Expert Opinion - Ultimate Issue. Parliament v. Conley
In Parliament v. Conley (Ont CA, 2021) the Court of Appeal considered the risk of an expert witness being believed by a jury on the 'ultimate' issue of credibility:
 Expert evidence carries with it the risk that a jury “will inappropriately defer to the expert’s opinion rather than carefully evaluate it”: White Burgess Langille Inman v. Abbott and Haliburton Co., 2015 SCC 23,  2 S.C.R. 182, at para. 17; R. v. Mohan, 1994 CanLII 80 (SCC),  2 S.C.R. 9, at pp. 21-22. The test for admissibility of expert evidence consists of two steps. First, the proponent of expert evidence must establish that four conditions are met in order to establish its admissibility: relevance; necessity in assisting the trier of fact; absence of an exclusionary rule; and a properly qualified expert. The second stage of the inquiry requires the trial judge to conduct a cost-benefit analysis to determine whether otherwise admissible expert evidence should nevertheless be excluded because its probative value is outweighed by its prejudicial effect.. Hamilton v. Bluewater Recycling Association
 The ultimate conclusion as to the credibility or truthfulness of a particular witness is for the trier of fact, and it is not the proper subject of expert opinion. The rationale for this policy is that credibility is a notoriously difficult problem, and a frustrated jury may readily accept an expert’s opinion as a convenient basis upon which to resolve its difficulties: see R. v. Marquard, 1993 CanLII 37 (SCC),  4 S.C.R. 223, at p. 248. In addition, one of the central dangers of expert evidence is that finders of fact, and juries in particular, may be too ready to rely on experts who appear to be knowledgeable, credible and reliable. Doherty J.A. explained this danger in R. v. Abbey, 2009 ONCA 624, 97 O.R. (3d) 330, at para. 90, leave to appeal refused,  S.C.C.A. No. 125:
Clearly, the most important risk is the danger that a jury will be unable to make an effective and critical assessment of the evidence. The complexity of the material underlying the opinion, the expert's impressive credentials, the impenetrable jargon in which the opinion is wrapped and the cross-examiner's inability to expose the opinion's shortcomings may prevent an effective evaluation of the evidence by the jury. There is a risk that a jury faced with a well-presented firm opinion may abdicate its fact-finding role on the understandable assumption that a person labelled as an expert by the trial judge knows more about his or her area of expertise than do the individual members of the jury. It is well established that a trial judge’s role as gatekeeper is not exhausted once a particular expert has been permitted to testify on the basis of their qualifications and the content of their report: see Bruff-Murphy v. Gunawardena 2017 ONCA 502, 138 O.R. (3d) 584, at paras. 62-66, leave to appeal refused,  S.C.C.A. No. 343; R. v. Sekhon, 2014 SCC 15,  1 S.C.R. 272, at paras. 46-47. Rather, the trial judge must protect the integrity of the process by ensuring that the expert does not overstep the acceptable boundaries in giving evidence. As Doherty J.A. stated in Abbey, at para. 62, it is essential that trial judges ensure both “[a] cautious delineation of the scope of the proposed expert evidence and strict adherence to those boundaries, if the evidence is admitted”.
 However, even when the content and scope of an expert’s evidence is delineated in advance, the expert may stray in the course of their oral testimony. As Hourigan J.A. noted in Bruff-Murphy, at paras. 62-63:
A trial judge in a civil jury case qualifying an expert has a difficult task. She must make a decision based on an expert report that will, in most cases, never be seen by the jury. While the report provides a roadmap of the anticipated testimony and specific limits may be placed on certain areas of testimony, the trial judge obviously cannot predict with certainty the nature or content of the expert’s testimony. The continuing gatekeeping role means that trial judges must not only continue to ensure that the expert’s actual testimony does not overstep the appropriate scope of the expert evidence; they must also include ensuring that the expert’s testimony continues to be independent in the sense that the expert does not become an advocate for the party by whom they are called.
Where, as here, the expert’s eventual testimony removes any doubt about her independence, the trial judge must not act as if she were functus. The trial judge must continue to exercise her gatekeeper function. After all, the concerns about the impact of a non-independent expert witness on the jury have not been eliminated. To the contrary, they have come to fruition. At that stage, when the trial judge recognizes the acute risk to trial fairness, she must take action. [Emphasis added.]
 As Hourigan J.A. made clear in Bruff-Murphy, the continuing gatekeeper role of a trial judge includes the continuation of the residual discretion to exclude evidence when they are not satisfied that the testimony’s probative value exceeds its prejudicial effect: at paras. 65-66; R. v. White, 2011 SCC 13,  1 S.C.R. 433, at para. 50; and R. v. Bingley, 2017 SCC 12,  1 S.C.R. 170, at para. 30.
In Hamilton v. Bluewater Recycling Association (Ont CA, 2016) the Court of Appeal confirms that expert witnesses may now give opinions on the 'ultimate issues' of a case, being fact-findings to be made by the judge or jury:
 The appellants submit that opinion evidence was improperly adduced by defence counsel during cross-examination of the appellants’ two expert witnesses, Travis Fricker, an accident reconstruction expert, and Dr. Christina Rudin-Brown, a human factors expert. The appellants say that these witnesses answered questions concerning their opinion as to the cause of the accident that were unrelated to their expertise and went directly to the ultimate issue for the jury. The appellants submit that there is a risk that the jury was “overwhelmed” by inadmissible opinion evidence.. R v Solleveld
 We disagree.
 First, there is no longer a general prohibition on the admission of expert evidence concerning the ultimate issue: see R. v. Mohan, 1994 CanLII 80 (SCC),  2 S.C.R. 9, at p. 24; R. v. Lucas, 2014 ONCA 561 (CanLII), 121 O.R. (3d) 303, leave to appeal refused,  S.C.C.A. No. 460, at para. 271; Hoang v. Vicentini, 2016 ONCA 723 (CanLII), at para. 62.
In R. v Solleveld (Ont CA, 2014) the Court of Appeal considered whether the expert's opinion came to close to the 'ultimate issue' that the fact-finder was charged with:
 Nor, in our view, should the impugned evidence have been excluded because it tread near the “ultimate issue” for the jury. . Whitfield v. Whitfield
 The Supreme Court of Canada has repeatedly affirmed that the common law rule precluding expert evidence on the ultimate issue no longer applies in Canada: R. v. Burns, 1994 CanLII 127 (SCC),  1 S.C.R. 656, at para. 25:
While care must be taken to ensure that the judge or jury, and not the expert, makes the final decisions on all issues in the case, it has long been accepted that expert evidence on matters of fact should not be excluded simply because it suggests answers to issues which are at the core of the dispute before the court: Graat v. The Queen, 1982 CanLII 33 (SCC),  2 S.C.R. 819. See also Khan v. College of Physicians and Surgeons of Ontario (1992), 1992 CanLII 2784 (ON CA), 9 O.R. (3d) 641 (C.A.), at p. 666 (per Doherty J.A).See also R. v. Bryan (2003), 2003 CanLII 24337 (ON CA), 175 C.C.C. (3d) 285, at para. 16: “there is now no general rule precluding expert evidence on the ultimate issue”.
 It was appropriate for the Crown to lead generic evidence as to legitimate and illegitimate international banking practices and documents. As Goudge J.A. observed in Bryan, at para. 19, (dealing with evidence about the indicia of possession of cocaine for the purpose of trafficking) where generic evidence is necessary to assist the jury, “it is hard to resist the same conclusion when that evidence is brought down to the specifics of this case.”
 Mohan holds, at paras. 24-25, that increased scrutiny of relevance and necessity are called for the closer one comes to the ultimate issue. In our view, the impugned evidence survives that increased level of scrutiny.  Professor Byrne’s evidence was highly probative of the issue the jury had to decide. Appropriate steps were taken to ensure that he did not usurp the function of the jury as the ultimate fact finder.  Professor Byrne explicitly testified that he could only comment on the characteristics of the documentation used and that he had no direct knowledge of whether any fraud had actually been committed. When Byrne was testifying, the trial judge told the jury:
It’s been very interesting to hear these comments but they are in a sense observations by this gentleman from a fairly specialized position. That doesn’t necessarily mean that the three people before you have committed such a crime. All right? These are if anything, I guess specialized generalizations… In his jury charge, the trial judge gave a similar instruction: “The professor emphasized that he could not venture an opinion in this case aside from the nature of the documentation he reviewed…” The jury was also instructed that they were not required to accept the opinions of expert witnesses and cautioned that “how much or little you believe or rely upon the expert’s opinion is entirely up to you.”
 We see no error in the admission or the treatment of the expert evidence. Accordingly, we would not give effect to this ground of appeal.
In Whitfield v. Whitfield (Ont CA, 2016) the Court of Appeal canvasses basic principles of the use of expert evidence, and in particular the tension that can exist between use of the expert's opinion evidence and the court's role as ultimate fact-finder:
 The role of an expert witness in the trial process has been the subject of much judicial and other consideration. It is well established that the purpose of expert testimony is to assist the trier of fact by providing information about a specialized field of inquiry which is likely to be outside the experience of a judge or jury: R. v. J.-L.J., 2000 SCC 51 (CanLII),  2 S.C.R. 600, 2000 S.C.C. 51, at para. 56.
 There is no question that expert evidence is essential to the litigation process and that many cases involving highly technical areas could not be tried without it: R. v. Abbey, 2009 ONCA 624 (CanLII), 97 O.R. (3d) 330, at para. 73.
 However, the nature of expert evidence and its allure of scientific infallibility give rise to the very real potential that it will “swallow whole the fact-finding function of the court”: Abbey, at para. 71. As a result, courts must remain vigilant to draw a firm line between the role of the expert and the role of the court to ensure that experts not be permitted to usurp the functions of the trier of fact: J.-L.J., at paras. 25-26. Indeed, the closer the expert’s evidence approaches an opinion on an ultimate issue, the stricter must be the application of this principle: J.-L.J., at para. 37.
 Nowhere is this distinction more important than in relation to the issues of credibility and reliability that a trier of fact is called upon to determine in every trial. Expert evidence on human conduct and the psychological and physical factors which may lead to certain behaviour relevant to credibility is permitted, provided the testimony goes beyond the ordinary experience of the trier of fact: R. v. Marquard, 1993 CanLII 37 (SCC),  4 S.C.R. 223, at pp. 248-49. Ultimate issues of credibility and reliability, however, are issues well within the experience of judges and juries and ones in respect of which no expert evidence is required: R. v. Béland, 1987 CanLII 27 (SCC),  2 S.C.R. 398, at pp. 415-16.
 These principles are illustrated in Marquard, which involved an allegation of aggravated assault of a child by her grandmother. At issue was the trial judge’s direction to the jury that they could rely on the expert evidence called to corroborate the truth of the child’s testimony. In ordering a new trial, the Supreme Court confirmed that the jury direction was erroneous because “the ultimate conclusion as to the credibility or truthfulness of a particular witness is for the trier of fact, and is not the proper subject of expert opinion”: p. 248. The court explained why this is so, at p. 248:
A judge or jury who simply accepts an expert’s opinion on the credibility of a witness would be abandoning its duty to itself determine the credibility of the witness. Credibility must always be the product of the judge or jury’s view of the diverse ingredients it has perceived at trial, combined with experience, logic and an intuitive sense of the matter.… Credibility is a matter within the competence of lay people. Ordinary people draw conclusions about whether someone is lying or telling the truth on a daily basis. The expert who testifies on credibility is not sworn to the heavy duty of a judge or juror. Moreover, the expert’s opinion may be founded on factors which are not in the evidence upon which the judge and juror are duty-bound to render a true verdict. Finally, credibility is a notoriously difficult problem, and the expert’s opinion may be all too readily accepted by a frustrated jury as a convenient basis upon which to resolve its difficulties. All these considerations have contributed to the wise policy of the law in rejecting expert evidence on the truthfulness of witnesses.