Essay on Assessing the Sanity of People with Mental Disorders for Criminal Behaviour

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Laws and regulations concerning the insanity defense have been under constant change. When is someone deemed legally insane? And how can we determine insanity based on psychology and law? This paper provides an overview of the different laws used to determine insanity over the decades and the implication of psychological input in court.

Criteria of the Insanity Defense in Court

The M'Naghten Rule

The M´Naghten rule, established in the 1840s, entails two criteria to determine whether someone eligible is for the insanity defense. The first criterion is that it has to be proven that they suffered from a mental disease at the time of the crime. Until there is sufficient proof, it is presumed that the defendant is sane. In addition, it has to be established whether the defendant can distinguish between right and wrong and therefore understands the severity of the crime. If the defendant does not understand the nature of his crime and the presence of a mental disease is proven, they cannot be found guilty of their crime and are eligible for the insanity defense based on the M´Naghten rule. The second criterion is heavily criticized as it is seen as overemphasizing the defect and cognition of reason. The rule also does not define when the capacity is deemed enough to be held accountable for the crime.

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The Irresistible Impulse Test

The irresistible impulse test was adopted in 1887 and expands the M´Naghten rule. It encompasses whether the defendant could control the impulses that led to the action. It must be proven that a mental disease/illness is present and that this illness must be the cause for the inability to control their actions to use this test for a criminal defense. To establish this, a testimony of a medical professional following a medical examination may be required. The evidence based on the testimony needs to prove the condition and its effect on behavior. It is criticized that the definition of insanity is too broad and therefore fails to establish which acts can be deemed uncontrollable or controllable. The rule also excludes everyone who was able to control their actions, which in turn can be deemed as too narrow.

The Model Penal Code (ALI)

To combine the most important criteria of the M`Naghten rule and the irresistible impulse test, the Model Penal Code was formed by the American Law Institute in 1962. This code states that one cannot be held accountable for a crime when one could not control their action or impulse. The M`Naghten rule was incorporated through the criteria of whether the defendant can understand right from wrong and awareness that their action was against the law.

The Durham Rule/Product Defect Rule

The Durham rule was founded in 1954 by the Circuit Court of Appeals (District of Columbia). The rule serves the purpose of determining criminal responsibility and gives mental illness a definition that can be used in court as an insanity defense. This, in turn, increased the importance of psychiatric testimony in court. The rule states that a person cannot be convicted of a crime if the act committed was due to a mental illness or defect. A mental defect is here defined as a condition that will not get better or worse, while a mental illness/disease can get better or worse. The Durham rule comes into effect when it can be considered that the criminal act committed would not have happened without the existence of a mental illness or defect, and the defendant suffered from it at the time of the crime. It has been criticized that no official medical diagnosis was necessary and that it gave too much power to the decision of the psychiatrist concerning whether the defendant is to be determined sane or insane. Their decision was able to determine the outcome of the court case. In addition, the definition of what is considered a defect is considered too broad. Considering the definition used for the Durham rule, a defect can be the result of a cognitive or physical deficit, such as an injury. Therefore, the Durham rule was abandoned in 1972, but it can be said that it was a forerunner to the current rules used to determine criminal responsibility.

The Rogers Criminal Responsibility Assessment Scales (RCRAS)

The Rogers Criminal Responsibility Assessment Scale (RCRAS) is a decision model used to asses insanity as well as cognitive impairment at the time of the crime. The test consists of five psychological variables significant to the subject (reliability, organic factors, psychopathology, cognitive control, and behavioral control). These five variables are assessed on a basis of 25 items that the subject has to rate on a 0-6 scale. The assessment scale has good inter-rater reliability and a kappa of approximately 0.73. Additionally, the RCRAS is applicable to persons of different variables such as gender, race, education, prior criminal activity, or clinical diagnosis.

Criteria of Insanity/Criminal Responsibility from a Psychologist's View

How Do Psychologists Assess Insanity?

For an insanity examination to be valid, three conditions must be met, according to Rogers, Wasyliw, and Cavanaugh (1984). The construct of insanity must be clearly defined. Further, the psychologist or clinician conducting the assessment must comprehend the legal as well as the psychological elements involved in the insanity defense. Lastly, the procedures to assess insanity must be proven reliable and valid in determining insanity.

Psychologists conduct interviews to asses insanity as well as use assessment tools such as the RCAS, which increases the validity and reliability of their initial assessment. These kinds of tests are also capable of detecting malingering, which influences the outcome of the assessment next to other biases.

The psychiatric interview should take into consideration both the statement and history of the subject, as well as outside sources. That way one might reach more dependable information regarding the subject as self-serving biases and malingering can occur. During the course of the interview, the psychologist tries to get an overview of the subject's mental health. Counting to that is whether delusions or hallucinations are present, their insight and orientation into their surroundings and relation with others, their memory, thought processes, and emotional reactions. It has to be taken into consideration that to fully encompass every aspect of the named areas of a mental examination might take several interviews. The psychologist or clinician conducting the interview might also make a physical examination to identify different causes, such as brain abnormalities.

Despite trying to identify the cause of the behavior, it is important to determine whether the person could control their behavior and whether that behavior is stable. Stability refers to whether the action was temporary or constant. When trying to understand the behavior, the psychologist also tries to differentiate between internal causes, like personality or free choice, and situational or external causes, such as the influence of others.

Psychological Input in Court

Psychological assessment of criminals in court was first conducted by medical practitioners. Only after the Second World War in the 1900s were psychologists allowed to assess criminal insanity and provide input in court. The role of the psychologist in court is to assist the judge and tier of fact in deciding on the ability of the subject to be held accountable for their crime. They do so in determining the mental state and the mental illness, if present, and present those to the court. This assessment helps to determine the applicability of the Modal Penal Code and the other rules, as well as to assess the mental state at the time of the crime. Only psychologists who are specialized and specially trained are allowed to testify in court. This training encompasses specialization in mental state examination, diagnosis of forensically relevant mental illnesses, and assessment of biases and malingering. When assessing the psychologist must be aware of the limits of their competency and must state so when testifying in court.

The Importance of Neuroscience and the Decision-Making Process

The field of neuroscience could prove to be a great help to forensic psychologists in determining whether a defendant can be deemed criminally insane. Test results could prove a mental disorder based on evidence in the form of brain scans and neurological mechanisms. In addition, it enables a deeper understanding of the disorders in a matter of how they influence the decision-making process. A study about schizophrenia found deficits in decision-making linked to the ventromedial and orbitofrontal cortex. Deficits included slower decision-making and making less optimal choices when the participant was provided with guiding information. This shows that neurobiology and tests on decision-making could provide useful insight into the severity of a mental disorder and, therefore, if the defendant is eligible for the insanity defense. Neuroimaging can also prove to be important as disruptions in the brain through tumors and hormonal or neurotransmitter imbalances can lead to criminal behaviors. These disruptions may directly influence aggression and impulse control through the amygdala and the prefrontal cortex, respectively. With neuroimaging techniques, these disruptions could be made visible and be used in court to reach the verdict of ‘not guilty because of insanity’.

The three phases of decision-making (generation of options, selection, and initiation) can be influenced depending on the mental disorder. Some disorders influence one of the phases, all three, or bypass the whole process. According to the DSM-V, one can infer what kind of influence a specific mental disorder has on the decision-making process. An example of the influence of a mental disorder might be a psychotic disorder that affects the first phase, the generation of options. Behavioral options might be generated and chosen that are not available or would not appear in a normal thought process. Another example that influences all three and constrains the decision-making process would be Tourette syndrome, which is characterized by involuntary tics. Such information should prove to be relevant in court, as the judge might conclude that the defendant is not guilty since the decision-making process was bypassed or strongly influenced by a mental disorder.


There will always be a debate on when someone can be deemed legally insane. It is difficult to determine when a person cannot be deemed in the right mindset to adhere to their crime. Therefore, the testimony of forensic psychologists plays a detrimental role in assessing this. Until recently, there was no way of proving that someone suffers from a mental illness, and forensic psychologists could only provide their statement to the court. This cannot be considered evidence in court. If the psychological assessment of a defendant would incorporate a neurological assessment using brain screening techniques on the suspicion of a mental disorder, it could provide scientific evidence that could argue in favor or against the defendant. Neuroimaging techniques would therefore provide increased validity and reliability in the process of determining insanity, as well as help determine a threshold for legal and moral insanity. Meynen (2013) suggests the combined work of psychiatrists, lawyers, and neuroscientists in reviewing and interpreting such new techniques.


The debate on legal insanity is and will be relevant for the future as the insanity defense is used in many court cases. In those cases, it is important to correctly determine whether someone can be deemed legally insane or not. The verdict influences the further proceedings of the accused. On the one hand, if one suffers from a mental disorder, they would be better helped at a mental hospital where the mental disorder could be cared for, treated, and observed by professionals or a prison that can provide these kinds of services. On the other hand, the insanity defense can be misused as the prison sentence might be less severe or the person is transferred to a mental institution. To avoid this, it is important for forensic psychologists and neuroscientists to revise and elaborate the defense with new scientific evidence to identify insanity with a higher validity and accuracy that would be applicable in court.


  1. Abrams, N. (1979). Definitions of Mental Illness and the Insanity Defense. The Journal of Psychiatry & Law, 7(4), pp.441-456. doi:10.1177/009318537900700404.
  2. Costanzo, M. L., & Costanzo, M. A. (2013). Teaching Clinical (and Nonclinical) Psychology Through Applications to the Legal System: Violence Risk Assessment and the Insanity Defense. Teaching of Psychology, 40(3), pp.252-256.
  3. Meynen, G. (2013). A Neurolaw Perspective on Psychiatric Assessments of Criminal Responsibility: Decision-Making, Mental Disorder, and the Brain. International Journal of Law and Psychiatry, 36(2), pp.93-99.
  4. Murray Ferguson & James R. P. Ogloff (2011). Criminal Responsibility Evaluations: Role of Psychologists in Assessment, Psychiatry, Psychology and Law, 18:1, pp.79-94, doi: 10.1080/13218719.2010.482952.
  5. Overholser, W. (1962). Criminal Responsibility: A Psychiatrist's Viewpoint. ABAJ, 48, pp.527-531.
  6. Rogers, R., Wasyliw, O. E., & Cavanaugh, J. L. (1984). Evaluating Insanity. Law and Human Behavior, 8(3-4), pp.293-303.
  7. Wechsler, H. (1954). The Criteria of Criminal Responsibility. U. Chi. L. Rev., 22, pp.367-376.
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