Introduction

The Durham Product Test

Introduction

A mental illness and a crime can exist in the same person without having anything to do with each other, and a rule of criminal responsibility has to say which of those people are excused. Consider two defendants who carry an identical diagnosis.

The product test, also called the Durham rule after Durham v. United States (D.C. Cir. 1954), answers by looking at what the illness did in each case.¹ Under that rule the defendant must be acquitted if the charged offense resulted from a mental disease or defect, and an offense qualifies as the product of such a condition when the defendant would not have committed the crime but for the disease or defect.² For the first defendant, the attack would never have happened without the delusion, so the offense is the product of the illness. The second would have forged the check anyway, so the same diagnosis explains nothing about the crime and excuses nothing. The reasoning follows from the structure of the rule: an illness relieves a defendant of responsibility only to the extent that it explains the act. The test therefore has two parts, a qualifying mental condition and a causal link between that condition and the unlawful act, and a diagnosis alone satisfies only the first.

The product test is one formulation of the insanity defense, the body of doctrine that excuses a defendant whose mental condition meets the jurisdiction's standard.³ In Clark v. Arizona the Supreme Court sorted American insanity standards into four strains: cognitive incapacity, moral incapacity, volitional incapacity, and the product-of-mental-illness test, which asks whether the criminal act stemmed from a mental disease.⁴ The product approach began in New Hampshire in 1871, and the D.C. Circuit adopted it in Durham in 1954.⁵ This chapter follows the rule in order. It starts with the dissatisfaction that produced it, then covers how the rule was announced and what its causal requirement demands, and ends with the criticisms that explain why it has largely disappeared. The first step is to see why courts were ready to replace tests built on a defendant's thinking or self-control with a test built on causation.