Competence and Compellability of Witnesses with Disabilities: A Defence Counsel’s Perspective

Questions of competence and compellability frequently arise in criminal trials where a witness has a mental or physical disability. From the defence perspective, these issues must be approached carefully. While the law strongly favours the reception of evidence, defence counsel has a duty to ensure that the accused receives a fair trial and that unreliable or unfairly prejudicial evidence is not admitted.

The Legal Framework

Canadian law begins from the presumption that all witnesses are competent to testify. The burden rests on the party challenging competency. For witnesses with mental disabilities, section 16 of the Canada Evidence Act governs the inquiry. The focus is functional rather than diagnostic: can the witness communicate the evidence, and can they promise to tell the truth?

Competence

Competence concerns a witness’s ability to understand questions and communicate answers about events. It is a low threshold. Defence counsel should be mindful that difficulties with memory, expression, or comprehension do not automatically render a witness incompetent. Instead, these issues often go to weight rather than admissibility.

Compellability

A competent witness may also be compellable, meaning they can be required to testify even if unwilling. A witness with a disability is not exempt from compulsion solely because of that disability. Defence counsel should therefore distinguish carefully between arguments about whether a witness can be compelled to testify and whether their evidence should ultimately be relied upon.

Witnesses with Disabilities and Section 6 of the CEA

Section 6 of the Canada Evidence Act authorizes courts to permit witnesses with physical or mental disabilities to testify using any means that make their evidence intelligible. This includes interpreters, communication aids, or other accommodations. From the defence perspective, accommodations are not a concession but a recognition that accessibility and trial fairness are complementary, not competing, principles.

The Competency Inquiry

Where a witness’s mental capacity is challenged, the court must conduct a focused voir dire. The inquiry is limited in scope and must not turn into a credibility assessment. Defence counsel should ensure that the inquiry remains confined to the statutory questions and does not improperly bolster the witness’s reliability in the eyes of the trier of fact.

Section 16(3): Promise to Tell the Truth

Section 16(3) has only two requirements for adult witnesses with mental disabilities: the ability to communicate the evidence and a promise to tell the truth. Courts have made clear that witnesses need not understand abstract concepts of truth, morality, or religion. Defence counsel should avoid overreaching challenges that risk appearing discriminatory or contrary to settled law.

Defence Preparation

When considering a challenge to competence, defence counsel should carefully assess whether the challenge advances the accused’s interests. A failed competency challenge may strengthen the Crown’s case by signalling judicial confidence in the witness. Where issues exist, counsel should consider whether they are better addressed through cross-examination rather than exclusion.

Practice and Procedure

Competency voir dires should be brief, respectful, and tailored to the witness’s abilities. Questioning must be patient, concrete, and adapted to the witness’s communication style. Defence counsel should also consider whether expert or lay evidence is necessary, bearing in mind that the witness herself is usually the primary source of evidence regarding competence.

Special Considerations for Defence Counsel

Defence counsel must balance vigorous advocacy with sensitivity. Courts are alert to the risk that competency challenges may be misused to exclude marginalized witnesses. Strategic restraint, grounded in law and fairness, will often best serve both the accused and the integrity of the trial process.

Conclusion

The law on competence and compellability reflects a strong commitment to inclusion and access to justice. From the defence perspective, the task is not to resist this commitment, but to ensure that accommodations and relaxed thresholds do not compromise the accused’s right to a fair trial. Properly understood and applied, these principles support both fairness and truth-seeking.