R v Mohan

Mohan set the foundational Canadian test for the admissibility of expert evidence. The proponent must show: (1) relevance, (2) necessity in assisting the trier of fact, (3) absence of any exclusionary rule, and (4) a properly qualified expert.

Mohan, a paediatrician, was charged with sexual assault on patients. He sought to call a psychiatrist to testify that his personality profile did not fit the perpetrator profile.

When is expert evidence admissible?

Excluded — necessity not met.

Four-part test: (1) relevance — logical and legal; (2) necessity — the subject matter is outside ordinary experience and the expert assists the trier of fact; (3) absence of an exclusionary rule (e.g. character, hearsay, privilege); (4) properly qualified expert. The trial judge acts as gatekeeper, weighing probative value against prejudicial effect.

Sopinka J held that profile evidence was not necessary because the issue (whether Mohan committed the acts) was within ordinary jury competence. Allowing it would risk distorting deliberation by importing pseudo-scientific personality typology.

The starting point for every expert-evidence ruling. Refined by White Burgess (2015), which adds an explicit duty of impartiality, and by JLJ and Trochym on novel science.

R v Mohan, [1994] 2 SCR 9, 1994 CanLII 80 (SCC).

Lamer CJ, La Forest J, L'Heureux-Dubé J, Sopinka J, Gonthier J, Cory J, McLachlin J, Iacobucci J, Major J