The Insanity Defense Reform Act of 1984 (IDRA) was signed into law by President Ronald Reagan on October 12, 1984,^[1] amending the United States federal laws governing defendants with mental diseases or defects to make it significantly more difficult to obtain a verdict of not guilty only by reason of insanity.

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The act removed the volitional component, that a defendant lacked capacity to conform their conduct to the law, from the ALI test.^[2]: 615 Defendants were exculpated only if "at the time of the commission of the acts constituting the offense, ... as the result of a severe mental disease or defect, [they were] unable to appreciate the nature and quality or wrongfulness of [their] acts."^[2]: 634 The law passed in the wake of public outrage after John Hinckley Jr.'s acquittal by reason of insanity in June 1982 for his attempted assassination of President Ronald Reagan.^{[2]: 634–635}

Prior to the enactment of the law, the federal standard for "insanity" was that the government had to prove a defendant's sanity beyond a reasonable doubt (assuming the insanity defense was raised). Following the Act's enactment, the defendant has the burden of proving insanity by "clear and convincing evidence".^[3] Furthermore, expert witnesses for either side are prohibited from testifying directly as to whether the defendant was legally sane or not,^[3] but can only testify as to their mental health and capacities, with the question of sanity itself to be decided by the finder-of-fact at trial.^{[citation needed]} The Act was held to be constitutional (and the change in standards and burdens of proof are discussed) in United States v. Freeman.^[3]

The Defense Reform Act was criticized by psychologist Lawrence Z. Freedman for being ineffective: "If the attacker is rational mentally, stable emotionally, and fanatic politically, he will not be deterred. Nor will an irrational, affectively disturbed individual be deterred."^[4]