Legal Definitions - McNaghten rules

The McNaghten rules are a legal standard used in criminal law to determine if a person can be held criminally responsible for their actions when they claim a mental illness prevented them from understanding what they were doing or that it was wrong. This standard, often referred to as the "right-and-wrong test," establishes that an individual is not criminally liable if, at the time of committing an offense, they were suffering from a "defect of reason, from disease of the mind" that prevented them from knowing either:

• The nature and quality of the act they were committing (meaning they didn't understand what they were physically doing or its consequences), OR
• That what they were doing was wrong (meaning they understood the act but believed, due to their mental condition, that it was morally or legally impermissible).

This test is a foundational element of the insanity defense in many jurisdictions.

Examples:

• Scenario 1 (Not knowing the nature and quality of the act): A person experiencing a severe psychotic episode believes they are in a video game and that the people around them are virtual characters. If this individual physically harms someone, genuinely believing they are merely interacting with a digital avatar and not a real person, the McNaghten rules might be applied. A court would assess whether their mental state prevented them from understanding the true nature and quality of their actions – that they were inflicting physical injury on another human being, not just pressing buttons in a game.

• Scenario 2 (Not knowing the act was wrong due to delusion): An individual suffering from a profound paranoid delusion genuinely believes that their neighbor is a secret agent from an enemy country, actively plotting to poison their water supply. In an attempt to "neutralize the threat," they damage the neighbor's property. While they understand they are physically breaking a window (the nature of the act), their severe delusion leads them to believe this act is not only justified but a necessary defense against a perceived imminent danger. Under the McNaghten rules, a court would evaluate whether this "defect of reason" prevented them from knowing that their act of property damage was legally and morally wrong, despite understanding the physical action itself.

• Scenario 3 (Not knowing the act was wrong due to severe cognitive impairment): A person with a severe intellectual disability, which qualifies as a "defect of reason from disease of the mind," is instructed by an older, manipulative acquaintance to take a package from a store without paying for it. The acquaintance tells them it's a "game" and that it's "okay" because the store has too much stuff. Due to their significant cognitive impairment, the individual genuinely does not grasp the concept of theft or that taking items without payment is legally and morally wrong. While they understand the physical act of picking up an item and leaving the store, the McNaghten rules would be applied to determine if their mental disability prevented them from understanding the wrongfulness of their actions.