Abstract
Most jurisdictions require that a mental capacity assessment be conducted using a functional model whose definition includes several abilities. In England and Wales and in increasing number of countries, the law requires a person be able to understand, to retain, to use or weigh relevant information and to communicate one's decision. But interpreting and applying broad and vague criteria, such as the ability 'to use or weigh' to a diverse range of presentations is challenging. By examining actual court judgements of capacity, we previously developed a descriptive typology of justifications (rationales) used in the application of the Mental Capacity Act (MCA) criteria. We here critically optimise this typology by showing how clear definitions-and thus boundaries-between the criteria can be achieved if the 'understanding' criterion is used narrowly and the multiple rationales that fall under the ability to 'use or weigh' are specifically enumerated in practice. Such a typology-aided practice, in theory, could make functional capacity assessments more transparent, accountable, reliable and valid. It may also help to create targeted supports for decision making by the vulnerable. We also discuss how the typology could evolve legally and scientifically, and how it lays the groundwork for clinical research on the abilities enumerated by the MCA.