Opinion Evidence & Experts
Generally, witnesses are not entitled to give evidence in relation to matters which are not within their direct knowledge. They are not entitled to give opinions on facts or draw inferences from them. In principle, this is a matter for the judge or where there is a jury, the jury as directed by the judge.
In modern times, with the advance of sciences, the role of the expert in the fact finding process has increased. Pathologist, forensic scientists, psychiatrists, DNA experts and fingerprint experts now play an important role in criminal proceedings. Medical practitioners and specialists, actuaries, accountants, surveyor and consultants commonly given expert evidence in civil cases.
Opinion evidence given by experts is an exception to the general prohibition on opinion evidence. Expert witnesses may assist the judge or jury with their expertise in drawing inferences which are outside of the knowledge and expertise of the judge or jury.
A witness put forward as an expert must prove his expertise. It must be an expert in the relevant discipline. The discipline must comprise a consistent although not necessarily a uniform body of knowledge. It should be a methodical and predictive body, with an appropriate basis in research. Courts may limit speculative opinions and require that expertise be validated by established theory and technique and proper scientific methodology.
Expert witnesses play an important role in many civil cases. In relation to cases of professional negligence, the evidence of expert witnesses is often critical. This is especially common in the case of medical negligence, where a negligence is unlikely to be found if there is a credible body of support for the course actually taken.
The evidence of expert raises issues of hearsay. Experts generally draw on research and publications within their field. They must identify their sources and methodology so that it can be tested. They are also likely to assume certain facts. These must be made explicit for challenge.
In theory, an ordinary, non-expert witness should narrate primary observed facts only. However, in practice, it would be impossible to describe many situations and behaviour at this low level of detail. Accordingly, judges exercise a degree of discretion and common sense in allowing a witness to give meaningful descriptive evidence.
Opinions on Credibility
A witness should not express a view on the ultimate issue in the case. This is reserved for the judge or jury. If the expert were to express an opinion on it, he would usurp the function of the judge or jury. Expert evidence should not be necessary on matters of everyday common sense and perception. If the judge and jury can reach their own conclusions without expert help, then opinion evidence is not appropriate. Many issues which arise in proceedings involve an assessment of human nature and behaviour, which are within the competence of the judge (or jury).
Historically, evidence was not allowed as to whether a witness was telling the truth or whether his testimony was reliable. The UK Courts have modified this principle and permitted expert evidence that the witness suffers from particular a mental disease or abnormality, such that he is incapable of giving a true and reliable account to the jury. The expert may only comment on whether the evidence is reliable in light of his mental condition.
A psychiatrist may not give evidence in relation to credibility. They may not comment in relation to truthfulness in the witness box. This will usually also be prohibited in any event, as evidence on a collateral issue (credibility).
Expert evidence may be useful to explain a person\’s mental condition or the fact that they suffer from a particular condition. The criminal courts are reluctant to receive psychiatric or psychological evidence in criminal trials, which purport to establish the accused’s intent and state of mind, for the purpose of criminal liability. However, expert evidence may be allowed in relation to the defence of insanity (and its equivalent in civil proceeding). The psychiatrist may give an opinion as to the accused’s capacity to understand the crime. He may not, however, comment on the accused intention or lack thereof at the time of the crime.
In criminal matters, DNA and forensic evidence have grown investigation in the last 40 years. DNA evidence fingerprinting and profiling potentially offers a unique identifier of identity. Although DNA evidence may be reliable in principle, there are significant risks of contamination and risks in relation to the chain of custody which may lead to overreliance and overconfidence. However, a number of prominent wrongful convictions based on apparently reliable forensic scientific evidence were revealed in the 1990s, in the UK.
Because of the danger of DNA evidence appearing conclusive, the judge should warn in the danger of conviction in exclusive reliance on such evidence, in the absence of corroboration or other appropriate direction.
Opinion evidence by experts is admitted more liberally in child welfare matters. Videotaped interviews of children by social workers are potentially admissible by the Childrens Act 1997.