Laws and Principles of Forensic Science:

 

1) Law of Individuality -

• This law states that “Every object whether natural or man-made has a distinctive quality or characteristic in it which is not duplicated in any other object,” in other words, no two things in this universe are alike.
• The most common example is the human fingerprints; they are unique, permanent, and prove the individuality of a person. Even twins do not have the same fingerprints.
• Consider grains of sand, salt, seeds, or man-made objects such as currency notes, laptops, typewriters, etc. They may look similar but a unique characteristic is always present between them.
• This principle is considered the most basic elementary unit of Forensic Science. Fingerprints, footprints, and tool marks obtained from the crime scene are studied and analyzed on the principle of individuality.

 

2) Law of Progressive Change -

• This principle states that “Everything changes with the passage of time and nothing remains constant.”
• The crime scene must be secured in time; otherwise, changes in weather (rain, heat, wind), or the presence of animals/humans, etc., can affect the crime scene.
• Bullet fragments may grow rust, firearm barrels loosen, shoes suffer wear and tear marks, wooden objects may suffer due to termites, etc. The longer the delay, the greater the changes.
• When samples are not durable, several complications occur in an investigation as the process of identification is affected due to variations in the main features of identification.
• Criminals undergo progressive changes with time. If not apprehended promptly, they may become unrecognizable except for their fingerprints or other characteristics of permanent nature.

 

3) Locard’s Principle of Exchange (Law of Exchange) -

• This law states that “As soon as two things come in connection with each other, they mutually interchange traces between them.”
• Whenever a criminal or his weapon/instrument makes contact with the victim or surroundings, traces are left at the crime scene, and traces from the area or person are also picked up (mutual exchange of matter).
• These traces are very helpful for investigation purposes, as they can be identified by experts and linked to their original source, resulting in the decisive linkage of the criminal with the crime scene and the victim. This law forms the basis of scientific crime investigation.
• This principle is validated in all cases where there is contact, such as fingerprints, tire marks, bullet residues, foot marks, hair samples, skin, muscles, bodily fluids, blood, pieces of clothing, etc.
• DNA analysis is a direct application of this principle, especially in cases where analyzed items are believed to have been handled by the perpetrator.

 

4) Principle of Comparison -

• For laboratory investigation, this law is very important.
• The law states that “Only the likes can be compared.” It emphasizes the need for providing samples and specimens that are comparable to the questioned items.
• For example, if a murder is committed using a firearm, then it is pointless to send a knife for comparison.
• Therefore, the essential condition of this principle is to supply specimens/samples of like nature for proper assessment with the questioned sample discovered from the crime scene.

 

5) Principle of Analysis -

• This principle states that “The quality of any analysis would be better by the collection of the correct sample and its correct preservation in the prescribed manner.”
• This approach leads to better results and prevents tampering, contamination, and destruction of samples.
• For instance, if a hard disk is collected in a paper bag, it can be damaged if exposed to a strong electromagnetic field, leading to poor results. Therefore, always use appropriate and effective collection and packaging techniques.

 

6) Law of Probability -

• This law states that “All identifications (definite or indefinite), made consciously or unconsciously, are based on probability.”
• The probability of the perpetrator having the same blood group as many other people is high, but the probability of the same match occurring in the case is low.
• If a woman with a bear tattoo on her right hand and an old injury mark on her head is reported missing, and an unknown woman is found murdered with these characteristics, the probability that the unknown corpse is the missing woman is high.
• The probability that the dead body belongs to another woman is one in millions.

 

7) Law of Circumstantial Facts -

• According to this law, “Facts cannot be wrong; they cannot lie or be wholly absent, but humans can and do.”
• This law emphasizes the significance of circumstantial facts and suggests that statements given by humans may or may not be accurate.
• In an investigation, facts identified and discovered are more accurate and reliable than any eyewitness testimony.