Introduction

Mental Disease Evidence Beyond Insanity

Introduction

When a defendant with a serious mental illness shoots someone, the first question most people ask is whether the defendant was legally insane. That question is real, but it is only one of the ways the illness can enter a criminal case. Under the Model Penal Code, legal insanity is a complete excuse: a person is not responsible for criminal conduct if, as a result of mental disease or defect, he lacks substantial capacity either to appreciate the criminality of his conduct or to conform it to the requirements of law.¹ The Code labels that claim an affirmative defense in § 4.03(1), which means it is something the defendant raises as a separate ground for acquittal, apart from the elements the prosecution must prove.²

The same evidence of mental disease or defect can matter without ever reaching that defense. It can bear on whether the prosecution has proved a required state of mind, since an offense that demands purpose or knowledge is harder to establish when an impairment makes it less likely the defendant actually had that mental state. It can also bear on the choice between death and imprisonment. The Code handles both uses in § 4.02, a separate provision from the insanity sections that sit beside it. Subsection (1) makes evidence of mental disease or defect admissible whenever it is relevant to prove that the defendant did or did not have a state of mind that is an element of the offense.³ Subsection (2), which the official text prints in square brackets, applies when a jury or court may determine or recommend death or imprisonment, and it makes evidence of impaired capacity admissible in favor of a sentence of imprisonment.⁴ The two tracks ask different questions. Insanity asks whether a defendant who may have satisfied every element should nonetheless be excused. The evidence track asks whether an element has been proved at all, or what punishment fits.

This chapter follows the evidence track. It begins by setting the defense track beside it so the differences in question, burden, and consequence are visible. It then teaches the § 4.02(1) rule that mental-disease evidence comes in when it is relevant to an element. Next come the constitutional and state-law limits on that rule, which Clark v. Arizona brought into focus and on which the states differ widely. The chapter ends with the capital-sentencing mitigation provision in § 4.02(2). The first step is to be clear about what the insanity defense is, so that the reader can see what the rest of this chapter is not about.