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Forensic Science and its Governing Principles and Laws

The word ‘forensic’ is derived from the Latin word ‘forensis’ that relates to a discussion or examination performed in public. Forensic science is the application of science and the scientific method to matters of law and resolution of legal conflicts. It is a multi-disciplinary subject which draws upon physics, chemistry, biology, computer science and other scientific principles and methods and is concerned with the recognition, identification, individualisation, and evaluation of physical evidence.

As society moves towards more scientific response to solving crime, significant advances have been made in the fields of serology, fingerprint and footprint analysis, handwriting analysis, ballistics and toxicology among others. Forensic scientists study and interpret the different types of evidence found at a crime scene. They employ techniques and tools for recovery and collection of crime scene evidence, so as to ensure that criminal evidence is recovered and retained without being contaminated and altered, packed and sent in a scientific and safe manner to the laboratory where the latest techniques are deployed and applied to extract prosecutable evidence that will link the evidence to the scene of crime and finally to the criminal so that he or she may be successfully prosecuted. Forensic scientists not only analyse and interpret evidence but also provide expert witness testimony in the Courts.

Forensic science has developed its own laws and principles. They guide the disciplines and methodologies of science in analysing the evidence impacting the proceedings in the Court of law. The following laws and principles are essential in crime scene investigation to link a suspect to the victim and the crime scene.

1. Locard’s Exchange Principle

Edmund Locard (1877-1966)
Edmund Locard (1877-1966)

Edmund Locard (1877-1966), a French scientist postulated the exchange principle in 1928 which asserts that every contact leaves a trace.

According to Locard, when a person or his instruments comes into contact with another person or object, a cross transfer of materials occur. They leave trace, and likewise pick up traces from the same contact. Such transfer or exchange may be large or small, visible or invisible, readily detectable or difficult to detect. It is the responsibility of the Investigating Officers to search, identify and collect such evidences.

Thus, a mutual exchange of traces takes place between the criminal, the victim and the objects involved in the crime. Every criminal can be linked to a crime by dust particles carried from the crime scene. For instance, in a case involving counterfeit coins, Locard asked the police to bring the three suspect’s clothing to his lab. After examination he recovered small metallic particles from the cloth material which made up the composition of the coins. Confronted with this evidence, the suspects were arrested and soon confessed to the crime.

2. Principle of Evidence Recovery

The principle of evidence recovery provides that no harm should be done to the evidence. Nothing should be added, lost, damaged or obliterated in the recovery process.

Careful attention should be taken to avoid contamination. Great care should be taken when there is a risk of losing or damaging evidence. Exhibit items need to be safely and securely packaged and transported to the laboratory.

3. Law of Individuality

Paul L. Kirk (1902-1970)
Paul L. Kirk (1902-1970)

Law of individuality is attributed to Paul L. Kirk (1902-1970) and provides that two objects may be indistinguishable; however no two objects are identical.

Individuality implies that every entity, whether person or object, can only be identical to itself and so is unique. It expresses that all articles or objects, man-made or natural, possess an individual character which under no circumstances is duplicated. Everything involved in a crime has an individuality which when established connects the crime to the criminal. The reasons for this could be either minor flaw present in the raw material, or imperfect stamping or variation in configuration of the crystals or substitution of some quantity of extraneous matter.

4. Law of Progressive Change

Law of progressive change provides that everything changes with the passage of time.

Change is inevitable. Different types of objects may take different time spans. Sample degrade with time, bodies decompose, firearm barrel loosen, tire tracks fade, metal objects rust. The scene of occurrence undergoes rapid changes.

5. Principle of Comparison

Principle of comparison asserts that only the likes can be compared. It emphasises the need of providing like samples and specimen for comparisons with the questioned items. A questioned hair can only be compared to another hair sample, likewise with blood samples, bite marks, tire marks, tool marks, etc.

Two objects are said to match when there are no unexplained, forensically significant differences between them.

If a comparison is conducted as the final and ultimate test, the rule is if in doubt, exclude. Whereas, if the comparison is conducted as a screening prior to the other tests, the rule is if in doubt, include.

6. Principle of Analysis

The principle of analysis stresses on the need of correct sampling and correct packing for effective use by experts. The quality of any analysis is determined by the quality of the sample under analysis, the chain of custody, and the expertise of the individual who analyses it.

The analysis can be no better than the sample analysed. Improper sampling and contamination render the best analysis useless.

7. Principle of Presentation

The principle of presentation provides that the laboratory report should be readily understandable and impartial. It should neither be understated, not overstated.

Complete disclosure should be made of all facts, assumptions, data, conclusions and interpretations.

8. Law of Probability

The law of probability asserts that all identification, definite or indefinite, is made, consciously or unconsciously, on the basis of probability.

Probability determines the chances of occurrence of a particular event in a particular way.

9. Law of Circumstantial Facts

The law of circumstantial facts has its basis in ‘facts do not lie, men can and do’.

The oral testimony depends upon the power of observation, assimilation and reproduction of the witness. It may be disturbed by rationality, external influence, suggestions, descriptions and opinions of others. Whereas factual evidence is free from these infirmities.

Thus, evidences given by victims or eye witnesses may not always be accurate. Sometimes they may intentionally lie or make up facts, or exaggerate or make assumptions or give evidence while having to rely on their poor senses.  On the other hand, evidence which gives a factual account e.g. based on investigation and evidence has a higher chance of being accurate and is more reliable.

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