Expert Evidence

• 2017onca502

  2017onca502

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• R v Mohan

  [1994] 2 SCR 9 · Supreme Court of Canada · sexual assault

  R. v. Mohan

  [1994] 2 S.C.R. 9
  Supreme Court of Canada — Sopinka J. | May 5, 1994

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  Background

  Dr. Chikmaglur Mohan, a practicing pediatrician in North Bay, was charged with four counts of sexual assault on female patients aged 13 to 16. The alleged assaults occurred during medical examinations in his office. The complainants had been referred to Dr. Mohan for various conditions, some psychosomatic in nature. All four complainants testified that Dr. Mohan did not wear gloves during intimate examinations and engaged in inappropriate sexual conduct including fondling and digital penetration.

  Dr. Mohan's defence strategy was unusual: he wanted to call a psychiatrist, Dr. Hill, to testify that based on his expert opinion, someone who committed these types of offences would belong to a "limited and unusual group of individuals" and that Dr. Mohan did not possess the psychological characteristics of such a group. Essentially, the defence wanted to argue that Dr. Mohan wasn't the *type* of person who would commit sexual assault.

  Issues

  1. When is expert evidence admissible in criminal trials?
  2. Can an accused person introduce expert evidence to show they lack the disposition to commit the alleged offences?

  The Expert Evidence Gatekeeping Rules

  The Supreme Court established the definitive test for admitting expert evidence in Canadian courts. Expert evidence is only admissible if it meets four strict criteria: (1) relevance — it must be logically connected to an issue in the case; (2) necessity — it must provide information outside the experience and knowledge of judge and jury; (3) absence of exclusionary rule — no other legal rule prohibits it; and (4) properly qualified expert — the witness must have genuine expertise in the relevant field.

  The Court emphasized that expert evidence carries special dangers. Dressed up in scientific language that juries don't easily understand, it can be given more weight than it deserves. There's also a risk that expert evidence will overwhelm the fact-finding process, turning trials into "battles of the experts" rather than careful examination of the actual evidence.

  Character Evidence Through the Back Door

  The Court ruled that Dr. Hill's proposed testimony was inadmissible character evidence disguised as expert opinion. The fundamental rule in criminal law is that the Crown cannot introduce evidence of an accused person's bad character to prove guilt, and by the same token, accused persons cannot simply call experts to testify that they have good character or lack criminal disposition.

  Dr. Hill's opinion — that sexual offenders belong to identifiable psychological groups and that Dr. Mohan didn't fit the profile — was essentially asking the jury to conclude that because of his personality traits, Dr. Mohan was unlikely to have committed these crimes. This violated the character evidence rules and risked misleading the jury with unreliable psychological profiling.

  Key Takeaway

  Expert evidence must provide necessary scientific insight, not dress up inadmissible character arguments in psychological clothing.

  This case established the framework that courts across Canada still use today to determine whether expert testimony should be allowed. For self-represented defendants, it's a reminder that expert evidence isn't automatically admissible just because someone has impressive credentials — it must genuinely help the court understand technical issues beyond common knowledge.

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• r v Mohan [1994] 2 SCR 9

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• R v Prasad

  [2002] O.J. No. 267

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