1. Understanding the charges
What the law requires. The defendant must understand, in broad terms, what they are charged with — what they are alleged to have done and the essential nature of the accusation. This is not a demand for legal sophistication. A defendant need not grasp the elements of the offence as a lawyer would, nor appreciate every evidential subtlety.
What it engages clinically. This limb draws on receptive language, basic comprehension, and a level of reasoning sufficient to hold and make sense of the allegation. It is most often in question where there is significant intellectual disability, an acquired brain injury affecting comprehension, a florid psychotic illness distorting the defendant’s understanding of reality, or a severe communication disorder. The assessor’s task is to establish not whether the defendant can recite the charge, but whether they genuinely apprehend what is being said about them.
2. Deciding whether to plead guilty or not
What the law requires. The defendant must be able to make a decision on plea and to communicate it. This involves understanding that there is a choice, that the choice has consequences, and being able to arrive at and express a position.
What it engages clinically. This is the limb that sits closest to the modern concept of decision-making capacity — the ability to understand relevant information, retain it, weigh it, and communicate a choice. Notably, the Pritchard test does not expressly frame this in capacity terms, which is one of the central criticisms of the criteria. In practice, this limb is engaged by conditions that impair reasoning or volition: psychosis affecting the defendant’s appreciation of their situation, severe depression affecting motivation and decision-making, or intellectual disability affecting the capacity to weigh consequences. A defendant who can state a plea but cannot meaningfully understand or weigh what it entails may fail this limb even though they appear, superficially, to be deciding.
3. Exercising the right to challenge jurors
What the law requires. The defendant must understand that they have the right to object to a juror and be able to exercise it. In modern practice this is a limited and infrequently exercised right, and it is rarely the limb on which a case turns.
What it engages clinically. The cognitive demand is modest: an understanding that the jury decides the case, that the defendant may object to a member of it, and the ability to act on that understanding. It tends to be in issue only where comprehension is globally impaired — severe intellectual disability or significant cognitive decline — such that the defendant cannot grasp the concept at all. Its main significance is as one of the conjunctive limbs: it must still be addressed, even if briefly.
4. Instructing solicitors and counsel
What the law requires. This is one of the most important limbs in practice. The defendant must be able to convey to their lawyers, intelligibly, the case they wish to advance and the matters they wish to raise in their defence. As the court made clear in M (John), this means being able to understand the lawyers’ questions, to apply their mind to answering them, and to convey their answers intelligibly. Importantly, the instructions need not be plausible, believable, or reliable — many defendants advance accounts that are none of these things, and it is the function of the trial to test them.
What it engages clinically. This limb engages expressive communication, working memory, attention, and the capacity to engage in a reciprocal exchange about one’s own situation. It is frequently the decisive limb. It is commonly in question where psychosis renders the defendant’s account incoherent or driven by delusion, where intellectual disability or brain injury impairs the ability to follow and respond to questions, or where a thought disorder prevents intelligible communication. The distinction the assessor must hold onto is between a defendant whose instructions are unusual, implausible or self-defeating (which does not establish unfitness) and one who cannot intelligibly instruct at all (which may).
5. Following the course of the proceedings
What the law requires. The defendant must be able to follow what happens during the trial sufficiently to participate in it — to track the evidence and the progress of the case in broad terms.
What it engages clinically. This limb depends on sustained attention, concentration, comprehension, and the stamina to maintain engagement over the length of a trial. It is engaged by conditions affecting attention and processing — significant cognitive impairment, dementia, the effects of brain injury, and active psychotic or affective illness that intrudes on the defendant’s ability to attend to proceedings. Crucially, this limb must be assessed in the context of the particular trial. A defendant might be able to follow a short, simple summary matter but not a lengthy, document-heavy, multi-handed trial. The complexity of the proceedings is part of the question.
6. Giving evidence in their own defence
What the law requires. The defendant must be capable of giving evidence on their own behalf, if they choose to. The question is one of capability, not of whether they will in fact testify.
What it engages clinically. This limb engages the ability to understand questions in examination and cross-examination, to retrieve and relate relevant information, and to respond coherently under the particular pressures of the witness box. It is engaged where psychosis, severe anxiety, cognitive impairment, or a communication disorder would prevent the defendant from giving an intelligible account when questioned. The courts have recognised this as a distinct and important limb — a defendant may be able to follow proceedings yet be unable, by reason of their condition, to give evidence in any meaningful way.
The role of special measures
Across all six limbs, a vital modern question is whether an apparent inability can be accommodated rather than treated as conclusive of unfitness. The use of an intermediary, simplified questioning, regular breaks, the removal of formality, and other adjustments may enable a defendant who would otherwise struggle to participate effectively. A rigorous assessment considers not only whether the defendant can presently do each of the six things, but whether reasonable adjustments would allow them to — because where they would, the proper course is to make the trial fair rather than to halt it.