Author: Ishita Saxena, a student of Symbiosis Law School, NOIDA


This legal article is a comprehensive study on the concepts of medical insanity and legal insanity. The article will be exploring the meaning of the terms and then analyse the legal standing on the two types of insanity and how it affects the capability of a person to perform civil or criminal acts. The author has performed doctrinal method of research. Secondary sources of research have been used in writing this article. The article aims to help the reader understand the difference between medical and legal insanity and their impact on the legal capacity of the person. This article will help the reader in forming a basic idea that a layman needs to understand before forming legal insanity as a defence for their acts.


Medical insanity is the derangement of the mind. Medically insane does not mean that the person in not in mental capacity to perform his acts but it means that person is of unsound mind. Insanity has been a good defence in civil as criminal suits. If the court deems the person to be insane and incapable of analysing the consequences of his act then the person may be acquitted from the charges. However, in the legal atmosphere it is required that person should be legally insane and not necessarily medically insane. Thus, it is essential to have a good understanding on the difference between these types of insanity.



The term insanity is a wide term. It suggests the incapability of the mind to function well. Either a person is sane or he is insane. But declaring a person insane requires some essentials to be followed. A medically sane person is also legally sane but while considering a medically insane person one has to look on other aspects as well. Certain rules had been prescribed so that a person can be accurately adjudged as medically or legally insane. 

Medical Insanity

Medical insanity means the weakness of the mind. It is a condition when individual’s cognitive ability or emotional functioning is impaired. It can be in various forms like anxiety, depression, and other psychiatric conditions. 

A person who is medically insane is not necessarily legally insane. An insane person who is not only without understanding but also those who are of unsound mind. This weakness of mind may be due to lack of development of mind or defective working of the mental system of the person. 

Mental insanity can take various forms like anxiety, mood disorders, personality disorders, psychotic disorders, eating disorders, trauma related disorders and may include substance abuse disorders.

Legal Insanity

Legal insanity, however, is a narrower term than medical insanity. It is a type of insanity when the person is not able to comprehend the legal consequences of their act then they can be termed legal insanity. A legally insane person is not able to understand the wrongfulness of his/her acts. Due to this inability of the person, it is necessary to provide them with assistance since they cannot by themselves provide any legal backing of their acts. They cannot even analyse the nature of their acts. 

A legally insane person cannot be held liable for his acts. The reason behind this assertion firstly, is that a person who is legally insane will not be able to analyse the legal consequences of his acts. In civil cases, such person might refuse to perform his part of obligations when they are to be performed. And in criminal cases, the person might not be able to understand that his acts might harm the persons in his surroundings. 

Test for determining legal insanity

According to Black Law’s Dictionary, have prescribed 4 main tests to find out whether a person can be adjudged as legally insane or in other words, can take the defence of legal incapacity due to unsound mind. 

The three main tests are:

  1. The McNaughten Rule
  2. The Irresistible Impulse test
  3. The Durham Rule 

The article will help the readers to get a better view of how to apply the tests by analysing the tests in detail.

The McNaughten Rule

This rule is used to establish the legal insanity of the person at the time of the commencement of the offence. The court established that a person cannot be found guilty because of insanity if at the time of the commencement of the offence he was:

  1. Unable to understand the nature and quality of his actions
  2. Unaware that what they were doing is wrong

It is required that both of these conditions should be fulfilled and the burden of proof is on the defendant to establish that during the time of the offence he was in such a stat of mind when he cannot adjudge the nature and wrongness of his acts.

The Irresistible Impulse Test

There are certain cases where the person knew that he was doing an act which is legally wrong however, he did the act because of an ‘irresistible impulse.’ According to this test, the person can be said to be insane if he was not able to stop himself even after knowing the nature and consequences of the act. The Irresistible Impulse test provides the defendant an immunity on the basis that due to such an impulse that he was not bale to resist his actions he can be said as legally insane and cannot be held responsible for his act.

Durham Rule

This rule was established in the case of Durham v. United States (1954). It is also known as the product test. In this case, the defendant was guilty of entering into a house and due to which he demanded the plea of insanity. The test has two main components:

  1. The defendant must be suffering from some mental disease
  2. If the criminal act is a direct act done under the causation of the mental disease.

Although the court did not explicitly explain the meaning of the first component, it is however, understandable that focus is on the objective mental reports of the person rather than person’s subjective cognition.

Although this test is practised only North Hampshire considering the broad idea given by the courts in this judgment.


The Indian Law on the defence of insanity is the section 84 of the Indian Penal Code. Now, Section 22 of the Bharatiya Nyaya Sanhita, 2023 establishes that any act done by a person of unsound mind is not an offence. It however, excludes person with mental retardation and includes abuse of alcohol and drugs. 

The Indian courts have also set precedents in this immunity of mentally unsound persons. 

In the case of Kannakunnummal Ammed Koya v. State of Kerela (1967), the court held that it is necessary to establish that at the time of commencement of the offence the person was actually insane, mere irresistible impulse forms no defence. 

One such important case in the legal history for establishing insanity as a defence is Bhikari v. State of U. P. it was held that the following are the essentials for establishing the defence of insanity:

  1. Burden of Proof on Prosecution: The prosecution bears the burden of proving the commission of an offense. This burden never shifts to the accused.
  2. Requirement of Intent: If intention is an essential element of the offense, the prosecution must prove this aspect as well.
  3. Burden on the Accused for Mental State: If the accused claims incapacity due to lack of knowledge about the nature of the act or its wrongfulness, the burden of proving this incapacity falls on the accused.
  4. Standard of Proof for the Accused: The standard of proof required for the accused to discharge their burden under Section 105 of the Indian Evidence Act, 1872, is not as rigorous as that of the prosecution. The accused needs to show, akin to a civil case, that the preponderance of probabilities favours their argument.

Differences between Legal Insanity and Medical Insanity

Medical insanity refers to the mental weakness of a person’s mind. This weakness can arise due to many factors like depression, anxiety, and other mental disorders. While, legal insanity is a narrower term than medical insanity. It includes people that are unable to comprehend the nature of their act. The person is not criminally or civilly responsible of their actions because due to some mental health problems they were unable to adjudge the nature of their act. 

Section 84 of the Indian Penal Code, Section 22 of the Bharatiya Nyaya Sanhita and Section 12 of the Indian Contract Act are the major provisions dealing with the immunity provided to insane person against being prosecuted. Medical insanity is however, determined by medical tests and is more inclined towards medical aspects rather than legal aspects.

Medical insanity is primarily related to individual’s health and fitness. However, legal insanity is primarily concerned with the individual’s ability to perform the actions that they were doing and being responsible towards their actions. It implies a lack of decision-making ability.

Medical insanity is diagnosed by medical procedures and tests and legal insanity is tested by following the rules established in the legal precedents. It is to be determined by finding out the circumstances and mental health of the accused.

Consequences for medical insanity are that the person is referred to a mental health specialist and sometimes to mental asylums. Consequences of legal insanity are that the person is not made responsible for the acts done by them during insanity and often sent to psychiatric institutions. Focus in both is to improve the mental health of the individual.

Medical insanity is found out on the basis of the medical reports of the individual and there is not strictness of burden of proof. To prove legal insanity, burden of proof is on the defendant to establish that at the time of commencement of the offence, the person was of unsound mind. 


Medical insanity and legal insanity although not very different from each other yet can be differentiated on the above discussed basis. While studying the differences between the two the reader will also be going through the intricate intersection of the two terms. That is the narrowness of the differences between the two types of insanity. While medical insanity is related to medical diagnosis and medical reports, legal insanity is related to mental health of the person at the time of the commencement of the offence. 

Legal insanity is a good defence in criminal and civil cases, as it is established in the Indian statutes and also in legal precedents. The article has addressed the tests applicable in India to affirm a person to be legally insane. Insanity is a wider topic and these two are the subsets of insanity with some overlaps. 

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