Where psychiatric evidence on mental impairment is conflicting (not unanimous), the trial judge is not required to give special directions warning the jury against applying common sense or the reasoning of sane persons to the accused's conduct. The special directions discussed in Taylor, Mizzi, Weise and Matusevich & Thompson are confined to cases where unanimous medical evidence supports the defence but the accused's conduct gives an appearance of rationality. A trial judge has no duty to direct a jury that one expert witness should be preferred over another on the basis of superior qualifications or experience. The defence of mental impairment under s 20 of the Crimes (Mental Impairment & Unfitness to be Tried) Act 1997 (Vic) does not require proof of psychosis or loss of contact with reality.
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