The criminal law generally assumes that most people have common mental and psychological capabilities sufficient to hold them responsible for the crimes they commit. But the criminal law does provide limited opportunities for a defendant to avoid or lessen his responsibility by demonstrating that one or more of his important human capacities was significantly impaired when he committed the criminal act.
Defenses such as insanity, infancy, intoxication, and diminished capacity are among the more important of these opportunities. These doctrines permit a defendant to claim that it would be unjust to punish him at all or as severely as a normal person because of his unusual limitations. They are fundamentally different from defenses like self-defense or necessity, which claim the defendant did the “right thing” in the situation. The defenses discussed in this chapter acknowledge that the defendant did not do the “right thing” but that, nonetheless, other policy considerations require that he be treated differently. For this reason many courts and scholars describe these defenses as “excuses” rather than “justifications.”