Sometimes courts will receive demeanour evidence, that is to say, evidence of how a person appeared or acted after or during an event. In its normal, natural everyday sense, the term “demeanour” refers to conduct, behaviour, a way of acting or to a person’s bearing, mien or outward manner.
Implicit in the reception of evidence of a person’s demeanour, however, are two invalid assumptions. The first assumes that for every action or event there is a normal reaction or manner of reaction. And the second is that an individual’s reaction actually reflects his or her inner emotional reaction or state:
R. v. Wall (2007), 2005 CanLII 80695 (ON CA), 2005 CanLII, 203 C.C.C. (3d) 232 (Ont. C.A.), at para. 49.
As a result of the invalid assumptions that underpin demeanour evidence, this evidence requires a predominance of probative value over prejudicial effect to be admissible:
Wall, at para. 50; see also R. v. Pannu, 2015 ONCA 677.
The evidence must be sufficiently clear to render its admission of value. As Doherty J.A. said in R. v. Trotta, 2004 CanLII 34722 (ON CA),  O.J. No. 4366, 190 C.C.C. (3d) 199 (C.A.), at para. 41:
The circumstances surrounding the proffered evidence must be such as to make that evidence sufficiently unambiguous and demonstrative of a relevant state of mind so as to overcome concerns that a trier of fact may too easily equate what is perceived to be an “unusual” reaction with a guilty mind.
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