How is a judge to evaluate a witness's credibility?
I suspect this approach is consistent with most common-law jurisdictions, but other answers for comparison would be great.
First, there is a distinction between credibility and reliability. R. v. T.T., 2020 ONCJ 368:
... I note the differences between credibility and reliability. Credibility relates to a witness's sincerity, whether he is speaking the truth as he believes it to be. Reliability relates to the actual accuracy of his testimony. In determining this, I must consider his ability to accurately observe, recall and recount the events in issue. A credible witness may give unreliable evidence.
And also, R. v. Morrissey (1995), 22 OR (3d) 514:
Testimonial evidence can raise veracity and accuracy concerns. The former relate to the witness's sincerity, that is, his or her willingness to speak the truth as the witness believes it to be. The latter concerns relate to the actual accuracy of the witness's testimony. The accuracy of a witness's testimony involves considerations of the witness's ability to accurately observe, recall and recount the events in issue. When one is concerned with a witness's veracity, one speaks of the witness's credibility. When one is concerned with the accuracy of a witness's testimony, one speaks of the reliability of that testimony. Obviously a witness whose evidence on a point is not credible cannot give reliable evidence on that point. The evidence of a credible, that is, honest witness, may, however, still be unreliable.
About credibility, see R. v. Gagnon, 2006 SCC 17, paragraph 20:
Assessing credibility is not a science. It is very difficult for a trial judge to articulate with precision the complex intermingling of impressions that emerge after watching and listening to witnesses and attempting to reconcile the various versions of events.
Other courts have listed some of the factors that might be relevant. R. v. J.T., 2019 BCCA 180 says the factors include:
- "a witness’s demeanor in the witness box, although it is recognized that it would be dangerous to rely wholly on demeanor to convict";
- "internal inconsistencies in a witness’s testimony at trial and/or external inconsistencies in a witness’s testimony with prior testimony or out-of-court statements";
- "any explanation by the witness for the inconsistencies";
- "any consistent evidence or corroborating evidence to support the witness’s evidence, although corroboration is not a requirement"; and
- "how a witness withstands cross-examination."
There are prohibited approaches to assessing credibility. Section 276 of the Criminal Code says that "evidence that the complainant has engaged in sexual activity... is not admissible to support an inference that, by reason of the sexual nature of that activity, the complainant... is less worthy of belief." It is an error to infer from a delay in reporting sexual assault that the complainant is less credible: "the trier of fact must not make an adverse inference on the complainant’s credibility based purely on the stereotype that any delay in disclosure indicates falsehood." R. v. D.D., 2000 SCC 43. It is also an error to "[judge] the complainant's credibility based solely on the correspondence between [their] behaviour and the expected behaviour of the stereotypical victim of sexual assault." R. v. A.R.J.D., 2018 SCC 6.
When it comes to determining a witness's credibility, judges and juries acting as finders of fact have several factors to take into account. For brevity, let's call those just "judges".
First and foremost, they will look at the witness's testimony itself. Does it make sense? Are there any inconsistencies? Are there any other witnesses who can corroborate the story?
Judges will also consider the witness's demeanor on the stand. Is the witness composed and relaxed, or are they nervous and fidgeting? Witnesses who seem nervous may be less credible, especially if they are changing their story from what they originally said.
Finally, judges will also consider any biases or ulterior motives the witness may have. For example, a witness who is related to the defendant may be more likely to give favorable testimony. All of these factors will be considered when a judge is determining whether or not to believe a witness's testimony.