If the issue of fitness to plead is raised, a judge is able to find a person unfit to plead. This is usually done based on information following a psychiatric evaluation.
In England and Wales the legal test of fitness to plead is based on R v Pritchard. The accused will be unfit to plead if he is unable to either:
to comprehend the course of proceedings on the trial, so as to make a proper defence;
to know that he might challenge any jurors to whom he may object;
to comprehend the evidence; or
to give proper instructions to his legal representatives.Prichard (1836) 7 C & P 303.
If the issue is raised by the prosecution, the prosecution must prove beyond reasonable doubt that the defendant is unfit to plead.Robertson 1 WLR 1767. If the issue is raised by the defence, it need only be proved on the balance of probabilities.Podola 1 QB 325.
In Scotland the test is based on HMA v Wilson, and has two elements: