Exploring functional and interpersonal evidence in complex capacity assessments

Student thesis: Doctoral ThesisDoctor of Philosophy


The Mental Capacity Act (2005) has increasingly come to be relied upon as a framework to protect the autonomy of individuals in England and Wales. Capacity assessments involve the application of legal criteria to psychological and psychiatric phenomena, resulting in a binary judgment as to whether the person can make their own decisions. These judgments have come under considerable scrutiny in the most complex cases.

For this thesis, I highlight two of the most contentious issues, focusing on the Mental Capacity Act (2005) in England and Wales. First, the legal concept of 'use or weigh' within functional capacity tests is poorly understood from a cognitive perspective. As a result, it can be conceptually difficult to distinguish between a person making a capacitous but unwise decision and a person who lacks this capacity because of a mental impairment.

Second, mental capacity and undue influence are treated under separate legislation with substantially different provisions. In other words, the law distinguishes between a person who cannot make a decision because of a mental impairment and an impairment caused by interpersonal factors. Tensions arise when these distinctions are unclear, and few studies having addressed such distinctions.

This thesis aims to:
1. Synthesise the broad research literature concerning undue influence, introduce the ways in which such influence has been conceptualised, and outline how it relates to capacity and decision-making autonomy in vulnerable adults.
2. Explore how interpersonal factors have been thought to impact mental capacity in Court of Protection (CoP) and High Court (HC) cases concerning vulnerable adults.
3. Synthesise the empirical literature concerning how insight is considered across neurological and psychiatric populations, to analyse associations between insight and demographic characteristics that are protected under the Equality Act (2010).
4. Explore how different professionals across various settings apply the ‘use or weigh’ criterion and consider interpersonal influences during capacity assessments.

The database search returned 20 published cases (Chapter 3) in which interpersonal influence from a named person was mentioned as being relevant to the person in question’s capacity.
Contrary to several readings of the law, findings reveal that the courts extensively consider how interpersonal factors operate on decision-making, particularly in terms of ‘use or weigh’. After content analysis, the typology generated five broad ways in which interpersonal influence may affect capacity. Intellectual disability (10/20 cases) and dementia (3/20 cases) were the most common impairments reported.

Across the entirety of the 207 clinical studies in the meta-analysis, insight was not strongly associated with any sociodemographic variable. (Chapter 4). Higher rates of insight were weakly associated with white (vs. Black) ethnicity, younger ages, being employed, and having more years of education and was not associated with marital status or sex. Several associations of insight with age and education varied between diagnostic groups.

A range of practical and conceptual challenges were reported during their capacity assessments of 611 professionals (Chapter 5). For example, most professionals were at least moderately concerned about undue influence, particularly when assessing older adults or people with learning disabilities, dementia, or brain injuries. During follow-up interviews (Chapter 6), professionals elaborated on nuances regarding both ‘use or weigh’ (e.g., the frontal lobe paradox) and the boundaries between capacity and undue influence.

Altogether, these findings outline how functional evidence (particularly ‘using or weighing’ and insight) and interpersonal evidence can complicate capacity assessments. The findings also address how such assessments vary between different patient groups, professional disciplines, and types of decisions. Implications for mental health and legal guidance are also
Date of Award1 Dec 2022
Original languageEnglish
Awarding Institution
  • King's College London
SupervisorGareth Owen (Supervisor), Anthony David (Supervisor) & Steve Fleming (Supervisor)

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