insanity defense
US federal affirmative defense under 18 U.S.C. § 17, with the burden on the accused.
The insanity defense is, in US federal law, an affirmative defense codified in 18 U.S.C. § 17, and it applies only within the American legal system — not under Danish law. The defense allows a defendant to be acquitted if it can be proven that, at the time the act was committed, the person suffered from a severe mental disease or defect that rendered them unable to appreciate either the actual nature of the act or its legal and moral wrongfulness.
It is essential to stress that mental illness in itself does not constitute a defense under the provision. The statute expressly specifies that a mental disease or defect does not otherwise constitute a defense. A direct causal connection is therefore required between the illness and the inability to comprehend the nature or wrongfulness of the act at the time of the offense.
The burden of proof rests on the defendant, who must meet it by so-called 'clear and convincing evidence' — a standard higher than the preponderance of the evidence but lower than 'beyond reasonable doubt'. This is unusual in criminal law, where the prosecution normally bears the burden of proof, and it reflects the legislature's intention to narrow the scope of the defense substantially.
The insanity defense attracted particular public attention in cases such as the Andrea Yates case in Houston in 2001 — a Texas state case, not a federal one — in which a mother drowned her five children and was initially found guilty, before an appeal ended in acquittal by reason of insanity. The case illustrated how the defense in practice requires documentation of a profound psychotic state, not merely a psychiatric diagnosis. Questions of mental state and responsibility likewise played a central role in the Billy Milligan case, where the defendant claimed to suffer from dissociative identity disorder with 24 personalities.
The defense is far more restrictive today than it once was. The broad 'irresistible impulse' criterion, which previously permitted acquittal if the defendant could not control their actions despite knowing they were wrong, is no longer recognized in federal law. The current statutory text sets out a narrow cognitive criterion and reflects the political and legal reaction to the acquittal of John Hinckley Jr. in 1982 following the assassination attempt on President Reagan.
