Insanity Defense in the United States of America

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The insanity defense can be characterized as the offender, who has mental defects or diseases can influence his ability to form mens rea – to have a guilty intent to commit a crime. It means that the defendant has committed the crime but there is lack of culpability, inability to think rationally, to define wrongfulness and consequences of actions performed, and to control the behavior due to mental illness. The following forms of the insanity defense are identified in the United States:

  1. The “M’Naghten Rule” or the “Right-Wrong” Test – that proves that there was a possibility that the offender did not realize or understand what he or she did, in addition, he cannot identify what is wright and what is wrong due to mental problems. This is the most commonly used form of test to identify the insanity defense in the United States. This test identifies the ability of the offender to understand and fully realize the nature of the crime committed and to define what was right and what was wrong during the time of committing the crime;
  2. The “Irresistible Impulse” Test can be identified as the process that reveals that the crime committed was the result of the defendant’s inability to control his inner impulses due to a mental disease. This test was used as a supplement to the “M’Naghten” Test that has its main focus on the cognitive element, such as awareness with the will of the defendant or his ability to control conduct. The offender should “prove” that he has a mind disease or some mental defects that do not allow him to control inner impulses when committing some criminal “wrong” things;
  3. The “Durham Rule” according to it, a committed crime was brought by the mental defect of the offender regardless of clinical diagnosis; there is no need to prove mental disease by medical diagnosis;
  4. The “Model Penal Code” Test for Legal Insanity or “Substantial Capacity” Test – a defendant in criminal case should have a proved relevant mental disease or schizophrenia that did not allow him to differentiate between right and wrong, or that led to a lack of impulse control during committing the crime. Therefore, such an individual can be named not guilty.

The most common defense applied in the United States is the M’Naghten insanity defense, in which the defendant is diagnosed to have “disease of mind” or “reason defect”. It means that the offender should possess some mental diseases, such as psychoses, schizophrenia, paranoia that do not let him or her to cognitively understand the criminal act by its nature and quality of the committed crime; the defendant does not differentiate between right and wrong legal and illegal acts.

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According to Schouten, the basic idea of the insanity defense is that not all criminal acts are committed by mentally or psychologically healthy people. Therefore, we should interpret the understanding of the fact that offenders who lack the ability to think rationally, who do not control their behavior and inner impulses, who cannot determine the wrongfulness and consequences of their actions, should not be pledged guilty for their “mental sickness.” (Schouten, 2012) Such individuals can possess danger to the society and themselves, therefore, they are being examined, monitored and controlled with the help of high-professional supervisors.

In the United States, we can point out some key issues when we can apply the procedure of insanity: availability, definition, and burden of proof. A forensic psychiatrist is evaluating the information about the defendant’s history of early crimes, his background, psychological portrait, the information about the time of committing a crime. After that, the psychiatrist is interviewing the defendant about possible understanding of what has happened, why the crime was committed. Moreover, various testimonies from the defendant, the victim, the court, the prosecutor are being analyzed. These testimonies may be influenced by the history of disease or new diagnosis, presence of legally relevant impairments. All of the above-mentioned helps the professional to make the decision whether the defendant is legally competent to stand trial or not.

Psychologist who is used as an expert witness to prove or disprove an insanity defense, move from factual data about an individual’s behavior to opinions of the prosecutor, the court of being guilty. Mainly, the empirical data is based on psychological tests, while another component is a diagnostic one during which they can decide, basing on their skills about the possibility that the offender has mental illnesses. Finally, the last testimony level can be observed as the ultimate factual question for the jury to solve. In accordance with the severity of a case, additional investigations, such as physical assessment or hospitalization. According to Levine, the forensic psychologist should have a qualified experience; have a PhD degree that allows him to be identified as a person, who is competent in a scientific sense. That gives him the opportunity to make a diagnosis of the existence and character of mental disorders and provide an opinion whether there was a causal connection between mental disease and committing the criminal act. (Levine, 1971)


  • Levine, E. R. (1971). Psychologist as Expert Witness in Psychiatric Questions. Clev. St. L. Rev. 378 Retrieved from
  • Schouten, R. (2012) The Insanity Defense. Psychology Today. Retrieved from

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