Learn about the Principles that Underpin Forensic Analysis

Forensic identification is based two main principles i.e. individuality and exchange.

The Individuality Principle

The principle of individuality as attributed to Paul L Kirk (1963) and is regarded as the building block for forensic science. Individuality implies that every entity, whether person or object, can only be identical to itself and so is unique. No two objects whether natural or artificial can be exactly the same. Kirk claimed that the aim of forensic science is to focus on the source of two items (questioned and known, or mark and print), which are thought to have come from a single source.  

As such, identification is concerned with establishing individuality from traces left at a crime scene rather than the sameness of two things. This means identification can be shown indirectly through the analysis of traces and samples e.g. no two fingerprints are the same.

The Exchange Principle

The exchange principle is attributed to Edmond Locard. The principle states that whenever two objects or subjects interact, some sort of trace will be left behind. This is generally at the crime scene. Trace materials include hairs, blood, fibres, and gunshot residues.

Locard suggested that there are many traces left behind and if interpreted properly they provide the most valuable information.

There are some other more general principles which apply to forensic science:

The Law of Progressive Change

Different objects change, although they may change across different time spans. For example, blood samples will eventually degrade. Some objects are more durable than others and may be relatively permanent, remaining mostly unchanged during identification.  If an object is very durable it may be quite easy use it for identification.  If it is less permanent and its main features change during the identification process it is not possible to answer the question of sameness.

The Law of Comparison

Different samples must only be compared to samples which are alike. In other words blood samples are compared to other blood samples, fibres are compared to other fibres, and so forth.

The Law of Analysis

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 Law of Circumstantial Facts

This is concerned with eyewitness testimony, victim statements, and so forth.  Anytime that people are called upon to provide evidence there is a chance that the evidence they supply is not accurate. This can be unintentional e.g. through mistaken observations, making assumptions or deliberate e.g. lying or exaggerating.  On the contrary, evidence which gives a factual account e.g. based on investigation and evidence has a higher chance of being accurate and is more reliable.


Law of Probability

Conclusions drawn from forensic analysis are dependent on the method used and its advantages and disadvantages. This all has to be taken into consideration.



In conclusion, forensic analysis depends on both the discovery of traces, and connecting them to individuals. If there are no traces found at a crime scene, it is impossible to identify suspects. If traces are found then provided these are analysed properly and the results interpreted in a suitable manner, they may be used as evidence.  

WHO CAN BENEFIT FROM THIS COURSE?

• A crime writer wishing to produce more authentic fiction.
• Anyone working in a legal office seeking to better understand aspects of their work
• Security guards learning to be more observant of the property they guard.
• Investigative journalists seeking to expand their understanding of criminal activity.
• Anyone considering a career in law enforcement or criminal law; to develop a fundamental understanding of the nature and scope of this subject prior to deciding on pursuing more in depth studies.
• Anyone else with a passion, or need to understand more about forensic science.