Legal Opinion

Legal facts shouldn’t be “alternative” ones

The “sufficient basis” requirement as an implied factor in Mohan

The Court in Mohan[1] implied — but failed to state clearly — the rule that the admissibility of expert evidence requires to show a sufficient basis for the knowledge used. This lack of proper clarification has led to inconsistency which is still present in the Canadian jurisprudence.

The trial judge in Mohan explicitly put the very basic assumption of the evidence at issue: the existence itself of any “unusual features” which would indicate “a member of a limited and distinguishable group.”[2] The judge was clear about the concern, “There is no scientific data available to warrant that conclusion. A sample of three offenders is not a sufficient basis.”[3] With no basis found, the trial judge stripped the evidence of its expert look and, as a result, rendered it inadmissible simply as a general character evidence, with nothing “expert” in its nature.[4]

Arguably, the Court of Appeal went astray, reversing the trial judge’s decision because of the logical relevance of the expert evidence.[5] Holding that the evidence “was admissible to show that the respondent was not a member of either of the unusual groups of aberrant personalities,”[6] Finlayson J.A. took the very existence of those groups for granted, relying basically on “all” of the expert’s “personal experience,”[7] however unspecific[8] it might be. The Court’s of Appeal argument, thus, missed the very gist of the problem: before deciding about the relevance of the expert knowledge, one must verify whether the knowledge exists at all. Therefore, by reversing the Court’s of Appeal decision, the Supreme Court inevitably had to have corrected this error, though, unfortunately, without stating it in an explicit way.

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