Criminal law, reflecting widely accepted "act theory," typically holds that responsibility depends on a defendant's ability to engage in reason-guided behavior. The criminal law excuses defendants with diminished rationality, such as the insane and those who kill in the heat of passion. Act theory, however, often provides vague, difficult-to-apply legal tests for juries because it often cannot specify how and to what degree rationality must be compromised. As a result, criminal law appeals to such notions as a "person of reasonable firmness" or "adequate provocation." Partly in response to these shortcomings, character or interpretive theorists have rejected act theory as a basis for criminal responsibility. They argue instead that excuse turns on explicitly normative preferences about actors' motivation, personality, or social position. Oddly, this position produces even vaguer and arguably capricious tests for juries to use when determining whether excuse should apply. This Article applies a straightforward economic cost-benefit analysis to clarify act theory so as to produce a more coherent and workable excuse doctrine. It accepts, as a starting point, that criminal responsibility turns on the capacity for reasonresponsive behavior or "practical reason." The Article points out, however, that actors can change their responsiveness to reason within relatively broad parameters. This insight makes explicit a moral judgment implicit in criminal responsibility: the law expects actors to achieve and maintain a certain capacity for practical reason so as to avoid criminal acts. Excuse is, therefore, appropriate when the cost of rendering one's behavior legal—by correcting faulty beliefs, illicit desires, or weak will— exceeds the avoided crime's injuriousness, considering the crime's probability, or foreseeability. Identifying the specific cost structures of "moral cognitive competence" provides a guide for more concrete, economic tests for the various legal excuses. In application, because of the high costs crimes, particularly violent crimes, impose, culpability would turn on whether actors have sufficient notice of their likelihood to commit crime but failed to improve their practical reasoning. This test offers a vastly simplified alternative to the standards the excuses currently employ, such as heat-of-passion's reasonable man or duress's person of reasonable filminess.