INTRODUCTION 

ORIGIN OF THE LAW OF INSANITY 

In this case, the accused McNaughten was suffering from a mental disorder called ‘persecution mania’. Because of this disease, the accused firmly believed that all the problems he is facing and all his sufferings are because of the then British Prime Minister Robert Pel. McNaughten then shot Mr.Drummond who was the private secretary of PM believing him to be the PM.
So he pleaded insanity and the House of Lords acquitted him. This generated a lot of public sentiment and lot of pressure was put on the House of Lords. They constituted a special committee of Judges and laid down the rules for plea of insanity and this was popularly known as McNaughten’sRules which is being followed till now.
The rules are as follows:
1. All are presumed to be sane, unless contrary is proved to the satisfaction of the Court. That is, the burden of proof is on the person who is claiming the defence of insanity.
2. To claim the defence, one should clearly show that at the time of commission of the act he was suffering from mental illness.
3. At the time of doing the act he was unable to know the nature of the act.

DEFENCE OF INSANITY IN INDIAN PENAL CODE 

Section 84 of the Indian Penal Code reads as follows,
‘Nothing is an offence committed by someone who is currently unable to know the nature of the act or does what is wrong or contrary to legislation due to a lack of a sound mind’. 
For raising the defence of insanity, the following elements should necessarily be established- 
1. The accused was in a state of unsoundness of mind at the time of the act. 
2. He was not aware of the nature of the act. He was unable to know if the act which he committed was either wrong or contrary to law. 

DIFFERENCE BETWEEN MEDICAL AND LEGAL INSANITY 

There are several types of mental ailments, but none is recognized in law, unless the elements of section 84 are satisfied. When a person is not insane but is imbalanced and excited, and is probably labeling under some kind of obsession or hallucination, Section 84 cannot be of any help to him. 
According to medical science, unsoundness of mind or insanity is a type of disease which messes with mental faculty of a person. It is some kind of a disease in brain or nervous system which leads to working of one or two functions of brain in an abnormal way or will not let them function at all. 

CONCLUSION

So what happens after an accused who raised the defence of insanity was acquitted by the Court? Will he be set free in the society? The answer is obviously in negative. The criminals who are acquitted for reason of their unsoundness of mind are sent to a psychiatric institution where they get treated for their mental unsoundness. They would be released from the psychiatric treatment only when they are deemed to be no longer dangerous to the society which rarely happens. However, on the other hand, a defendant who received the guilty but mentally ill verdict will be sentenced in the same way as if he were found guilty of the offence. After necessary treatment if the defendant was deemed cured, then he will be required to serve out the rest of his sentence.

References:

[1] https://blog.ipleaders.in/insanity-defence-indian-penal-code/
[2] http://www.legalserviceindia.com/legal/article-3140-defense-of-insanity.html
[3] https://www.pbs.org/wgbh/pages/frontline/shows/crime/trial/faqs.html

This blog is authored by Reema