To use Lack of Capacity due to Mental Disease or Defect as an affirmative defense, what must the defendant prove?

• A. That they had no intent.
• B. That they lacked substantial capacity to appreciate the wrongfulness of their conduct or to conform conduct to the law.
• C. That they were intoxicated.
• D. That the mental illness was diagnosed.

Answer: (B)

The fundamental idea is the two‑part mental capacity standard used for the insanity/defense based on mental disease or defect. To use this defense, the defendant must show that, because of a mental disease or defect, they lacked substantial capacity in two areas: to appreciate the wrongfulness of their conduct and to conform their conduct to the requirements of the law. In other words, the mental disorder must have impaired their ability to know that what they were doing was wrong or to control their behavior to fit the legal rule. This two‑prong test is why the described option is correct: it explicitly covers both staying mindful of wrongness and being able to act within the law.

Simply having a mental illness diagnosis by itself or being intoxicated doesn’t automatically meet this standard, and lacking intent is not the same as lacking substantial capacity to understand wrongfulness or to conform conduct. The defense hinges on impairment at the time of the act and the specific inability to reach those two capacities.