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Is this patient not guilty by reason of insanity?

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How to assess a defendant’s mental state at the time of the offense



Police find Mr. B, age 45, at home after he called 911 to report that he killed his wife. Covered in blood, he confesses immediately and is holding the knife he used to stab her. Police arrest him without resistance and charge him with murder.

Three months later, Mr. B presents for a sanity evaluation. He has a history of schizoaffective disorder and has required three past psychiatric hospitalizations. Urine and serum toxicology studies the day of the killing were negative for alcohol and drugs.

Was Mr. B legally sane or insane when he committed this offense? As psychiatrists, we are often called on to assess competence to stand trial and sanity at the time of a crime. In a previous article (Current Psychiatry, June 2006), we described how to evaluate whether a mentally ill criminal court defendant is competent to stand trial. This article introduces the process for conducting a sanity evaluation.

What is sanity?

“Sanity” is a legal—not clinical—term related to a plea of “not guilty by reason of insanity.” A sanity evaluation—a mental health professional’s specialized assessment—may be entered into evidence at a criminal trial to help a judge or jury determine whether a defendant is criminally responsible for an alleged offense.

Approximately 1 in every 100 defendants charged with a felony raise an insanity defense.1 A criminal defendant who pleads not guilty by reason of insanity asserts that he committed the offense and asks the court to find him not culpable because of his mental state when the offense occurred. Competence to stand trial, by comparison, focuses on the defendant’s present mental state (Table 1).

Before starting a sanity evaluation, determine the standard that applies in the jurisdiction where the alleged offense occurred. Federal and state courts have restricted insanity standards the past 20 years. Some states, including Idaho and Nevada, abolished the insanity defense. Others adopted “guilty but mentally ill” standards, which hold mentally-ill defendants criminally responsible for their actions.2

All insanity standards require that the defendant had a mental disease or defect at the time of the offense, but the terms “mental disease” and “mental defect” do not equate with particular DSM-IV-TR “mental disorders.” Rather, courts can interpret which diagnoses qualify for determining sanity.

Courts usually rule that serious psychotic and mood disorders qualify as a mental disease and mental retardation qualifies as a mental defect. Mental disorders that usually do not qualify include personality disorders, paraphilias, and voluntary intoxication.

Table 1

How competency and sanity assessments differ

Presence of mental illnessYesYes
Mental statusCurrent mental stateMental state at the time of the offense
Purpose of evaluationAbility to stand trialCriminal responsibility
Variation in laws by jurisdictionMinor variationGreat variation

History of insanity defense

Many early codes of law provided exceptions to criminal responsibility for the mentally ill. Modern insanity standards are based on English common law (Table 2).

Table 2

From ‘wild beast’ to ‘irresistible impulse’: Milestones in the insanity defense

Wild beast1724Most strict standard; required total deprivation of memory and understanding
Irresistible impulse1840More liberal standard; required that “…some controlling disease was…the acting power within him which he could not resist…”
M’Naughten rule1843Required that the defendant not know the nature/quality or the wrongfulness of the offense.
American Law Institute’s Model Penal Code standard1955Combined M’Naughten Rule with irresistible impulse
Federal Insanity Defense Reform Act1984Stricter standard that dropped the irresistible impulse standard after attempted assassination of President Reagan
‘Wild beast.’ The “wild beast” standard was established in 1724 in Rex v. Arnold. Arnold, the mentally-ill defendant, was found guilty after he shot and wounded Lord Onslow. Arnold’s death sentence was reduced to life in prison after Lord Onslow himself advocated for this change.

To be found insane under the wild beast standard, the defendant had to be “totally deprived of his understanding and memory, so as to not know what he is doing, no more than an infant, a brute or a wild beast.”3

‘Irresistible impulse.’ The “irresistible impulse” standard was first used successfully in 1840 in the trial of Edward Oxford, who attempted to assassinate Queen Victoria. In Regina v. Oxford, the court recognized that “if some controlling disease was…the acting power within him which he could not resist, then he will not be responsible.”4

The M’Naughten rule—perhaps the most famous standard—was established in 1843. M’Naughten suffered from paranoid delusions that the prime minister of England was plotting against him; he planned to kill the prime minister but mistakenly killed his secretary. The examiners who evaluated M’Naughten testified that he was insane, and the jury concurred. The public and royal family were incensed, however, and appellate judges reviewed the verdict and insanity standard.


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