General Principles

• Author: Eric V. Gottardi/Jennifer A. MacLellan/Michael Lacy/Robin Flumerfelt
• Pages: 3-20

3

1

General

Principles

I. Overview ................................................ 4

II. Qualification of an Expert .................................. 5

III. Duty of an Expert ......................................... 5

IV. The Courts’ Shifting Attitude Toward ExpertEvidence ............ 6

V. Admissibility of Expert Opinion Evidence ...................... 11

A. The Threshold Stage ................................. 12

B. The Gatekeeping Stage ............................... 14

C. Statutory Provisions Relevant to Admissibility:

CriminalCode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

VI. Canada Evidence Act ...................................... 19

VII. Conclusion .............................................. 19

© [2022] Emond Montgomery Publications. All Rights Reserved.

4 Part I Principles and Procedure

I. Overview

Experts play a vital role in our criminal justice system. Their testimony often has a

powerful influence over the outcome of a trial. In some cases, their testimony can

provide the sole basis for the court to identify the accused to the exclusion of all other

suspects; in other situations, an expert’s testimony may provide an alternative ex-

planation of events that can exonerate the accused. In a different vein, where mental

illness is in issue, expert evidence may be used to raise or rebut the argument that

the accused was not criminally responsible on account of mental disorder. There are

countless ways that an expert can impact the trajectory of a case.

While expert testimony can be instrumental in promoting the court’s truth-seek-

ing functions, it also poses significant dangers. As recognized by the Supreme Court

of Canada, expert evidence has the potential to distort the fact-finding process and

confuse the jury.1 The impressive credentials of an expert witness may lead the trier

of fact to accept their evidence as authoritative or to give the evidence more weight

than it deserves.2 Expert evidence is far from infallible; it is capable of being tainted,

tailored, unsubstantiated, couched in scientific terms, or based on unreliable facts

and ultimately debunked science.3 These shortcomings are not simply a theoretical

matter; expert evidence is a leading cause of wrongful convictions in our criminal

justice system.4 Taken together, the prominence and influence of expert evidence has

far-reaching implications for the administration of justice in Canada.

The power of expert witnesses stems in part from their unique position in the

justice system. Many of the rules that limit the scope of a lay witness’s testimony

do not apply to an expert witness. For example, unlike ordinary witnesses, who can

typically only testify as to facts personally observed, experts are not required to have

first-hand knowledge of the facts that form the basis of their opinions.5 In addition,

ordinary witnesses are generally prohibited from providing any opinion evidence; it

is the job of the trier of fact to decide what secondary inferences are to be drawn from

the evidence.6 Expert witnesses stand in direct contrast to this general rule. Since the

14th century, common law courts have recognized that certain exceptional matters

require the application of special knowledge that lies outside the experience of the

1 R v Mohan, 1994 CanLII 80, [1994] SCJ No 36 (QL) at para 19.

2 Ibid.

3 Federal, Provincial and Territorial Heads of Prosecutions Committee Working Group on the

Prevention of Miscarriages of Justice, Report on the Prevention of Miscarriages of Justice (2004),

online: Department of Justice <https://www.justice.gc.ca/eng/rp-pr/cj-jp/ccr-rc/pmj-pej/

index.html> at 115 [Report].

4 Ibid.

5 Sidney N Lederman, Alan W Bryant & Michelle K Fuerst, The Law of Evidence in Canada,

4thed (Toronto: LexisNexis, 2014) at 784.

6 Ibid; see also R v DD, 2000 SCC 43 at para 49.

© [2022] Emond Montgomery Publications. All Rights Reserved.