Background. Based on the developing clinical and legal literature, and using the framework adopted in draft legislation, capacity to make a valid decision about a clinically required blood test was investigated in three groups of people with a ‘mental disability’ (i.e. mental illness (chronic schizophrenia), ‘learning disability’ (‘mental retardation’, or intellectual or developmental disability), or, dementia) and a fourth, comparison group.

Methods. The three ‘mental disability’ groups (N = 20 in the ‘learning disability’ group, N = 21 in each of the other two groups) were recruited through the relevant local clinical services; and through a phlebotomy clinic for the ‘general population’ comparison group (N = 20). The decision-making task was progressively simplified by presenting the relevant information as separate elements and modifying the assessment of capacity so that responding became gradually less dependent on expressive verbal ability.

Results. Compared with the ‘general population’ group, capacity to make the particular decision was significantly more impaired in the ‘learning disability’ and ‘dementia’ groups. Importantly, however, it was not more impaired among the ‘mental illness’ group. All the groups benefited as the decision-making task was simplified, but at different stages. In each of the ‘mental disability’ groups, one participant benefited only when responding did not require any expensive verbal ability.

Conclusions. Consistent with current views, capacity reflected an interaction between the decision-maker and the demands of the decision-making task. The findings have implications for the way in which decisions about health care interventions are sought from people with a ‘mental disability’. The methodology may be extended to assess capacity to make other legally-significant decisions.