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This Article calls for the creation of a generic partial excuse for diminished rationality from mental disability. Currently, most jurisdictions recognize only one partial excuse: the common law heat-of-passion defense. Empirical research demonstrates that populations with delusions experience similar impairments to decision-making capacities as people confronted with sudden, objectively adequate provocation. Yet, current law affords significant mitigation only to the latter group, which only applies in murder cases. Adoption of the Model Penal Code’s “extreme mental or emotional disturbance” (EMED) defense could extend mitigation to other forms of diminished responsibility. However, examination of jurisdictions’ adoption and utilization of the EMED defense shows that, of the few states that have adopted it, most have rejected its diminished responsibility potential. Instead, most retain key features of heat of passion such as requiring an external provoking event, rendering the defense inapplicable to many delusion-driven crimes. A better solution would be to create a generic partial excuse for diminished rationality from mental disability. Over the decades, several prominent scholars have offered proposals for generic partial excuses for partial responsibility, but, as of yet, none has inspired legislative action. This Article’s proposal differs from prior proposals in four key respects. First, it limits its purview to rationality impairments from mental disabilities, a traditionally recognized form of diminished blameworthiness. Second, to be workable and attractive to states, this proposal recommends that states draw definitions of partial responsibility from existing statutory frameworks, namely existing insanity or Guilty But Mentally Ill (GBMI) standards. Such an understanding of partial responsibility should carry greater local legitimacy, and the popularity of GBMI verdicts with legislatures and juries may mean that extending those statutes into the realm of partial responsibility would be more palatable to state legislatures than wholly new language. Third, in light of the realities of mental disorder and its lived experience, our proposal does not advocate for a lesser degree of mitigation for defendants who contributed to their irrationality through failure to comply with medical directives. Fourth, our proposal draws from GBMI statutes and partial responsibility standards outside the United States to suggest sentencing, treatment, and post-sentence options to accompany a partial responsibility verdict and respond to any possible threat to public safety. This Article examines the first two distinctive components of the partial excuse; the third and fourth aspects of the proposal will be developed in a future work.