linked with the crime scene and the victim.

This principle is demonstrated in almost all the cases where there is a contact such as fingerprints, foot marks, tyre marks, bullet residues, hair sample, etc.

Law of Probability

This law states that, “All identifications definite or indefinite, made consciously or unconsciously are on the basis of probability.

The term probability is mostly misunderstood. It determines the chances of occurrence of a particular event in a particular way out of total number of ways in which an event can take place or fails to take place with equal facility.

For example, an unknown woman is found murdered in a farm. The dead body of that woman had a gold plated tooth, healed foot fracture and a tattoo on her left arm. A woman with the similar characteristics was also reported missing. There are high chances that the unknown corpse is of that missing woman?

The probability of that corpse being of another woman will be 1 in millions and the identity of that unknown corpse is established.

Law of Circumstantial facts

According to this law, “Facts do not lie, men can and do.”

Facts cannot be wrong, it cannot lie, it cannot be wholly absent. Therefore the importance of circumstantial facts is good for oral evidence.

In a case where a person is in armed forces is known to carry his duty till 10 Pm and resumes in the morning at 9 am. He secretly slips out of the unit at night kills a person and returns back secretly to join the duty on time. By circumstantial evidence, he can prove his presence in the unit at that time and escape the punishment.


Forensic science thus is used for recognition, individualization, identification of pieces of evidence collected from the crime scene using these principles of forensic science in the court of law.

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Diana Fredson · 25/10/2021 at 9:25 am

nice page…five stars

Somnath Maiti · 22/11/2021 at 7:35 pm

Too much good it is…. very helpful…

suraj jena · 07/01/2022 at 11:13 am

An awesome page need to be really appreciated wo much detailed explanation i have never ever read❤️

William James · 02/02/2022 at 12:32 pm

Thanks for the article. A forensic engineer job description usually includes studying materials, devices, and structures in order to investigate why these fail when they do.

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