What legal defense can be claimed if someone could not understand their actions due to mental incapacity?

The legal defense applicable in this scenario is insanity. This defense is grounded in the idea that a person who is mentally incapacitated at the time of committing a crime does not possess the necessary cognitive ability to understand the nature of their actions or to distinguish right from wrong. Under the insanity defense, the individual is deemed unable to form the intent required to be held criminally responsible due to their mental state.

Insanity laws vary by jurisdiction, but they typically require that the defendant demonstrate their mental illness affected their understanding at the time of the crime. This can involve psychological evaluations and expert testimony to establish the presence and impact of a mental disorder, which distinguishes it from other defenses that do not rely on mental incapacity.

The other options are distinct legal concepts that do not apply in this context. Involuntary intoxication refers to a state where a person is rendered unable to make rational decisions due to being intoxicated against their will. Coercion involves being forced to act against one's will through threats or intimidation but does not relate to mental incapacity. Duress similarly involves being compelled to commit an act against one's will due to immediate threats or harm, which is not relevant in cases where mental incapacity is the focus.