Although the defences of abnormality of mind and mental illness may both deal with mental health problems and their effect on the ability of the accused to form a criminal intent, they are not the same defence.
Abnormality of mind must proceed from an arrested development or retardation of mind while mental illness must proceed from a defect of reason.
Abnormality of mind is only a partial defence, meaning that the accused will not be found guilty of the capital offense of murder but of homicide only. The mental illness defence, however, is a full defence and it results in an acquittal of the accused.
The defence of mental illness is strictly. The mental illness of the accused must be a diagnosed mental disease or disorder that leaves him totally bereft of the ability to use his reason with any sense or composure. It must be such that the accused had no recognition that his actions could produce the victim’s death or that what he has done is morally or legally wrong. This is the difference between abnormality of mind and mental illness.
If an accused has an abnormality of mind, he still retains some mental judgment despite his retardation or arrested mental development to know that his acts are wrong although he cannot fully understand or appreciate the extent of their immorality or illegality.
The accused with an abnormality of mind has the capacity to know that he can hurt the victim with his actions, but his mental retardation and arrested mental development prevent him from fully realizing that his actions directly caused the death of the victim.
In both these defences the prosecution begins its presentation of evidence with the presumption that the accused was of sound mind and in full possession of his mental faculties. It is the accused who must prove his abnormality of mind or mental illness with medical evidence. Both these defences must be proved by the accused.
Once the accused succeeds in proving mental illness, the jury will find that the accused is not guilty by reason of his mental illness. On the other hand, if the accused succeeds in proving an abnormality of mind, he will still be found guilty albeit, he will not be found guilty of murder but of the lesser crime of homicide.
Disclaimer : This article is just a summary of the subject matter being discussed and should not be regarded as a comprehensive legal advice for you to defend yourself alone. If you are charged with criminal offences, it is recommended that you seek legal assistance from criminal lawyers.