This one is fairly common, and actually makes sense when viewed through a lay person’s eyes. However, it’s not true. In order to appreciate why incompetency and mental illness are not synonymous, it makes sense to return to the legal definition of competence, which is found in the U.S. Supreme Court decision of U.S. vs. Dusky:
“The test (of competence) must be whether he (the defendant) has sufficient present ability to consult with his attorney with a reasonable degree of rational understanding and a rational as well as factual understanding of the proceedings against him.”
The key word here is ability. Mental illness (to include cognitive impairments, such as from a head injury) is a necessary, but not sufficient, condition of being found incompetent. This is because competence does not depend on one’s state, but rather one’s ability, and a mental health problem, by itself, does not tell us whether an individual has the ability to assist his counsel and/or possess a rational understanding of the proceedings against him.
Put another way, a defendant can be psychotic (to include schizophrenia), mentally retarded, blind, deaf, amnesic, etc., and still be competent to stand trial. This is because the primary question is not what symptoms an individual is currently experiencing, but to what degree those symptoms are impairing the defendant’s ability to (again) assist their attorney with their defense, and to rationally understand the proceedings against them. Consistent with this, courts have recognized for years that although an individual may be psychotic (a broad term suggestive of the presence of such symptoms as hallucinations, delusions, disorganized thinking, and other experiences that indicate a lack of grounding in reality), they may still be competent. According to Grisso, some studies have shown that anywhere from 10-25% of defendants who have been found competent to stand trial were experiencing psychotic symptoms at the time.
Many jurisdictions (including the federal system) have added a requirement of “the presence of a severe mental disease or defect” (or similar variation). This doesn’t change the above fact, because the onus is still, ultimately, on the defendant’s abilities, and not on their overall mental health. What this does change, however, is that a defendant without a significant mental health diagnosis cannot be found incompetent. Again, necessary but not sufficient. Therefore, in assessing an individual’s competency to stand trial, the evaluator will assess the individual’s mental health, including diagnostic considerations, but will then assess the individual’s capacity to assist counsel and understand the proceedings.
One other point - as I’ve stated before, the bar is pretty low for assessing competency to stand trial, while pretty high for actually being found incompetent. This is because the presence of a mental health issue (or even the suggestion of such an issue) is enough to get a judge to order an evaluation. So, many defendant’s will meet this very low threshold. Conversely, to be found presently incompetent to stand trial is a fairly high threshold, since the word reasonable in the Dusky decision is a level that is not all that hard to meet. Part of what is going on is that defendants are being referred based on a different standard than that of their assessment. They are being referred based on having a mental health issue, but being assessed on the abilities outlined above. So, what you ultimately get are many defendants being evaluated for competency, with most of them being found competent. Think of it as the “better safe than sorry” mentality, but it does help to ensure that the trial process is fair and accurate.