Background. People with intellectual disabilities who have been victims or other witnesses of crime have had limited access to the criminal justice system, often on the basis of assumptions about their incapacity to be interviewed by the police and to give evidence in court. The aim of this study was to assess their capacity to be witnesses in court.

Methods. Forty-nine men and women with intellectual disabilities, all of whom were potential witnesses of ill-treatment, were assessed in order to provide advice, initially to the police, about their capacity to be interviewed for judicial purposes. The assessments included evaluations of each person's intellectual ability, memory, acquiescence, suggestibility, and their ability to explain concepts relating to the oath.

Results. Only 37 (76%) were able to complete the assessments. Most of those with a Full Scale IQ score of [ges ] 60 had a basic understanding of the oath, compared with only a third of those with IQ scores between 50 and 59, and none of those with IQ scores < 50. Nevertheless, some of the people who were unable to demonstrate an understanding of the oath did understand the words ‘truth’ and ‘lie’, especially when asked about these concepts in relation to concrete examples.

Conclusions. While intellectual ability appears to be the best overall predictor of the capacity of people with intellectual disabilities to act as witnesses, confining witnesses to those who could explain the meaning of the oath would mean that a number of persons who might be interviewed by the police and subsequently appear in court could be excluded from the judicial process.