In general, the admissibility of evidence is dependent on whether the evidence is relevant to a fact in issue in the case. In DPP v Kilbourne the court held that evidence is relevant if it is logically probative or disapprobative of some matter which requires proof. The principle of relevance that governs admissibility is based on practical considerations such as time, cost, convenience, and, importantly, serve to disallow evidence where its prejudicial effect on a party outweighs its probative value. Relevant evidence may however still be excluded on the basis of common law and statutory rules of exclusion, such as, for example, the rules that unconstitutionally obtained evidence and hearsay evidence is excluded.
Against this general background, the question now becomes: what about opinion evidence?
Opinion evidence is an inference, impression, conclusion, or belief that a witness gives in support or opposition of an issue to be proved in court. In line with the general rule stated above, the constitutional court in Helen Suzman Foundation v President of the Republic of South Africa held that any opinion that is expressed on an issue that the court can decide without having to receive such opinion is in principle inadmissible because of its irrelevance. In other words, opinion evidence will only be admissible if the witness is in a better position than the court to give an opinion.
Opinion evidence can be categorised into expert evidence and layperson evidence. Admissibility, however, is not dependent on the distinction as such but is determined by the general rule. Procedurally, the distinction is noteworthy since a party is required to give notice of its intention to rely on expert evidence in civil cases. In criminal cases, the prosecution is required by the Constitution to disclose expert opinion evidence to the accused prior to the commencement of the trial.
Examples of where the opinion of a layperson may be relevant and admissible is where a witness can give evidence of the approximate age of a person, the state of sobriety of a person and the general condition of a thing. If a witness is unable to provide reasons for their opinion, this will not affect the evidence’s admissibility but rather the weight to be attached to the evidence.
Expert evidence, on the other hand, is often used to assist courts where the issues to be determined are of a scientific or complex nature, as is often the case with, for example, chemistry, ballistics, psychiatry, etc. Courts will however not simply admit evidence because the person testifying is an expert in the field. The courts have developed various principles to aid in determining whether expert evidence is admissible in a particular case.
In Holtzhauzen v Roodt the court formulated the following principles in this regard:
The rules regulating admissibility of evidence serve to help our courts to only focus on facts upon which the success of a party depends in law. Policy and practical considerations may assist courts with this task. Ultimately, the courts should uphold the fairness and integrity of the trial by ensuring that parties are not unfairly prejudiced by having to defend themselves against irrelevant evidence adduced against them.