Mental state

Last Updated: 28 Jan 2021
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1. Why is a defendant’s mental state at the time of the offense (MSO) important from a legal standpoint?

From a legal standpoint, the relevance of the mental state of the defendant is important because it determines whether or not the act was done with any capability, aptitude, or ability.  These factors are relevant because they determine if the defendant acted with consciousness in the commission of the offense.  The commission of an offense without any knowledge of the gravity of the situation or under a state of insanity is an exempting circumstance that exonerates the defendant from any criminal liability.

The mental state is also important during the custodial investigation wherein the defendant is read his Miranda Rights.  This is particularly important because the law provides that the accused has the right to know of the nature of the charges against him and the possible consequences of any answer which he may give during such questioning.  A mentally incapable or incompetent person will be unable to comprehend these matters and therefore it is important to determine his capacity.

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Another perspective is that the mental state of the defendant also determines his competency to stand trial (CST), Competency to refuse an insanity defense, Competency to confess, Competency to waive rights to silence, Competency to dismiss counsel, Competency to serve a sentence, and Competency to be executed.  Under the Constitutional requirement of due process, the defendant has a right to know the charges against him and also to cross examine any witnesses that will be presented as well as the right to defend himself in court.  Without a determination of the competency of the defendant, such rights of the defendant may be violated which is against the Constitutional guarantee of due process.

2. What is the role of a forensic psychologist in such cases?

The role of a forensic psychologist in such cases is to determine the mental competency of the defendant.  The in-depth knowledge of human cognitive and emotional functioning that the forensic psychologist possesses enables him to arrive at a conclusion on whether or not the defendant is competent to stand trial.  The forensic psychologist can also conduct and insanity examination to determine whether or not the act was done with criminal intent (mens rea) by the defendant.

The different forensic assessment instruments that the forensic psychologist utilizes in order to determine the competency of the defendant are the following, Competency Screening Test (CST), Competency Assessment Instrument (CAI), Interdisciplinary Fitness Interview (IFI) and the MacArthur Structured Assessment of the Competencies of Criminal Defendants (MacSAC-CD).

3. What mistakes might a forensic psychologist make when conducting this type of evaluation?

The most common error that a forensic psychologist might make during these types of evaluations involves the area of predicting the human behavior of the defendant.  While the different tests and evaluations that the forensic psychologists use may be relatively accurate in determining the competency of the defendant to stand trial and at the commission of the offense, the difficulty lies in determining how long the incompetency will last and whether or not the defendant is liable to commit future crimes in case the defendant is released or exonerated.

At the present there is no way of determining whether or not the defendant may commit future crimes.  It must also be remembered that the forensic psychologist conducting the examination must also be well versed with the pertinent provisions of law applicable to the case.


Adler, J. R. (Ed.). (2004). Forensic Psychology: Concepts, debates and practice. Cullompton: Willan.

Ribner, N.G.(2002). California School of Professional Psychology Handbook of Juvenile Forensic Psychology. Jossey-Bass. ISBN 0-7879-5948-0

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Mental state. (2017, Feb 14). Retrieved from

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