Legal Definitions - M'naghten rule

The M'naghten rule is the oldest legal test for criminal insanity, originating in England in 1843. Under this test, a defendant is considered legally insane if, at the time of committing a crime, they either did not know the nature and quality of their act, or knew what they were doing but did not know it was wrong. This rule remains a standard for insanity defenses in many jurisdictions.

The M'naghten rule is a legal standard used in some jurisdictions to determine if a defendant can be held criminally responsible for their actions due to mental illness. It is one of the oldest and most stringent tests for criminal insanity.

Under the M'naghten rule, a person is presumed to be sane unless they can prove that, at the time they committed the criminal act, a mental disease or defect prevented them from:

• Not knowing the nature and quality of their act: This means the person did not understand what they were physically doing. For example, they might have believed they were performing a completely different action than what actually occurred.
• Knowing that what they were doing was wrong: This means the person understood the physical act they were committing, but due to their mental illness, they did not know that the act was morally or legally wrong.

If a defendant successfully proves either of these conditions, they may be found "not guilty by reason of insanity" and typically committed to a mental health facility rather than prison.

Here are some examples illustrating the M'naghten rule:

• Example 1 (Not knowing the nature and quality of the act): Imagine a person suffering from a severe psychotic episode who believes they are a character in a video game. In an attempt to "level up," they physically attack a stranger on the street. When questioned, they genuinely perceive the stranger as a virtual enemy and the act as part of a game, not as a real-world assault on another human being.

  Explanation: In this scenario, the individual physically committed an assault. However, due to their profound delusion, they did not understand the true nature and quality of their actions – they believed they were interacting within a game, not harming a real person. This would satisfy the first prong of the M'naghten rule.

• Example 2 (Knowing the act, but not knowing it was wrong): Consider an individual with a deeply entrenched paranoid delusion who believes that a specific government agency is implanting harmful devices into people's brains through their local water supply. Convinced that they are saving lives, this person sabotages a local water treatment plant. They understand that they are tampering with the plant's machinery, but in their delusional state, they believe this act is morally righteous and necessary, not a criminal act of destruction.

  Explanation: Here, the person knew they were physically interfering with the water treatment plant. However, their severe delusion led them to believe that this act was not wrong, but rather a heroic and justified action to prevent harm. This would satisfy the second prong of the M'naghten rule, as they did not know their actions were wrong.