Oath helping forbidden

Oath helping, having a witness attest to the credibility of another, was part of the early common law. One reads of cases from time immemorial where dozens of witnesses were called solely to say "if X swears an oath X ought to be believed". Such evidence has not be admissible since at least the Restoration.
Difficulties emerge, however, with regard to evidence regarding peculiarities in testimony that might be misunderstood to impact on credibility. Thus it may be proper to call an expert witness to explain that an abused child would, say, not come forward with a complaint right away. Sometimes such testimony slides into evidence that the child is telling the truth.

Such an slippage is, as today's decision in R. v. P.G., 2009 ONCA 32 makes clear, improper. The Court holds:

[16]          First, expert evidence is not rendered inadmissible simply because it may bear on the very factual issue to be decided by the trier of fact. Second, an expert witness may not offer an opinion as to the veracity of any witness, except in very exceptional cases. Third, expert opinion as to what factual inferences or conclusions should be drawn from the evidence concerning the "behaviour and symptomatology" of a child is admissible even though it indirectly enhances the credibility of the child's evidence. Fourth, the trial judge may control the format in which the expert evidence is given by excluding an expert's conclusory statements where the expert is able to express the opinion in less conclusory terms without detracting from its accuracy.
[17]          These principles, taken together, allow the Crown to present expert evidence that will assist the trier of fact while protecting the accused from the trier of fact placing undue weight on the expert's opinion. There exists, however, potential tension between the second and third principles. The second principle is that the expert is not permitted to express an opinion about the veracity of any witness. Despite this, the third principle states that the expert is permitted to offer an opinion (based on the child's "behaviour and symptomatology") that the child has been sexually abused even though it "indirectly enhances the credibility of the child's evidence."
[18]          The third principle qualifies but does not annul the second principle. While the scope permitted an expert witness by the third principle is broad, the court should be vigilant in ensuring that the expert in expressing his or her opinions does not violate the second. As Doherty J.A. noted in Khan v. College of Physicians and Surgeons of Ontario, though there is strong value and need for evidence in cases of child sexual abuse, a trier of fact "[f]aced with the often intractable problem of trying to decide who is telling the truth in cases of alleged child abuse ... may seek refuge in the apparent security and objectivity of the expert's opinion evidence." Major J. made much the same observation in R. v. D. (D), [2000] 2 S.C.R. 275 at para. 53, that there is a danger that "faced with an expert's impressive credentials and mastery of scientific jargon, jurors are more likely to abdicate their role as fact-finders and simply attorn to the opinion of the expert in their desire to reach a just result."

James Morton
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