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Expert Evidence in Canada, A Primer: R. v. Mohan

The use of expert evidence in criminal cases requires a degree of caution. While expert evidence tendered by either the Crown or the Defence can be a powerful tool to prove guilt or innocence, there is also an inherent danger that the trier of fact will place an overemphasis on the evidence.

In criminal jury cases, there is a division between the finder of fact, and law. The role of the jury is to listen closely to the evidence and make a determination as to what actually occurred in the case. The jury is then required to apply their findings of fact to the law directed by the Judge. In judge alone cases, the trial judge fulfills both roles.

The law of evidence in Canada generally holds that witnesses are only permitted to testify as to their own observations. For example, in a bank robbery case, a lay witness may properly testify as to distinct markings that they notice on the accused, as well as what they remember from the particular event. It would be improper for a lay witness to go a step further and state at trial, “in my opinion, the person seated with Defence counsel committed the robbery.” This is a conclusion that is properly the role of the jury or the trial judge to determine after assessing all of the evidence in the case.

Expert witnesses are an exception to the general evidentiary rule that prohibits opinion evidence. Individuals who are properly qualified as “experts,” are entitled to present opinions and conclusions on issues that they are qualified to comment upon.

Against this background, we can see the potential problems that expert evidence may raise. There are many different types of experts that may be called to testify in a given case. When people think of “experts” they typically think of doctors, psychiatrists, pathologists, and various forensic scientists. In drug cases, however, it is not uncommon for the Crown to seek to call police officers to testify as “experts.”

In drug cases, it is not uncommon for the Crown to call police officers with experience in drug related matters to testify as to: the importance of debt lists or other drug paraphernalia to the likelihood that the accused is engaged in drug trafficking; drug consumption rates; counter surveillance techniques; drug packaging; and the wholesale and street value of various drugs.

The overarching danger of expert evidence is that the finder of fact will simply accept the opinions and conclusions of the purported “expert,” to the exclusion of all other evidence presented in the case.

In light of the potential risk to an accused’s fair trial, the Supreme Court of Canada, in a case called R. v. Mohan, outlined the criteria that must be considered before a witness may give expert evidence at trial.

These criteria are:

a)     Relevance of the evidence;

b)    The necessity of the evidence in assisting the trier of fact;

c)     The absence of any exclusionary rule to the reception of evidence;

d)    The proposed expert being properly qualified.


A) Relevance

Before a piece of evidence may be admitted at trial it must be relevant. According to Canadian evidence law, evidence will be relevant if it tends to make the fact in issue more probable than it would be without the proposed piece of evidence.

Even if evidence is found to be relevant, it might still be excluded if the judge finds that the admission of the evidence would cause more prejudice than its ultimate benefit.

In the case of expert evidence, courts must be alive to the danger that expert evidence will be misused and will distort the fact-finding process. This may result in the exclusion of evidence even if it meets the threshold of logical relevance.

B) Necessity

Because experts are permitted to provide “opinion” evidence, the Court clarified that the evidence must be necessary before it can be admitted. In determining necessity, the Court explained, “What is required is that the opinion be necessary in the sense that it provide information which is likely to be outside the experience and knowledge of a judge or jury.”

There used to be a general rule of evidence called: “ the ultimate issue rule.” This particular rule prevented witnesses from providing opinions or conclusions that touched upon the “ultimate” issue in the case—usually the ultimate question of guilt or innocence. An example would be a police expert testifying: “based on the facts, it is my opinion that Mr. X was involved in drug trafficking.” This is an ultimate conclusion, and properly the role of the jury to determine. The police expert might, however, be permitted to testify that: “drug traffickers typically have scales, debt lists, and large sums of money in their possession.”

The Supreme Court held that although the ultimate issue rule no longer generally applies, the dangers contemplated by the rule are still relevant. As such, an expert’s opinion that approaches the ultimate issue in the case must be carefully considered to make sure that it is truly necessary for the jury to hear. If the judge or jury can determine the issue based on their knowledge and experience, the expert ought not to be permitted to give the evidence.

C) Absence of Any Exclusionary Rule

Even if particular expert evidence is logically relevant to the issue at trial and is necessary, the evidence may still not be admitted if its reception is prohibited by another exclusionary rule. For example, if the particular expert evidence relies upon hearsay, it may be excluded. Again, particular expert evidence may be excluded if it’s prejudicial value outweighs is probative (beneficial) effect.

D) Properly Qualified Expert

Before a witness will be permitted to give expert evidence, they must be properly qualified as an expert. This means that the proposed expert must have acquired special or peculiar knowledge through study or experience in respect of matters on which he/she seeks to testify.

The Courts have regularly endorsed select scientific fields as areas within which experts may provide expert evidence. These areas include: pathology, psychiatry, toxicology, anthropology, etc. The particular expert, however, must still be qualified. This is usually done by submitted a C.V. of the “experts” qualifications in a particular subject/area. In certain cases, the Defence or Crown may seek to challenge the witness’s qualifications to give a particular expert opinion. In many cases, the Crown or Defence will concede that a particular witness is, in fact, a properly qualified expert.

Where a witness is sought to testify in an area considered to be “novel science,” special scrutiny is required and an examination of the witness’s qualifications and experience will be conducted before they will be allowed to give expert evidence.