It is a time-honored rule of common law (and civil law) jurisdictions that witnesses ought only to relate their personal observations of events. They are not to attempt to enter their opinions into evidence in a court of law. The major exception to this is the expert witness who, because of their knowledge or experience in a specific area, is allowed to give opinion evidence. Generally, the rationale behind this is that judges can't possibly be knowledgeable in all areas of human activity and in any event, many cases before the courts turn on a scientific issue or upon special knowledge. Examples abound but the whole realm of medicine and especially cases where medical malpractice is alleged, can only be resolved through referral to expert evidence.
In R v Abbey, Justice Dickson of the Supreme Court of Canada wrote:
"With respect to matters calling for special knowledge, an expert in the field may draw inferences and state his opinion. An expert's function is precisely this: to provide the judge and jury with a ready-made inference which the judge and jury, due to the technical nature of the facts, are unable to formulate.
"An expert's opinion is admissible to furnish the Court with scientific information which is likely to be outside the experience and knowledge of a judge or jury. If on the proven facts a judge or jury can form their own conclusions without help, then the opinion of the expert is unnecessary."
In R v Mohan, Justice Sopinka of that same court added:
"Admission of expert evidence depends on ... relevance; necessity in assisting the trier of fact; the absence of any exclusionary rule;
(and) a properly qualified expert."
As Justice Jacob of the High Court of Justice of England and Wales noted in Pearce, at ¶61:
"Most (but not all) expert witnesses ... belong to some form of professional body or institute."
As Thomas Woods wrote in an article on topic in 2002:
"The only reason an expert is permitted to give opinion evidence is
because the facts that form the basis of the opinion are beyond the ken of the lay observer. Thus, in admitting expert opinion into evidence and giving it weight, the court (to that extent) delegates its fact-finding function to a witness."
The common law rule on expert evidence is often codified. For example, this from the 2010 English Civil Procedure Rules, at §35.3 and 35.4:
"It is the duty of experts to help the court on matters within their expertise. This duty overrides any obligation to the person from whom experts have received instructions or by whom they are paid.
"No party may call an expert or put in evidence an expert’s report without the court’s permission. When parties apply for permission they must identify the field in which expert evidence is required.... If permission is granted it shall be in relation only to the expert named or the field identified ...."
Or, this from the British Columbia Supreme Court (Civil) Rules of Court, 11-2(1):
"In giving an opinion to the court, an expert appointed under this Part by one or more parties or by the court has a duty to assist the court and is not to be an advocate for any party."
A proposed expert witness has to first be qualified; a process of submitting the expert's qualifications to the court, usually done by having the witness depose that his c.v., then tendered to the court, is truthful in all regards. If the judge's accepts the witness as an expert within the stated area of qualification, that witness can then, and exceptionally, give opinion evidence.
The judge is not bound to any expert evidence and often has to weigh and prefer the evidence of one expert against that of another.
In giving their evidence, expert witnesses are not expected to act as advocates but, rather, as objective vehicles of specialized knowledge. If the court perceives advocacy, the expert's evidence can be discredited or rejected. Where an expert's evidence is tainted with partiality, that expert is sometimes referred to as a hired gun.
To save time, some jurisdictions allow the presentation of expert witness statements in writing. See, for example, Rule 11-6 of the (Civil) Rules of Court of the Supreme Court of British Columbia.
A few practice points. An expert's evidence must state the facts upon which the opinion is based. It will be up to the party producing the expert, preferably before but possibly after the expert, to prove the facts upon which the expert opinion is tendered in to evidence.
As Justice Lowry wrote, at ¶14-15 of Rowe:
"[T]he statement of the opinion of an expert witness for which the rule provides proves no
more than the fact that the opinion is adduced to prove. It is not evidence of, and does not prove, the
facts upon which the opinion is based. They must be proven in some other way. ...
"Expert witnesses will often be able to testify to some or all of the facts necessary to the opinion
offered and can prove such from their own knowledge. But it is, of course, not necessary that an expert
witness have any personal knowledge of the facts...
"[T]he opinion must be based on stated facts or hypotheses to be proven
by evidence. In the present case the facts were not stated. And certainly the expert
could not find facts in any event on hearsay, unsworn and untested.
"It will usually be
for counsel to prove facts in Court upon which the opinion is based."
In an old English case, Pope v St. Leger
, the issue before Justice Holt was the applicability of the chess rule piece touche, piece joue
applied in Backgammon, as a player touched his piece but then refused to play it and sought to play another piece. To answer the question of whether "he was bound to play it", the judge called in the groom-porter
, an officer of the Royal court who had, as one of his tasks, to decide disputes which arose in gaming.