The plaintiff must prove, on the balance of probabilities, that it was Tony
[Doe] and/or Father Horwath who sexually assaulted him. You heard his evidence about this issue. You also heard circumstantial evidence related to the
opportunity each of Tony [Doe] and Father Horwath had or did not have to
sexually assault the plaintiff. You also heard evidence about the plaintiff’s
behaviour while at Maryvale and following, including the observations and
opinions expressed by Dr. Smith.
 In her review of the evidence, the trial judge did not
refer to any evidence from Dr. Smith, or opinions that he had
expressed, nor did she mention the evidence of Dr. Smith when
she provided instructions about damages.
 Objection was taken during the pre-charge conference
to the trial judge’s instruction that the jury could consider
Dr. Smith’s observations and opinions in determining whether
the sexual assaults had occurred. Defence counsel specifically
objected to the inclusion of any reference to Dr. Smith’s evidence
in relation to whether or not the plaintiff had been sexually
assaulted and argued that no mention should be made of
Dr. Smith’s evidence in relation to that question. This reference,
however, was not removed, and no change was made to this part
of the charge before it was delivered.
 A trial judge’s decision with respect to the admissibility of
expert evidence is entitled to deference and reversible on appeal
only if there is an error in principle or a material misapprehen-
sion of the evidence, or if the decision is unreasonable: see R. v.
Abbey (2009), 97 O.R. (3d) 330,  O.J. No. 3534, 2009 ONCA
624, at para. 97, leave to appeal to S.C.C. refused  S.C.C.A.
No. 125; R. v. Shafia,  O.J. No. 5627, 2016 ONCA 812, 341
C.C.C. (3d) 354, at para. 248, leave to appeal to S.C.C. refused
 S.C.C.A. No. 17.
 As I will explain, I am of the view that the trial judge erred
in admitting Dr. Smith’s opinion evidence on the issue of liability
(whether the sexual assaults occurred) and causation (whether
Mr. Imeson suffered any harm as a result).
(1) Dr. Smith’s role as a participant expert
 The appellant says that the trial judge erred by permitting
Dr. Smith to provide expert opinion evidence as a participant
expert. The appellant submits that, at most (and though unnec-
essary), Dr. Smith was properly qualified only to opine on