Everyone is presumed to be sane; sometimes this isn’t the case. Due to this, the defence of insanity was created. The defence of insanity was created user common law in mid 1800s. The rules of the defence are based on case of M'naghten. Insanity is a general defence which can be used where the mens rea of the offence is required. The defence is not available for strict liability offences, such as driving. If the defendant is found insane then he will not be found guilty by reason of insanity and a different special conviction will be applied. It was seen in the case of M'naghten - the first insanity case. The defendant was found not guilty of murder by use of the insanity defence, and he was sentenced to life imprisonment in a mental institution. The M’Naughten case led the judges to create the M’Naughten Rules; three essential points which the defendant has to prove. A defect of reasons. A disease of mind. Not knowing the nature and quality of the act and that it was legally wrong.

This states that the defence will be available for defendants’ whose minds are impaired. However, if the defendant is capable of reasoning but he failed to use this power, then this doesn’t count as a defect of reason. Case example - Clarke 1972 It was held that the defect of reason must be more than absent-mindedness or confusion.

The disease can be either: Mental or Physical however physical disease must be affecting the mind. Case example - R v Kemp 1957 The defendant was suffering from a physical illness which was affecting his memory and understanding. He was found not guilty by reason of insanity. The case of Kemp developed the theory that the defendant must have an internal factor that existed at the time of the offence. Case example - quick 1973 The diabetic defendant’s insulin was classed as an external factor, and therefore, he couldn’t rely on the insanity defence.

This can be caused by a state of unconsciousness or impaired consciousness or due to the lack of understanding or awareness due to a mental condition whilst conscious. There will be no defence if the defendant knew that what he had done was legally wrong – even if he had a mental illness. Case example - Windle 1952 The defendant said, “I suppose they’ll hang me for this,” which meant that he understood that what he did was wrong.

As mentioned before, if a defendant successfully pleads insanity, then he is found not guilty by reason of insanity. Up until 1991, a successful plea would have led to a judge sentencing the defendant to a mental institution and this wasn’t available for all offences – such as epilepsy or diabetes. The Criminal Procedure Insanity and Unfitness to plead Act 1991 extended the options to: A hospital order, a supervision order and an absolute discharge. However for murder, the judge must still impose a hospital order and the defendant can only be released if the Home Secretary give permission to do so.