In diminished responsibility cases under s 304A of the Criminal Code (Qld), the trial judge must direct the jury on the meaning of 'substantially impaired' — at minimum, that the impairment need not be total but must be more than trivial. Expert psychiatric witnesses must be instructed on the correct legal meaning of 'substantially impaired' before forming their opinions on the ultimate issue; opinions formed without such instruction are inadmissible as irrelevant or misleading. Whether further direction beyond the 'more than trivial but less than total' formulation is required depends on the circumstances of the particular case.
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