Psychological Medicine, 1999, 29, 1367–1375. Printed in the United Kingdom  1999 Cambridge University Press Assessing the ability of people with a learning disability to give informed consent to treatment K.ARSCOTT, D.DAGNAN  B.STENFERT KROESE From the Tizard Centre, University of Kent at Canterbury ; West Cumbria Healthcare NHS Trust, Community Learning Disabilities Service, Workington, Cumbria ; and School of Psychology, University of Birmingham ABSTRACT Background. People with a learning disability are increasingly being encouraged to take a more active role in decisions about their psychological and medical treatment, raising complex questions concerning their ability to consent. This study investigates the capacity of people with a learning disability to consent in the context of three treatment vignettes, and the influence of verbal and memory ability on this capacity. Methods. Measures of verbal ability, memory ability and ability to consent to treatment (ACQ) were administered to 40 people with a learning disability. The ACQ consisted of three vignettes depicting a restraint, psychiatric or surgical intervention. These were followed by questions addressing people’s ability to understand the presenting problem ; the nature of the proposed intervention ; the alternatives, risks and benefits ; their involvement in the decision-making process ; and their ability to express a clear decision with a rationale for treatment. Results. Five people (125%) could be construed as able to consent to all three vignettes; 26 (65%) could be construed as able to consent to at least one. The questions that were most difficult to answer concerned a participants’ rights, options and the impact of their choices. Verbal and memory ability both influenced ability to consent. Conclusions. This study introduces a measure that may enable clinicians to make more systematic assessments of people’s capacity to consent. A number of issues surrounding the complex area of consent to treatment are also raised. INTRODUCTION Questions regarding capacity to consent to treatment frequently arise for people with a learning disability (the current UK term broadly equivalent to the US definition of Mental Retardation) and health professionals are in- creasingly called upon to make determinations about an individual’s competence in this area. However, universally accepted standards or guidelines for establishing an individual’s ability to consent to treatment have been lacking (Law Commission, 1991 ; Venesey, 1994 ; Murphy & Clare, 1995). The Law Commission (1991, 1993, 1995) has attempted to clarify the legal position  Address for correspondence : Dr Katy Arscott, Tizard Centre, University of Kent at Canterbury, Canterbury, Kent CT2 7LZ. in England and Wales, stating that capacity re- quires an ‘ understanding in broad terms of the nature and likely effects of what is to take place ’ (Law Commission, 1991, p. 29). Current legal definitions for capacity can be found in case law and in the Mental Health Act Code of Practice (Department of Health and Welsh Office, 1993, para. 15.10). The latter provides some guidance, suggesting that to be able to consent an individual must possess an ‘ adequate knowledge of the purpose, nature, likely effects and risks of [the] treatment including the likelihood of its success and any alternatives to it’ (p. 55). However, this applies only to individuals with a mental disorder as defined by the Act, and it is unclear what criteria should be used to assess ‘adequate’ knowledge. In case law (see Re C, 1367