Principles of Forensic Science with Examples

Principles of Forensic Science with Examples

BY SIFS India | April 28, 2024

Principles of Forensic Science with Examples

Every subject has a set of principles, and so does forensic science. The laws and principles of natural sciences like physics, chemistry, and biology form the basis of the forensic science discipline.

It makes use of all the principles and has developed its own set that investigators follow during investigations to assist legal bodies in the court of law.

All these principles play an essential role in linking a suspect or a criminal with the victim and the crime scene.

Now let us discuss each one in detail.

Principles and Laws of Forensic Science

Law of Individuality

It is the most basic one, and as per this, no two objects, whether naturally occurring or man-made, are similar or have duplicate characteristics.

All objects present in this universe are distinct, and even if they appear alike, there are features that, if examined minutely, will reveal the differences between them.

For example, firearms created in the same factory with the same machine, at the same time, by the same workers, and using the same material are different from each other in their class and individual characteristics.

The same goes for coins. Coins of the same mint and denomination might look exactly alike to the naked eye but acquire individuality during manufacturing that is not easy to observe but can be spotted if seen with high concentration.

Several other pieces of evidence, like toolmarks, fingerprints, footprints, lip prints, etc., found at the scene of a crime are examined based on this principle.

Law of Progressive Change

As per this principle, with time, everything undergoes progressive change. Whatever is present today will not remain the same in the coming time, and its appearance will change. 

The speed with which the change occurs may vary depending on the object or sample.

That is why it is advised to secure the crime scene immediately to protect evidence from natural calamities (like rain, heat, and storms),  human or animal intervention, etc. 

The longer the delay, the greater  the changes that affect the identification process during investigation as the main identifying features of the sample wear out.

So if the evidence is not identified, preserved, and sent to the forensic lab at the right time, it will deteriorate and not give the desired results upon investigation.

Similarly, a bullet might get rusted, tissue samples might get degraded if not preserved immediately, and criminals might become unrecognizable if not apprehended in time, except for a few features like fingerprints that remain unchanged.

Principle of Exchange

This was given by a French scientist, Edmond Locard. As per this principle, whenever two objects come into contact with each other, a mutual exchange of material takes place between them.

Whenever a criminal or his weapon of crime comes into contact with the victim or the objects at the crime scene, they leave behind some traces and also pick up traces from the victim or the surroundings, either knowingly or unknowingly.

These traces, or physical evidence, later help investigators identify suspects. A forensic expert analyzes these traces and helps in linking them to the original source, thereby developing a link between the criminal, the victim, and the crime scene.

For example, an investigating officer might receive footprints, fingerprints, bullet residues, blood samples, soil, tools, hair samples, skin, bodily fluids, cloth pieces, etc. to examine.

The officer is required to establish exactly the places and objects with which the perpetrator or tool came into contact during the crime and the points of contact.

Principle of Comparison

From a lab analysis and examination point of view, this principle holds immense importance. As a result, only similar things can be compared to each other, and different things cannot be used for comparison.

You need to have like samples and specimens to examine the questioned sample found at the crime scene for authenticity.

For example, if a forensic lab expert receives a hair and a fiber sample for comparison, he cannot proceed as both differ from each other in appearance, chemical composition, color, shape, size, biological characteristics, texture, etc.

Also, if human blood is discovered at the crime scene, then sending animal blood for comparison is useless.

If a firearm is used, then there is no requirement to send a knife to the lab for comparison.

Principle of Analysis

Investigating officers usually found various physical pieces of evidence from the crime scene that needed to be sent to the forensic lab for analysis and a final opinion by the expert about the evidence.

However, to get the best results out of the examination, it is necessary to gather the correct sample, properly preserve it, pack it using appropriate techniques, and transport it carefully to the lab.

This methodology of collecting samples will help in avoiding damage caused by tampering, contamination, and sample degradation.

For example, a hard disk collected in a paper bag can get damaged due to a strong electromagnetic field if it falls within its range.

Law of Probability

Probability refers to the likelihood of a specific event happening in a specific manner out of a number of ways in which the event has an opportunity to occur or not occur compared to all possible outcomes.

For example, if a forensic scientist receives a biological fluid as physical evidence, the chances of it belonging to a particular person will vary.

Also, if a bank is robbed by a gang, investigators need to calculate the probability that the gang could be held responsible.

Principle of Linkage

As per this principle, facts don’t lie, but people can. The human statement might or might not be accurate, but physical evidence recovered from the crime scene can easily help forensic experts link a suspect to a victim. Also, the evidence found can sometimes link directly to the suspect and sometimes indirectly.

Example of direct linking

Fingerprints found at a crime scene are the most valuable piece of physical evidence to directly link a suspect to a crime scene.

Toxicological evidence like animal and plant poison, chemical poison, viscera, etc. can be used to directly link a person to the crime scene.

Example of indirect linking

Physical evidence like tool marks, tire marks, footwear impressions, etc. indirectly points toward the suspect.


These principles of forensic science lay the foundation for the subject. With these, you learn the significance physical evidence holds, how to collect the right evidence from the crime scene, preserve it accurately, and transport it to the laboratory to get the best results out of the forensic examination.

These principles guide you, starting from investigating the crime scene appropriately to presenting the findings of the forensic expert during court proceedings, which helps in convicting the accused.


Q. What are the 7 laws of forensics?

A. Law of individuality, Principle of comparison, Law of circumstantial facts, Law of probability, Principle of exchange, Law of progressive change, Principle of analysis

Q. Who gave the principle of exchange in forensic science?

A. Edmond Locard gave the principle of exchange, according to which substances undergo exchange every time two objects come into contact with one another.

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