A special thanks to Judge Wayne Gorman. Determining who is telling the truth is not easy. Inherently, judges or juries can make mistakes driven by implicit bias or by myths about factors that suggest someone is telling the truth. Judge Gorman came across a recent New Zealand case that raises the issue of how juries should be instructed on demeanor:
In Taniwha v. The Queen  NZSC 121, September 8, 2016, the accused was convicted in a trial before a jury of a number of physical and sexual assaults. In addressing the jury, the prosecutor referred to the complainant’s reaction when shown a photograph and suggested that her reaction “wasn’t made up, that wasn’t faked.”
On appeal, the accused argued that the trial judge ought to have provided the jury with a “tailored demeanour direction” (i.e., explaining that demeanour in the witness stand should not be over-emphasized).
The appeal was dismissed. The Supreme Court of New Zealand concluded that “the Judge’s failure to give a tailored demeanour direction in summing up to the jury” did not give “rise to a miscarriage of justice” (at paragraph55).
In rendering its judgment, however, the Supreme Court suggested that there is “research which indicates that a person’s demeanour when giving evidence in court generally provides little or no assistance to a fact-finder charged with determining whether or not the witness is telling the truth. A witness who presents as confident, articulate and honest may be mistaken or dishonest; a witness who presents as diffident, hesitant or awkward may be telling the truth and their evidence may be accurate. Not only can appearances be deceptive, but fact-finders may over-estimate their ability to recognise those who are truthful from those who are not, by, for example, relying on unreliable behaviours such as fidgeting or looking away.”
The Supreme Court suggested that in instructing a jury on demeanour, the judge should identify “factors which will help jurors to determine whether a witness is telling the truth. This should reflect the particular circumstances of the case as far as they can be assessed in advance, but could include reference to considerations such as”:
(a) Whether the witness’s evidence is consistent with the evidence of other witnesses which the jury has accepted.
(b) Whether the witness’s evidence is consistent with objective evidence such as documents or text messages, and if it is not, what explanation is offered for any inconsistencies.
(c) Whether the witness’s account is inherently plausible – does it make sense? Is it likely that people would have acted in the way suggested?
(d) Whether the witness has been consistent in their account over time and, if not, why not?
The Supreme Court also suggested that “a direction along the following lines could be given”
I must warn you, though, that simply observing witnesses and watching their demeanour as they give evidence is not a good way to assess the truth or falsity of their evidence. For example, a witness may not appear confident or may hesitate, fidget or look away when giving evidence. That doesn’t necessarily mean that their evidence is untruthful. The witness may be understandably nervous giving evidence in an unfamiliar environment in front of unknown people. Or there may be cultural reasons for the way a witness presents. On the other hand, a witness may appear confident, open and persuasive but nevertheless be untruthful. And remember that even an honest witness can be mistaken.
Things like gestures or tone of voice may sometimes help you to understand what the witness actually means. But you should be cautious about thinking that they will help you much in determining whether or not the witness is telling the truth.