Laws / Principles of Forensic Science

Laws / Principles of Forensic Science

Objectives

1. Law of Individuality

2. Principle of Exchange

3. Law of Progressive Change

4. Principle of Comparison

5. Principle of Analysis

6. Law of Probability

7. Law of Circumstantial Fact

Law of Individuality

• The most basic law on which forensic science works is the Law of Individuality or Law of Uniqueness. Paul L. Kirk is credited with establishing this law in 1963.

• It states that every object, natural or man-made, has individuality which is not duplicated in any other object. It is unique. Neither nature has duplicated itself nor man can. (B.R. Sharma)

• Simply stated, no object, item, or person can be exactly the same, although they may look similar physically but not at the structural level.

• It is the fundamental law of forensic science. Anything and everything involved in a crime is unique — the crime scene, culprit, modus operandi, weapon, clues, etc.

Examples

1. No two fingerprints are the same whether they belong to two different persons, twins, or two fingers of the same person.

2. No person is identical; everyone has unique psychology, personality, dress, voice, walking pattern, etc.

3. Handwriting of a person is also unique and cannot be exactly the same for two individuals.

Principle of Exchange

• This is another very important law given by Edmond Locard, a French scientist.

• It is also known as Locard’s Principle of Exchange.

• It states that whenever two entities come in contact, a mutual exchange of traces takes place.

• Simply put, whenever a crime occurs there will be some clues because something will connect the crime scene to the culprit.

Examples

• When an accident occurs between two vehicles, there is an exchange of paint, glass fragments, and marks.

• In cases of rape, semen stains, hair, saliva, etc. can be exchanged between the culprit and the victim. The culprit’s tissues may also be found under the victim’s nails due to resistance.

• While walking on muddy ground, mud or dirt transfers between the shoes and the ground, which can help confirm a person's presence at that place.

Law of Progressive Change

• This law states that “Everything changes with time.”

• Nothing in the world is permanent. Just like life and death, anything that comes to life will eventually end.

• In relation to a crime scene, no evidence, dead body, or crime scene remains permanent; it will eventually deteriorate or get destroyed.

Examples

1. A dead body decomposes over time due to natural or unnatural processes.

2. An outdoor crime scene may be disturbed by wind, rain, animals, or people.

3. A fingerprint on paper becomes difficult to develop after a certain period of time.

Principle of Comparison

• This principle states that “Only likes can be compared.”

• It emphasizes the necessity of providing corresponding samples or standards for comparison with the questioned item.

Examples

1. Black hair can only be compared with black hair, not with brown or blond hair.

2. An AR bullet can be compared only with another AR bullet, not with shotgun ammunition.

3. Blood can only be compared with blood, not with other body fluids like sweat, semen, or urine.

Principle of Analysis

• This principle states that “The analysis can be no better than the sample analyzed.”

• It emphasizes the importance of correct sampling and proper packaging for accurate results.

• If a sample sent for analysis is contaminated, there is a high chance of incorrect results.

Examples

• If an investigating officer parcels cloth pieces containing both blood and semen together in a rape case and asks for semen blood group determination, results may vary because the blood may belong to another person. Antigens from blood may contaminate the semen stain.

• In DNA analysis, if semen is not collected properly and not dried before packing, results may vary.

• If sand from a shoe and reference soil samples are not collected properly, they may match falsely.

Law of Probability

• This law states that “All identification, definite or indefinite, is made consciously or unconsciously on the basis of probability.”

• In simple terms, everything in forensic science is based on probability.

• Probability is a mathematical concept that determines the chance of occurrence of a particular event among all possible outcomes.

Examples

• There are chances that a fingerprint may be present at the crime scene.

• The discovered drug could be cannabis or cocaine.

• A semen sample could belong to Person A or Person B.

• The person who fired the shot may be approximately 5’7” tall.

Law of Circumstantial Fact

• Also known as “Facts Do Not Lie.”

• This law focuses on the importance of circumstantial evidence such as camera recordings, fingerprints, voice recordings, etc., which can be used as proof in court.

• Eyewitnesses may change their statements, but physical facts remain reliable.

Examples

1. If a person fires a gun and a cartridge case is recovered, it can be used as circumstantial evidence to link the bullet to that particular gun.

2. In a hit-and-run case, CCTV recordings provide more reliable evidence than eyewitness accounts.

3. In a case where it is questioned whether a man assaulted a woman, physical injuries, recordings, or marks on the body can prove the fact.

Laws / Principles of Forensic Science