Dangers of Expert Evidence

R. v. Singh, 2014 ONCA 791:

[31]       It is important not to lose sight of the scope of a trial judge’s ‘gate-keeping’ function in relation to expert evidence.  A ruling in favour of the admission of expert evidence does not discharge the trial judge’s obligation to protect against the misuse of such evidence.  In R. v. Abbey, 2009 ONCA 624, 97 O.R. (3d) 330, at para. 62, Doherty J.A. of this court warned:

A cautious delineation of the scope of the proposed expert evidence and strict adherence to those boundaries, if the evidence is admitted, are essential.  The case law demonstrates that overreaching by expert witnesses is probably the most common fault leading to reversals on appeal. [Citations omitted.]

[32]       More recently, in affirming the importance of the Abbey caution and the oversight role of a trial judge concerning expert evidence, Moldaver J. of the Supreme Court, writing for the majority, stressed in Sekhon, at paras. 46 and 47:

Given the concerns about the impact expert evidence can have on a trial – including the possibility that experts may usurp the role of the trier of fact – trial judges must be vigilant in monitoring and enforcing the proper scope of expert evidence.  While these concerns are perhaps more pronounced in jury trials, all trial judges – including those in judge-alone trials – have an ongoing duty to ensure that expert evidence remains within its proper scope.  It is not enough to simply consider the Mohan criteria at the outset of the expert’s testimony and make an initial ruling as to the admissibility of the evidence.  The trial judge must do his or her best to ensure that, throughout the expert’s testimony, the testimony remains within the proper boundaries of expert evidence. …

The trial judge must both ensure that an expert stays within the proper bounds of his or her expertise and that the content of the evidence itself is properly the subject of expert evidence.  [Citations omitted; emphasis added.]

[33]       Justice Moldaver continued, at para. 48, by noting that trial judges “are accustomed to disabusing their minds of inadmissible evidence.”  Nonetheless, he emphasized:

It goes without saying that where the expert evidence strays beyond its proper scope, it is imperative that the trial judge not assign any weight to the inadmissible parts.