The Insanity Defense and What It Requires

The insanity defense asks a question about a single moment. Not whether the defendant is ill, not whether the defendant is ill now, but what the defendant knew or could appreciate while acting. Every formulation in use shares that structure. They differ in what has to be missing, how severe the condition must be, and who has to prove it, and those differences decide cases.

The federal formulation

Section 17 of title 18 states the test in one sentence: it is an affirmative defense that, at the time of the commission of the acts constituting the offense, the defendant, as a result of a severe mental disease or defect, was unable to appreciate the nature and quality or the wrongfulness of his acts. The section then adds that mental disease or defect does not otherwise constitute a defense, and places the burden of proving the defense by clear and convincing evidence on the defendant.

Three features of that sentence are load-bearing. The condition must be severe, which excludes the ordinary run of diagnoses that do not disable understanding. The disability must be an inability to appreciate rather than a difficulty in controlling, which removes the volitional branch that once existed in federal law. And the burden and standard are unusual: most affirmative defenses are proved by a preponderance, and clear and convincing is a demanding allocation to place on a defendant.

Ohio's definition is narrower still. It provides that a person is not guilty by reason of insanity only if the person proves that, at the time of the offense, the person did not know, as a result of a severe mental disease or defect, the wrongfulness of the person's acts. The nature and quality branch is absent, so a defendant who understood that the conduct was wrong cannot succeed no matter how disordered the surrounding thinking was.

The formulations states use

Four families of test appear in American law. The cognitive test asks whether the defendant knew the nature and quality of the act or knew that it was wrong. The volitional addition asks separately whether the defendant was able to control the conduct even knowing it was wrong. The substantial capacity formulation combines them, asking whether the defendant lacked substantial capacity either to appreciate the criminality or wrongfulness of the conduct or to conform conduct to the requirements of law. A product formulation, used in one state, asks whether the act was the product of a mental disease or defect.

The word substantial in the third formulation is not decorative. A cognitive test written as an absolute inability sets a higher bar than one written as a lack of substantial capacity, and the same expert testimony can satisfy one and fail the other. Whether the defense reaches volitional impairment is the other major dividing line, and the federal system and many states removed it deliberately on the ground that the distinction between an irresistible impulse and an impulse not resisted cannot be drawn reliably.

States that replaced the defense

A small number of states abolished the affirmative defense and admit mental condition evidence only where it bears on the mental state the offense requires. Kansas provides that it is a defense that the defendant, as a result of mental disease or defect, lacked the culpable mental state required as an element of the crime charged, and states that mental disease or defect is not otherwise a defense. Idaho provides that mental condition is not a defense to any charge of criminal conduct, while preserving the admission of expert evidence on a state of mind that is an element of the offense.

The Supreme Court held that this approach does not violate due process, reasoning that no single formulation of the insanity defense is so entrenched in the nation's traditions that a state must adopt it, and that a state remains free to consider mental illness at sentencing and in civil commitment. The dissent argued that a rule which convicts a person who could not understand that the act was wrong abandons a principle common to the common law. Both positions are argued in legislatures that continue to revisit the question, and this publication takes no side between them.

The practical difference is narrower than the doctrinal one. Most offenses require an intent to do the physical act rather than an understanding of its wrongfulness, and a defendant who deliberately performed the act formed that intent even while deeply disordered. Evidence that would establish a defense under an appreciation test therefore frequently establishes nothing under a mens rea approach, which is the same limitation that constrains diminished capacity evidence generally.

Procedure and consequences

The defense is subject to notice. Federal rules require written notice of an intent to assert insanity within the time for filing pretrial motions, and separate notice of an intent to introduce expert evidence of a mental condition bearing on guilt or on punishment in a capital case. Notice triggers the government's examination, and failure to give notice or to submit permits exclusion of the defense expert. What the defendant says in that examination is governed by the rules described in statements made during an evaluation.

Success carries its own consequence. An acquittee is not released; the verdict routes the person into a commitment proceeding governed by the post-acquittal statute, with the burden on the acquittee to establish that release would not create a substantial risk. And nothing about the defense concerns present capacity, which is answered separately under the competency standard. A defendant can be entirely competent to stand trial and legally insane at the time of the offense, and the two findings sit together without contradiction.