This paper examines the insanity defense as an affirmative legal strategy used primarily in murder and attempted murder cases. It traces the defense's historical evolution from 18th-century common law through modern statutory reform, explains the court process defendants must navigate when pleading not guilty by reason of insanity, and analyzes four major legal tests—M'Naughten Rule, Irresistible Impulse test, Durham Rule, and Model Penal Code Rule—that courts use to determine criminal responsibility. The paper also addresses burden of proof requirements, the 1984 Comprehensive Crime Control Act, and public skepticism about the defense, concluding that while the insanity defense remains legally important, it is rarely successfully invoked in practice.
To claim insanity is an affirmative defense used by a percentage of defense teams, primarily in cases where the defendant has been charged with murder or attempted murder. The defense attorney must examine all available evidence to determine whether this extreme defense is appropriate to pursue. The insanity defense can be difficult for a defendant to prove. When this defense is claimed, it initiates a process in which the defendant is evaluated by at least two, and possibly more, licensed psychiatrists to determine whether the defendant was competent at the time of the alleged offense and whether they are competent to stand trial.
The insanity defense has a significant historical foundation. This defense reflects a compromise between society's desire for punishment and its recognition of mental illness. On one hand, society believes that everyone must be punished for wrongdoing, including capital punishment. On the other hand, society recognizes that mentally ill people should receive treatment rather than traditional punishment. The insanity defense embodies this compromise. It reflects society's belief that the law should not punish defendants who cannot control their actions due to mental illness that impairs their judgment.
In the 18th century, legal standards for the insanity defense varied significantly. Some courts wanted to determine whether the defendant knew the difference between "good and evil," while others only wanted to know whether the defendant knew what they did. This inconsistency caused major problems in the justice system. In the 19th century, the insanity defense took a different turn. It became generally accepted that "insanity" was a question of fact to be decided by a jury or three-judge panel.
To understand the insanity defense fully, one must understand the court system and the process a defendant must follow to receive a fair trial and either be acquitted or be found guilty beyond a reasonable doubt. The basic state court process follows this sequence: alleged criminal activity, an investigation into the activity, filing of a criminal complaint, and an initial appearance before a judge, which usually occurs in municipal court. After that hearing, the defendant may waive a preliminary hearing (probable cause hearing) or set a date for one. If the hearing proceeds, it may result in grand jury presentation, or if probable cause is not found, the criminal complaint will be dismissed. After grand jury presentation, the result is either a no bill (ending the case) or an indictment.
Once an indictment is issued, an arraignment is scheduled. This is where the defendant must appear and bond is typically set. During arraignment, a defendant has the opportunity to plead not guilty, guilty (which is rarely done), or not guilty by reason of insanity. If a not guilty plea is entered, it allows for trial motions to be filed and some motions to be set for hearings. Once all hearings are complete, the case proceeds either to trial or to a plea bargain. If a plea bargain is reached, sentencing follows; if a trial takes place, the jury waits for a verdict, and if the defendant is found guilty, sentencing follows. If a guilty plea is entered, sentencing usually follows the plea unless otherwise requested.
If a not guilty by reason of insanity plea is made, a motion is filed by the defense and the defendant is scheduled for several evaluations. The prosecutor will usually have an evaluation completed by a court psychiatrist. The defendant has the right to have an independent evaluation completed. Depending on the evaluations' outcomes, the prosecutor may elect to have the defendant evaluated privately as well. Once insanity is claimed, if the defendant is found not competent to stand trial, a psychiatrist makes this determination and presents it to the judge, who must decide whether the defendant is treatable and can stand trial later, or whether the defendant is treatable but unable to stand trial.
If the defendant is treatable and able to stand trial, the defendant is usually housed in a mental facility for treatment. Once treatment is complete, the defendant is re-evaluated. If found able to be tried for the alleged crime, the standard court process is then followed. If the defendant remains unable to be brought to a state of "reality" despite treatment, the judge orders commitment to a mental health facility for a period of time depending on the alleged crime. When the commitment period ends, the defendant is re-evaluated for release.
One may ask: what is the approach of the insanity defense? The first approach concerns the burden of proof. The insanity defense is an affirmative defense, meaning the burden of proof falls solely on the defense. The defense must show that the defendant meets the insanity defense criteria by a preponderance of the evidence. This does not mean the defendant must prove innocence of the alleged crime, only that they were insane at the time the crime was committed.
By pleading not guilty by reason of insanity, the defendant requests that the court order an evaluation by a psychiatrist. At this time, the prosecutor may request an evaluation. Once all testing is completed, a hearing is held. At the hearing, both the prosecutor and the defense question the evaluators about their recommendations. It then falls to the court to decide whether the defendant is mentally disabled (legally incompetent) and unable to stand trial, whether the alleged crime was committed due to temporary insanity, whether the defendant is able to be treated, or whether the defendant is competent to stand trial. When making this decision, the court must examine not only the evaluators' recommendations but also the evidence presented by the defense to explain why the defendant may have acted that way and why they should be found not guilty by reason of insanity.
Evidence presented could include past police reports, previous evaluations the defendant may have had at other times in their life, past and current school records, and medical records, including current or past medications. The insanity defense has long been rooted in common law under the assumption that an "insane person may not be held criminally responsible for their conduct." The insanity defense is based on the theory that the perpetrator of an alleged crime must act with a "guilty mind," or mens rea, which means "criminal intent." This, combined with the actus reus (the criminal act), creates the completed crime. However, if the defendant is incapable of possessing the mental state leading to culpability, then a vital element of the crime is missing, and there is "no crime."
It is important to understand that "insanity" is a legal term, not a medical term. The issue before the court on a plea of not guilty by reason of insanity is whether a defendant should be held criminally responsible for their conduct. Insanity does not encompass all medical abnormalities that medical science recognizes. A person who is mentally ill or intellectually disabled may not be considered "insane" for purposes of the insanity defense. Terms such as "mental disease or defect" do not include voluntary intoxication. Persons commonly referred to as "sociopaths" or "psychopaths" usually do not qualify for the insanity defense. These distinctions make it significantly more difficult for the defense to prove that the defendant is not guilty by reason of insanity.
In order to determine insanity, the fact-finder—the court or judge—is guided by one of the tests developed over the years to help determine if the defendant was "insane" at the time of the offense.
The first test is the M'Naughten Rule, the oldest and most famous test, founded in 1843 and still used in some jurisdictions today. The M'Naughten Rule was named after a Scottish woodcutter who murdered the secretary to the Prime Minister, Sir Robert Peel, in a botched attempt to assassinate the prime minister himself. During his trial, the jury acquitted him, finding him not guilty by reason of insanity. At sentencing, he was committed to a mental institution for life. This acquittal caused such an uproar that courts were ordered to develop a stricter test for insanity. Eventually, his case became the basis of the law governing legal responsibility in insanity cases in England, and was later adopted without modification by American courts. This test is considered the "right/wrong" test. Under this rule, an individual will be found insane if, "at the time of the committing act, the party committing the act was laboring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing, or, if he did know it, that he did not know what he was doing was wrong."
The second test is the Irresistible Impulse test, used to address behavior beyond the control of a defendant. Under this test, a defendant will be found innocent if "the defendant cannot control his conduct even if he knew what he was doing and knew if it was wrong." This test is usually used when the alleged crime is what we call a "crime of passion."
The Irresistible Impulse test, unlike the M'Naughten Rule, takes a person's control into consideration. As psychiatrists agree, it is possible to know one's behavior is wrong but still be unable to stop oneself from committing the act. To address this issue, some states have modified the M'Naughten test by adding an "irresistible impulse" provision. This provision absolves a defendant who can distinguish right from wrong but is nonetheless unable to stop themselves from committing an act they know is wrong. This is also known as the "policeman at the elbow" test: Would the defendant have committed this crime even if a policeman were standing at their elbow?
The third test is the Durham Rule, based on what is called the "product test," and named after Monte Durham. Mr. Durham was a twenty-three-year-old who had been in and out of many prisons and mental institutions. In the 1950s, growing dissatisfaction with the M'Naughten test led to criticism from both the legal and psychiatric professions. In 1954, an appellate court discarded the M'Naughten Rule and the Irresistible Impulse test in favor of a broader, medically based test. In Durham v. United States, the U.S. Court of Appeals for the District of Columbia ruled that a defendant could not be found criminally responsible "if his unlawful act was the product of mental disease or mental defect." This rule looks beyond what the defendant knew or did not know at the time of the offense, unlike the M'Naughten Rule, and concentrates on the outcomes and what inspired them. It is more "expert-driven" than the other tests and represented the first major break from the right/wrong M'Naughten Rule.
The Durham Rule, however, proved to be vague and difficult to apply. Many were concerned that this test would exonerate far more defendants than ever before. Critics worried that defendants might begin to use alcoholism or other disorders whose symptoms include antisocial behavior as excuses for their crimes. After twenty-two states rejected the Durham Rule, a panel of federal judges overturned the ruling in favor of the Model Penal Code test in 1972.
This brings us to the fourth test: the Model Penal Code Rule. The drafters of the Model Penal Code formulated an insanity test combining elements of the M'Naughten Rule with the Irresistible Impulse test. In 1972, the American Law Institute, a panel of legal experts, developed this rule. This test states: "A person is not responsible for criminal conduct if, at the time of such conduct, as a result of mental disease or defect, he lacks substantial capacity either to appreciate the criminality of his conduct or to conform his conduct to the requirements of law." This new test was based on a District of Columbia Circuit's decision in the federal appellate case, United States v. Brawner.
This standard is admittedly extremely vague. It leaves a number of factors up to the jury to determine based on the facts of the case and the testimony of all experts involved. Approximately half of the states have adopted the Model Penal Code rule for the insanity defense.
"1984 federal reform and evidentiary standards"
"Skepticism and high-profile insanity defense trials"
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