abnormality of the mind ( R v Byrne ); drug personality disorder (Celebici Trial); involuntary intoxication ( R v Galbraith) ; mental weakness and low intelligence ( Lord Dea's decision ) ; minority ( R v Raven ); physical deformities such as blindness and being a deaf-mute ( R v Pritchard). In the treatise "Partial Defences To Murder" more mitigating factors are added i.e. sufficient provocation by the offended party ( R v Smith ); immediate vindication of a grave offence to himself or his relatives (Table 7); Incomplete self-defence where there is no reasonable necessity of the means employed by the culprit (R v Martin); passion or obfuscation (Case 113); disease or injury (Note 17); jealousy, mercy killing, depression, relationship of victim to the accused (Table 7). The list goes on and on.
Insanity is a plea or defence by which the accused at the time of the commission of the act, "was laboring under such a defect of reason, arising from a disease of the mind, as not to know the nature and quality of the act he was doing or, if he did know it, that he did not know that what he was doing was wrong (The M'Naghten Rules). Insanity totally exempts the culprit from criminal liability unless he does it during a lucid interval. If so, he is wholly liable for the crime unless there are mitigating factors attending the crime.
Diminished Responsibility is defined as a plea or defence in which the accused at the moment of the commission of the crime suffers from some "form of mental unsoundness or mental aberration or weakness of mind", so much so that his "mind is so affected that responsibility is diminished from full responsibility to partial responsibility" ( HM Advocate v Savage).
Comparison and Contrast
1. Both insanity and diminished responsibility are mental states. In insanity, there is a mental disorder or a mental disease which causes the deranged person to be deprived completely of reason, discernment or freedom of the will at the time of the commission of the crime. In insanity, there is an absence in the agent of crime of any of all the conditions that would make an act voluntary. On the other hand, in diminished responsibility, there is a mental debility or aberration of the mind or a temporary mental capacity or a temporary mental impairment (Scottish Law Commission 2). Here, there is some degree of reason, discernment or freedom of the will albeit such is beclouded and weakened by the presence of any of the mitigating factors hereinabove mentioned.
2. Insanity totally exempts the offender from criminal liability because the insane person is totally deprived , at the time of the performance of the crime, of discernment or reason or intelligence and is unable to distinguish between right or wrong. In diminished responsibility, as a rule, there is no exemption from criminal liability but there is instead a mitigation or extenuation of criminal responsibility