Psychological Evaluations & the Law: Competency to Stand Trial and the Insanity Defense

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  • 0:07 Abnormality
  • 1:05 Competence
  • 3:27 Insanity Defense
  • 6:17 Lesson Summary
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Lesson Transcript
Instructor: Natalie Boyd

Natalie is a teacher and holds an MA in English Education and is in progress on her PhD in psychology.

Psychology and the law intersect in many ways. In this lesson, we'll look at two important parts of a trial that forensic psychologists participate in: competency hearings and the insanity defense.


Jonathan hears voices. They tell him to do things, and when he doesn't, they keep pestering him. He thinks that if he can just do what the voices tell him, they'll leave him alone. But lately, they've been asking him to do things that he really doesn't want to do, like hurt a girl in his class. He really likes the girl, Sylvia, but if he doesn't hurt her, he's afraid the voices will never go away. He doesn't know what to do.

Abnormal psychology is the study of abnormal thoughts, behaviors, and actions. In this field, psychologists study the causes and treatments of all sorts of psychological disorders.

Jonathan is suffering from a mental disorder that might lead to him hurting someone. But what happens if he does? The legal system and psychology interact in many different ways. Let's look closer at some of the ways that the court system intervenes, including the concepts of competence and the insanity defense.


Forensic psychology is the intersection of psychology and the judicial system. Forensic psychologists may hold a variety of roles within the legal system. One is the evaluation of criminals to determine if they are competent to stand trial.

Let's say for a moment that Jonathan listens to the voices in his head and hurts Sylvia. And let's say that he doesn't try to hide it, and the district attorney has a lot of evidence against him. That should be an easy case to try, right?

Well, not necessarily. In order to prosecute him, someone like Jonathan has to be evaluated by a psychologist to determine whether or not he is competent to stand trial. What does this mean? He has to understand the nature of the charges against him and be able to cooperate with his defense lawyers.

For example, imagine that Jonathan's defense lawyer explained to him what he was being charged with and what might happen if he's found guilty. But no matter how much explaining is done, Jonathan just doesn't understand what's going on. Maybe he's so lost in his own world of voices that he just doesn't understand what's real and what's not anymore. In that case, he might be found incompetent to stand trial.

What does this mean for Jonathan? Well, the law varies from state to state, but many criminals who are incompetent to stand trial are committed to state mental institutions and periodically reevaluated. Sometimes, this reevaluation leads to release from the mental institution, and sometimes it means that they are found competent and tried at that point.

Not being able to tell the difference in reality and hallucinations or delusions is one reason a person may be found psychologically incompetent to stand trial. One other reason that a person may not be competent to stand trial is if he has a severe intellectual disability, which keeps him from functioning at a normal mental level. Other reasons include not being able to read or write and even severe memory loss.

Note that a competency hearing to determine fitness for trial only looks at the patient's current state of mind. Let's say that Jonathan was completely sane when he hurt Sylvia, but soon afterwards, he had a psychological break and wasn't competent to stand trial. He would still be incompetent to stand trial, even though he was functioning normally at the time of the crime.

Insanity Defense

But what happens if he wasn't functioning normally at the time of the crime? What happens then? Imagine for a minute that Jonathan understands his charges and is able to help his defense lawyer. But at the time of the crime, he didn't know what he was doing. In that case, his defense team might argue that he is not guilty by reason of insanity.

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