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M'Naghten rule
Under the M'Naghten rule, a defendant is not criminally responsible if, at the time of the act, a defect of reason from disease of the mind meant that he did not know which of the following?
- correctA. The nature and quality of the act, or that what he was doing was wrong
- B. The identity of the person he was harming
- C. That he would be caught and punished
- D. That his behaviour was unusual compared with others
Explanation
Key fact
M'Naghten asks about knowledge, not about control. Not knowing the nature of the act, or not knowing it was wrong.
The nature and quality of the act, or that what he was doing was wrong. Those are the two limbs, and either one suffices.
Why
The rule requires a defect of reason arising from disease of the mind.
The first limb is not knowing the nature and quality of the physical act.
The second is knowing the act but not knowing that it was wrong.
It is a purely cognitive test - what the person knew.
Every person is presumed sane until the contrary is proved, and the burden is on the defence.
Why not the others
- B. The identity of the person he was harming — is not the question. Mistaking identity is a factual error, not a defect of reason.
- C. That he would be caught and punished — is not part of the test. Taking steps to avoid detection may in fact suggest the defendant did know the act was wrong.
- D. That his behaviour was unusual compared with others — is not a legal test of responsibility. Eccentricity is not insanity.
Remember
Did he know what he was doing, and did he know it was wrong? Two questions, and either answered no is enough.
Go deeper
The rule is criticised for ignoring irresistible impulse - a person may know an act is wrong and still be unable to stop, and M'Naghten gives them no defence.
Durham and the American Law Institute tests were attempts to widen it; India retains the M'Naghten formulation.
The doctor's role is to give evidence about the mental state at the time of the act. Whether that meets the legal test is a decision for the court, not the psychiatrist.
Reddy KSN, Murty OP. The Essentials of Forensic Medicine and Toxicology, 36th ed. (Jaypee, 2025); Modi's A Textbook of Medical Jurisprudence and Toxicology, 28th ed. (LexisNexis, 2025).