The Insanity Defense: How Courts Judge Criminal Mental State

Less Than 1% of Criminal Cases Even Attempt It

The insanity defense is invoked in fewer than 1% of felony cases in the United States, and it succeeds in only about 25% of those attempts. That translates to roughly 0.25% of felony defendants being found not guilty by reason of insanity (NGRI). Despite its rarity, the defense generates enormous public attention and controversy, particularly after high-profile cases like John Hinckley Jr.’s 1982 acquittal for the attempted assassination of President Ronald Reagan.

Legal insanity is not the same as mental illness. A defendant can be severely mentally ill and still be found legally sane. The distinction hinges on whether the mental condition prevented the defendant from understanding the nature of the act or knowing it was wrong at the time of the offense. That narrow standard excludes most psychiatric conditions.

Historical Foundations: The M’Naghten Rule

The modern insanity defense traces to an 1843 English case. Daniel M’Naghten shot and killed Edward Drummond, secretary to British Prime Minister Robert Peel, believing Drummond was Peel himself. M’Naghten suffered from paranoid delusions that the Tory party was persecuting him. The jury acquitted him on grounds of insanity.

Public outrage following the verdict prompted the House of Lords to establish formal standards. The resulting M’Naghten rule requires the defendant to prove that, due to a “defect of reason, from disease of the mind,” they either did not know the nature and quality of the act or did not know that what they were doing was wrong.

The ALI Standard and Its Compromise

The American Law Institute (ALI) proposed a broader test in 1962 as part of the Model Penal Code. Under the ALI standard, a defendant is not responsible if, due to mental disease or defect, they lacked “substantial capacity” to either appreciate the criminality of their conduct or conform their conduct to the requirements of law. The word “substantial” was deliberate—it acknowledged that mental illness exists on a spectrum rather than as an absolute binary.

This test dominated American courts through the 1970s. Then came Hinckley.

The Hinckley Aftermath

John Hinckley Jr. shot President Reagan in 1981 and was found not guilty by reason of insanity in 1982 under the ALI standard. The verdict sparked a legislative backlash. Within three years, Congress passed the Insanity Defense Reform Act of 1984, which narrowed the federal standard back toward M’Naghten by requiring defendants to prove they were “unable to appreciate” the wrongfulness of their conduct. The act also shifted the burden of proof to the defendant, requiring clear and convincing evidence.

• Before Hinckley, the prosecution bore the burden of proving sanity in most federal courts
• After 1984, defendants must prove insanity by clear and convincing evidence in federal cases
• Idaho, Kansas, Montana, and Utah abolished the insanity defense entirely (though this faces ongoing constitutional challenges)
• Many states created “guilty but mentally ill” verdicts as an alternative—these result in conviction with a recommendation for treatment

The Evaluation Process

When a defendant raises the insanity defense, the court typically orders psychiatric evaluation by one or more forensic mental health professionals. Both prosecution and defense may retain their own experts. Evaluators review the defendant’s mental health history, interview the defendant, administer psychological testing, and analyze the circumstances of the offense.

The evaluation focuses on the defendant’s mental state at the time of the crime, not at the time of trial. This retrospective assessment is inherently challenging. Evaluators must reconstruct a mental state that may have existed months or years earlier, often relying on incomplete records and the defendant’s own potentially unreliable account.

Malingering: The Faking Problem

Studies estimate that 15–20% of defendants evaluated for insanity exaggerate or fabricate symptoms. Forensic evaluators use validated instruments specifically designed to detect malingering. The Structured Interview of Reported Symptoms (SIRS-2) is widely considered the gold standard. It presents questions that appear face-valid to malingerers but produce response patterns that genuine mental illness does not.

• Genuine psychotic symptoms follow recognizable clinical patterns that are difficult to convincingly fake
• Malingerers tend to endorse rare or absurd symptoms at rates far exceeding genuine patients
• Consistent symptom presentation across multiple evaluations is one marker of authenticity
• No single test is definitive—evaluators rely on converging evidence across multiple measures

Outcomes After a Successful Defense

A verdict of not guilty by reason of insanity does not mean freedom. In most jurisdictions, the defendant is committed to a psychiatric facility for treatment. The commitment period often exceeds the prison sentence the defendant would have received if convicted. A 1991 study found that NGRI acquittees in New York spent an average of 3.5 years in confinement, compared to an average prison sentence of 2.8 years for defendants convicted of comparable offenses.

Release requires the acquittee to demonstrate that they no longer pose a danger to themselves or others. This determination involves periodic review by treatment teams and judicial hearings. John Hinckley Jr. was committed to St. Elizabeth’s Hospital in Washington, D.C., and was not granted unconditional release until June 2022—41 years after the shooting.

Public Perception Versus Statistical Reality

Surveys consistently show that the public believes the insanity defense is used far more frequently and succeeds far more often than it actually does. A 2007 study found that respondents estimated the defense was raised in 37% of felony cases (actual: less than 1%) and succeeded in 44% of attempts (actual: about 25%). This perception gap fuels legislative hostility toward the defense and contributes to jury skepticism. The gap between public belief and courtroom reality remains one of the widest in all of criminal law.

This article is for informational purposes only and does not constitute legal advice.