Presenting Expert Opinion Evidence

• Author: Justice Cameron Gunn/Mona Duckett/Patrick McGuinty
• Pages: 181-206

181

Presenting

Expert

Opinion

Evidence

11

I. The Preconditions to Expert Opinion Evidence ................. 

II. Conducting the Direct Examination of an Expert: Qualifying the

Expert Witness ........................................... 

A. Presenting Evidence of the Expert’s Impartiality and

Objectivity Is of Fundamental Importance ............... 

B. Bias and Impartiality in the Context of a Police Expert

Witness Who May Carry an Interest in the Litigation ....... 

III. Conducting the Direct Examination of an Expert: Adducing the

Expert’s Substantive Evidence ............................... 

A. Asking the Witness Hypothetical Questions Based on a

Specic Factual Framework ............................ 

B. Ensuring That the Expert Does Not Tread Outside Their

Area of Expertise .................................... 

C. Staying Away from Adducing Anecdotal Expert Evidence .... 

D. Being Careful About the Language Used by an Expert ...... 

E. Making the Expert’s Evidence Understandable ............ 

IV. Challenging the Expert’s Qualications ....................... 

V. Cross-Examining the Expert Witness .......................... 

Copyright © 2023 Emond Montgomery Publications. All Rights Reserved.

182 Part II Witness Presentation

The purpose of this chapter is to assist counsel in properly presenting the evidence of

an expert witness in an eective yet ethical manner. This, in turn, will allow an expert

witness to serve their very purpose, which is to provide the trier of fact with unbiased

and impartial evidence to assist them in the search for the truth. The guidance given

in this chapter on the presentation of an expert witness is tightly intertwined with

some of the principles outlined in Chapter 5, Preparing the Expert Witness, and, as

such, the chapters should be read together.1

Gone are the days when an expert witness served as a hired gun or advocate for the

party that retained them.2 Canadian law is haunted with examples of expert witnesses

who have directly and exclusively contributed to wrongful convictions and miscar-

riages of justice.3 As a result, expert witness evidence must be dealt with delicately.

Both prosecutors and defence counsel play an important role in this process. For

prosecutors who rely on expert witness evidence in a criminal trial setting, their duty

to the court and to the accused as well as their ethical obligations must govern their

every question. The mismanagement of expert witness evidence is extremely danger-

ous. Although expert evidence can assist a trier of fact in its fact-finding mission, it

is equally capable of misleading a fact finder and distorting the trial process. For that

reason, the law has carefully but forcefully placed an obligation on expert witnesses

to set aside any bias and provide a “true” impartial opinion based only on the facts

before them and their expertise. Put simply, an expert has an overriding obligation to

assist the court, not the party who calls the expert.

In R v Mohan,4 SopinkaJ articulated the dangers behind expert opinion evidence,

all of which remain relevant today:

There is a danger that expert evidence will be misused and will distort the fact-finding

process. Dressed up in scientific language which the jury does not easily understand

and submitted through a witness of impressive antecedents, this evidence is apt to

be accepted by the jury as being virtually infallible and as having more weight than it

deserves. As LaForestJ. stated inR. v. Béland, [1987] 2 S.C.R. 398, at p. 434, with

respect to the evidence of the results of a polygraph tendered by the accused, such evi-

dence should not be admitted by reason of “human fallibility in assessing the proper

weight to be given to evidence cloaked under the mystique of science.”5

1 In addition to this chapter and Chapter 5, Preparing the Expert Witness, see E Gottardi et

al, Qualifying and Challenging Expert Evidence (Toronto: Emond, 2022) for an excellent and

thorough overview of all areas of the law surrounding expert witnesses in Canada.

2 Bru-Murphy v Gunawardena, 2017 ONCA 502, leave to appeal refused, [2017] SCCA No 343

(QL) at para 1; Mouvement laïque québécois v Saguenay (City), 2015 SCC 16 at para 106.

3 See e.g. Inquiry into Pediatric Forensic Patholo in Ontario: Report (Toronto: Queen’s Printer,

2008) (Stephen T Goudge, Commissioner) [Goudge Report].

4 R v Mohan, [1994] 2 SCR 9, 1994 CanLII 80.

5 R v Mohan, [1994] 2 SCR 9 at 21, 1994 CanLII 80.

Copyright © 2023 Emond Montgomery Publications. All Rights Reserved.