The difference between the common law insanity defense and the mistake defense is

• A. Insanity negates mental state; mistake concerns beliefs about conduct
• B. Insanity is always a defense; mistake never
• C. Insanity requires knowledge; mistake requires ignorance
• D. Insanity is a form of privilege; mistake is a right defense

Answer: (A)

The main idea here is how mental state affects liability under two different defenses. Under the common-law insanity defense, the defendant’s mental condition is so impaired that they cannot form or appreciate the required mental state for the crime—often to know the act is wrong or to understand the nature of the act. Because that mental state element isn’t satisfied, liability for the offense cannot attach.

A mistake defense, by contrast, looks at the defendant’s belief about the facts surrounding the act. If a reasonable mistake about those facts would negate the required mental state for the offense, liability can be avoided. In short, insanity disrupts the mind’s capacity to meet the criminal mens rea, while a mistake defense hinges on erroneous beliefs about the conduct or its circumstances.

Why the other statements don’t fit: saying insanity is always a defense ignores that it’s only available when the mental incapacity meets the standard and isn’t automatic in every case; saying insanity requires knowledge or that mistake requires ignorance misstates the nature of these defenses (insanity concerns impaired perception of reality and wrongfulness, not simply knowledge, while mistake concerns beliefs about facts rather than broad ignorance); and describing insanity as a privilege or mistake as a right defense uses wrong labels and concepts for how these defenses function in practice.