Should Expert Testimony Be Allowed in Court?

Allowing experts to testify in courts is a decision most countries have struggled with. In the end, the majority of court systems around the world have determined very broad criteria for the admissibility of an expert's testimony. The criteria used in several countries are sometimes referred to as the "common knowledge rule". The "Common knowledge rule" basically allows for experts to testify on anything out of the realm of common knowledge. This tricky, however, as it must be determined what can be considered common knowledge. The differences in criteria for allowing expert testimony between countries such and England and Wales and Canada are mostly a result of differences in the interpretation of what is common knowledge.

In England and Wales, the landmark case of R v. Turner (1975, Q.B. 834) originally set the standards for what was considered "common knowledge" in the field of psychology. The ruling in this case with regard to psychological expert testimony was psychiatrists or psychologists were only needed to testify if there was a need to explain a mental abnormality, or a direct interpretation of the criminal's state of mind or intent. In essence, this case labeled normal human behavior as "common knowledge", unwarranting of translation by an expert in the court. This case still stands as a general guideline for accepting expert testimony. However, more exceptions to this rule have been made recently. In 1993, in the case of R V. Sally Loraine Emery, expert testimony was allowed to explain post-traumatic stress disorder and battered woman syndrome because it was determined the "General public" was not educated on these topics. Even more recently, some experts have even been allowed to testify on complex issues of eyewitness testimony, although these cases are few in number. England and Wales essentially still follow the ruling of R v. Turner, but the courts are now open to broader interpretation of this ruling and exceptions are generally determined on a case by case basis.

In some aspects, Canada has placed stricter guidelines for the admissibility of expert testimony, yet in some aspects, Canada may be the most liberal in admission. Unlike England and Wales, Canada has yet to allow expert testimony on eyewitness identification. The courts of Canada have also refused testimonies on the operation of memory. In the 1994 case of R v. Mohan, four criterions were outlined for the admissibility of expert testimony: relevance; necessity in assisting the trier of fact; absence of an exclusionary rule; and properly qualified expert (Kapardis, pg. 201). Once again, this outline of admissibility can be interpreted as a common knowledge rule and is somewhat broad. The courts of Canada have determined, as in the case of R v. Johnston (1992, 69 CCC 395), new scientific evidence (without the general approval of experts in the field in which it originates) may be translated by expert witnesses. It is in this manner the courts of Canada may be considered more stringent and more lenient in their interpretation of the "common knowledge" rule.