Legal Definitions - M'Naughten rule

The M'Naughten rule is a foundational legal test used in criminal cases to determine if a defendant was legally insane at the time they committed a crime. It is often referred to as the "right and wrong" test.

Under the M'Naughten rule, a person is considered legally insane if, at the time of committing the act, they were suffering from a mental disease or defect that prevented them from understanding either:

• The nature and quality of the act they were doing, or
• That what they were doing was wrong.

If a court or jury finds that a defendant met this standard, it means they lacked the necessary criminal intent (mens rea) because they could not comprehend the wrongfulness of their actions. While it is a historical test, it remains influential and is still used in some jurisdictions, though many others have adopted more modern standards that incorporate broader psychological considerations.

Here are some examples illustrating how the M'Naughten rule might be applied:

• Example 1: Delusional Belief

  Imagine a person, Mr. Davies, who suffers from severe paranoid schizophrenia. He genuinely believes that his neighbor is a demonic entity sent to destroy the world and that he has received a divine command to "exorcise" this entity by physically harming them. If Mr. Davies were to assault his neighbor, his defense attorney might argue that, under the M'Naughten rule, he was legally insane. The argument would be that due to his profound delusion, Mr. Davies did not understand that his act was morally or legally wrong; he believed he was performing a righteous, necessary act to save humanity, not committing a crime.

• Example 2: Profound Cognitive Impairment

  Consider Ms. Chen, an elderly woman with advanced dementia who wanders into a stranger's home, believing it is her own, and begins to rearrange furniture, causing significant damage. When confronted, she becomes agitated and strikes the homeowner, genuinely not understanding that she is in someone else's house or that her actions are illegal or harmful. If charged with assault and property damage, her defense could invoke the M'Naughten rule, arguing that her severe cognitive impairment prevented her from understanding the nature of her actions (that she was in a stranger's home, damaging property) or that striking someone was wrong in that context.

• Example 3: Psychotic Break and Misperception of Reality

  Suppose Mr. Evans experiences an acute psychotic break, during which he perceives a group of strangers on the street as menacing figures about to attack him with weapons, even though they are simply waiting for a bus. In a state of extreme terror and delusion, he lashes out, injuring one of the individuals, believing he is acting in desperate self-defense. If Mr. Evans is charged with assault, his legal team might argue that, at the moment of the incident, his severe mental state rendered him unable to understand the true nature of his actions (that he was attacking innocent people) or that his actions were wrong, as he genuinely believed he was fighting for his life against an imminent threat.