Introduction

Model Penal Code Insanity Test

Introduction

The Model Penal Code states its rule for excluding criminal responsibility in a single sentence at § 4.01(1): "A person is not responsible for criminal conduct if at the time of such conduct as a result of mental disease or defect he lacks substantial capacity either to appreciate the criminality [wrongfulness] of his conduct or to conform his conduct to the requirements of law."¹ The American Law Institute drafted that sentence, and the rule travels under several names. The substantial-capacity test, the Model Penal Code test, the MPC test, and the ALI test all refer to § 4.01, so an exam question using any of those labels is asking about this one formula.² A substantial minority of states adopted the Code's approach as one of the four main definitions of insanity, alongside M'Naghten, the irresistible-impulse test, and the Durham product test.³ Since 1984, many jurisdictions, including the federal courts, have narrowed the defense toward a test resembling M'Naghten, although portions of the substantial-capacity test continue to be used. You should therefore expect to meet the Code's formula in modified form as well as in its original wording.⁴

The sentence is best read as a set of parts that each do a separate job. It opens with a condition of the defendant's mind, a mental disease or defect, and then requires that the impairment arise "as a result of" that condition rather than from some other source. The impairment is one of capacity: the ability to appreciate that the conduct is criminal (the Code brackets "wrongfulness" as an alternative to "criminality"), or the ability to conform the conduct to the law, and the defendant need lack only one of the two. The lack need only be substantial, which is a lower bar than total incapacity, and it must exist "at the time of such conduct," so the inquiry looks at the moment of the act and not at the defendant's condition before or after it. The test therefore asks two questions in order. The first is whether a recognized disorder existed, and the second is how far that disorder impaired the defendant's understanding or self-control when the act occurred. A diagnosis answers only the first question, so a defendant whose disorder left both capacities substantially intact remains responsible. The test is built this way because responsibility presupposes the ability to grasp that conduct is wrongful and to govern it, and the Code aims its inquiry directly at those abilities.

Section 4.03(1) adds a procedural half to this substantive formula by classifying mental disease or defect excluding responsibility as an affirmative defense.⁵ That classification decides who raises the defense and how it is handled at trial, questions this chapter takes up once the elements are in place. A successful defense also has an unusual result. Instead of an ordinary acquittal and release, it produces a special verdict of not guilty by reason of insanity, usually followed by commitment to a mental institution.⁶ The Code did not write § 4.01 on a blank slate, because each choice in its wording answers a weakness in the rules it replaced. The history therefore comes first, and the opening section below traces what the drafters were reacting to before the chapter turns to the elements one at a time.