In which case was the principle of substantial impairment of responsibility established?

The principle of substantial impairment of responsibility was established in the case of R v Byrne. In this landmark decision, the court examined the effects of mental illness on a defendant’s ability to understand their actions and form intent. Byrne, who had a history of mental illness, argued that he should not be held fully responsible for his actions due to his impaired mental state at the time of the crime. The ruling recognized that significant mental impairment can affect a defendant's capacity to reason, understand, or control their conduct, thereby mitigating their culpability.

This principle allows for a defence of diminished responsibility, which can lead to different outcomes in cases involving serious criminal charges. The acknowledgment of mental health's role in criminal responsibility is crucial in ensuring that justice is administered fairly, considering the complexities of human psychology and its impact on behavior.