The American legal system currently tends to excuse sleepwalking killers, particularly based on the involuntary act defense, more so than the insanity defense. By contrast, the law generally does not excuse psychopathic murderers. However, the status quo may not be the optimal or most just solution to this legal dilemma; depending on one’s philosophical beliefs regarding the tension between society’s interest and the accused’s rights, one can identify various flaws within the prevailing application of the insanity defense in cases involving sleepwalking and psychopathy. As the law is constantly evolving, there is space for growth in this area, especially with the advancement in neuroscience, which can offer more insight into sleepwalkers’ and psychopaths’ brains.

In approaching the complex questions of whether and how the law should excuse sleepwalking killers and psychopaths from punishment, this article turns to relevant findings from neuroscience for support and focuses on one particular approach, that of the Model Penal Code (MPC) insanity defense. We begin with an overview of the relevant criminal law doctrine in contextualizing the MPC’s approach to insanity defense. We then apply the relevant MPC section, § 4.01, to the sleepwalking killer and psychopath contexts, bearing in mind relevant studies and findings in neuroscience and related scientific disciplines, as well as their limitations at this stage. Part of the analysis considers the U.S. Supreme Court’s latest insanity law decision, Kahler v. Kansas (2020), in relation to the subject matter of this article.

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