What legal concept allows defendants to argue they were not mentally capable of committing a crime?

The correct answer is the insanity defense. This legal concept allows defendants to assert that they were not mentally capable of understanding the nature of their actions or distinguishing right from wrong at the time they committed the crime. Essentially, if a defendant can successfully demonstrate that they were suffering from a severe mental disorder that impaired their judgment, they may be deemed not criminally responsible for their actions.

The insanity defense hinges on mental health assessments and often involves expert testimony to establish the defendant's mental state during the commission of the crime. This defense is rooted in the principle that punishment is only justified when an individual has the capacity to understand the consequences of their actions. Thus, mental incapacity becomes a crucial factor in determining culpability under the law.

In contrast, terms like capacity defense and affirmative defense are broader and may encompass various defenses that might not solely relate to mental health. Self-defense is a completely different legal argument used in situations where a defendant claims they acted to protect themselves from imminent harm. Therefore, the insanity defense specifically addresses mental competency in relation to the commission of a crime, making it the correct concept for this context.