Competency to stand trial is a standard which must be met if someone is to stand trial. If someone is competent to stand trial, it means that he or she understands the purpose and nature of the proceedings and can assist the defense. If someone does not meet this standard, proceedings are terminated. In the United States, competency to stand trial has been ruled to be an aspect of due process of law, and many other nations also have similar protections in place.
If at any point during criminal proceedings there is a doubt about the defendant's competency to stand trial, a competency evaluation must be completed. A common cause to question someone's competency is mental illness which might make it difficult for someone to understand the proceedings or which would hinder preparation of defense. People with intellectual disabilities may also be deemed incompetent to stand trial.
During the competency evaluation, a mental health professional meets with the defendant. The evaluation is designed to test whether or not the defendant meets a standard of rationality and is capable of understanding why proceedings are happening, how they will be conducted, and what the potential consequences of the proceedings might be. Competency hearings can also be requested before someone waives legal rights, to confirm that the individual understands these rights and what happens when they are waived, and when someone wishes to plead guilty.
Having an intellectual disability or mental illness does not necessary mean that someone's competency to stand trial will be called into question or that someone cannot stand trial. Only an evaluation can determine this. However, some courts have been criticized for trying people who appear to lack an understanding of the proceedings by people who have said that the competency hearing was not extensive enough or was not conducted properly. Concerns have also been raised from another perspective by people who are worried that competency hearings might be used as a delaying or stalling tactic by the defense.
Competency to stand trial is sometimes compared to an insanity defense, but the two are different concepts. An insanity defense is used during a trial by someone who has been deemed competent to stand trial. In this defense, it is argued that at the time the crime was committed, the defendant was not of sound mind, and must be not guilty by reason of insanity because the defendant did not realize the repercussions of his or her actions. In these cases, the defendant's mental state at the time of the trial allows him or her to consult with a lawyer, to understand the charges and proceedings, and to participate in the trial.