A party may call an expert witness to testify about facts relevant to the credibility of one of their witnesses where those facts are likely to be beyond the experience of the trier of fact. The rule against oath-helping prevents the expert from going so far as to testify that the witness is likely to be telling the truth.
In some cases, factors relevant to the credibility of a witness are beyond the ordinary experience and understanding of lay triers of fact. Without the assistance of experts, lay triers of fact are apt to make erroneous assumptions about credibility. For example, laypersons may not appreciate that children cannot be expected to notice time and place the way that adults do, or that sexually abused children are prone to fantasize or to retract their allegations.8Or they may not appreciate that the professed inability to recall can be the result of hysterical amnesia.9
Where common experience does not provide the tools needed to assess the credibility of a witness, a party will be entitled to call an expert to provide that information, even though the testimony of the expert does nothing more than support the credibility of another witness.10This practice is not without its limits. The modern rule against oath-helping prevents expert witnesses from offering the opinion that a particular witness is telling the truth. The expert can provide background information relevant to the credibility of a witness, but not information directly about the credibility of what a witness is saying.11
 See, for example, R. v. J.(F.E.) (1990), 74 C.R. (3d) 296 (Ont. C.A.).
 R. v. Clark (1983), 35 C.R. (2d) 357 (Ont. C.A.).
 The law was once more guarded about admitting such evidence. See, for...