Where an offender relies on mental illness to reduce moral culpability and moderate deterrence, professional evidence about the condition and its effect on behaviour is required; bare assertions by counsel of a historical diagnosis, without professional evidence of the condition's effect on the offending, do not provide a sound basis for the sentencing court to conclude that conduct was significantly explained by untreated mental illness. The general state of an offender's mental health remains a relevant mitigating factor even without professional evidence.
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