The report published by the Commission on Assisted Dying advocates a change in the law to allow assisted suicide and cites possession of the mental capacity to make a request as a ‘key element that should underpin a safeguarded framework for assisted dying’; but this assertion is based on evidence that presents unclear and inconsistent concepts of mental capacity and little discussion about the standards and frameworks that should be used to assess this capacity.
Within the submitted evidence there were two key areas of consistency among the experts. Firstly that mental capacity should be a central safeguard and secondly that advance decision making for those likely to lose capacity in the future is not appropriate for assisted suicide: capacity should be present at the time the decision is being made. But between and sometimes within expert submissions, there was a lack of consistency in the definition and boundaries of the concept of mental capacity, and the interface of capacity with other areas that might have a bearing upon its determination, particularly motivation, voluntariness, autonomy, rationality and the presence and severity of mental disorder, specifically depression.
The Commission on Assisted Dying has strongly recommended that any assisted suicide legislation be closely regulated and safeguarded. Mental capacity determination as set out in the Mental Capacity Act 2005 has surface validity for fulfilling this safeguarding role but deeper exploration of the evidence informing these recommendations shows that the ways in which mental capacity is conceptualised are diverse. Several of the experts expressed ideas that were not consistent with the principles of the Mental Capacity Act, particularly the presumption of capacity.
The Act makes it clear that if there is no demonstrable disorder of mind or brain then the patient is free to make whatever decision they choose regardless of whether this is wise, unwise or no decision is made at all. In order for the Commission to recommend assessment of capacity for all patients there may be an implicit normative judgment about the decision to request assisted suicide being strongly indicative of a disorder of mind or brain, but this potentially introduces a problem-that the decision itself implies that capacity may be impaired but in order to proceed with assisted suicide it must be demonstrated that it is not. The conceptual shift toward demonstrating presence rather than lack of capacity, reflected in the recommendation from the Commission that capacity be assessed formally in all cases is also potentially problematic because within the Mental Capacity Act there is no clear definition of mental capacity (only a lack of capacity) and no clarity or guidelines on what would constitute sufficient mental capacity to decide to undergo assisted suicide.
The Mental Capacity Act 2005 test of capacity only applies to England and Wales. Internationally, most jurisdictions base their capacity laws on a ‘functional’ approach which is decision and time specific rather than ‘outcome’ or ‘status’ based approach and capacity is presumed;  however, different jurisdictions use different components for capacity determination; for example in the US, the capacity test is based on national case law and evaluates dimensions of ‘understanding’ , ‘appreciation’ , ‘reasoning’ and ‘expressing’ a choice. Also, there are a number of instruments used to assess capacity  for example the Macarthur Competency Assessment Tool (MacCAT-T) developed in the US .
Of the jurisdictions internationally where assisted suicide is legal, all include mental capacity as part of their safeguards  but only the Oregon and Washington statutes give an explicit definition of mental capacity . Guidelines for mental health professionals accompanying the Oregon Death with Dignity Act (DWDA) outline the capacity evaluation process, but these acknowledge that this process is difficult, especially in determining the impact of mental disorders on decision making ability .
Challenges in mental capacity determination are not unique to the situation of assisted suicide. In healthcare, both refusals (e.g., refusal of further life sustaining treatment) and requests (e.g., requests for gender reassignment or living organ donation) require a determination of mental capacity which can often involve detailed and wide-ranging assessment in order to reach a satisfactory conclusion. Assessment of factors such as motivation and voluntariness will often form part of a comprehensive assessment of decision making in these circumstances.
In her paper examining mental capacity using an anthropological approach, Doorn  argues that the available literature focuses on criteria for the assessment of competence without elaborating on what it is to be competent or incompetent to make a decision. She describes ‘thin’ and ‘thick’ conceptualisations of capacity which correspond to a more cognitive conceptualisation based on ‘negative’ autonomy (self determination with freedom from the interference from others) and a richer conceptualisation which acknowledges values (both of the patient and clinician) and is based on ‘positive’ autonomy (the potential for self development and fulfilment). She argues that assessment tools used to measure capacity have their roots in a ‘thin’ conceptualisation which does not acknowledge the ‘value ladenness’ of capacity decisions but rely on narrower cognitive abilities. This view is not without criticism [33, 34] but is echoed by other authors who argue that a value neutral or value free conceptualisation of capacity is potentially problematic in practice  and that capacity assessment is inherently normative and irreducible to a set of objective criteria . The findings of this study showed that a cognitive conceptualisation was more frequently endorsed by those strongly in favour of assisted suicide which would appear to be consistent with the value of self-determination, but among the experts there were a number of normative judgements being made about reasons for requesting assisted suicide e.g. being a burden on others. Ideas about ‘reasonable’ and ‘unreasonable’ reasons for requesting assisted suicide further emphasise the subjectivity potentially inherent in the process.
The interface between mental state and mental capacity continues to present challenges and this issue is far from resolved. Even within an assessment framework emphasising cognitive elements of mental capacity, depression may have a significant bearing in terms of their ability to use and weigh the relevant information, but how far this might be tolerated and the patient still be found competent to make the decision to request assisted suicide remains unclear.
Depression is common in palliative care  and desire for hastened death is strongly associated with depression in palliative populations . In Oregon it has been shown that depression is not always appropriately identified in patients requesting assisted suicide . There is evidence to suggest that treatment of depression can reduce the wish for hastened death  and that antidepressants are effective in patients with life threatening illness .
Strengths and limitations
This study analysed data that were not originally gathered for the purpose of examining concepts of mental capacity. This could be seen as both a limitation and a strength. Because the study used secondary analysis of these data there was no opportunity to further examine concepts or directly compare similar data. Had they been interviewed with mental capacity as the main focus, the experts may have presented different ideas and perspectives and different conclusions may have been reached. However, the strength of these data is that they provided an opportunity to examine the experts’ ‘naturalistic’ ideas about capacity and to analyse the points of convergence with and divergence from current legal, clinical and philosophical constructs.
The experts presenting to the Commission were invited by the Commissioners because of particular interest or expertise in areas related to the subject being examined. Few were experts in mental health and even fewer experts in mental capacity determination. This sample can therefore not be considered to be representative of current thinking about mental capacity but the responses do show a range of ideas about mental capacity from several different backgrounds, disciplines and ethical standpoints.
The authors acknowledge that one’s ethical standpoint on the legalisation of assisted suicide can have a bearing on individual ideas about mental capacity, particularly the standard required for possession of capacity . Reflexivity is an important element of analytic rigour in qualitative methodology  which allows the research to be placed in appropriate context so that conclusions can be judged in light of this context. The researchers analysing and reporting this data have a particular interest in mental capacity assessment and three of the authors (AP, MH and RMC) are clinicians who frequently assess mental capacity as part of their roles and are familiar with the challenges of applying the legal framework of the Mental Capacity Act 2005 in complex clinical situations including end of life decision making. We take the position that legalisation of assisted suicide is a matter for society to decide through due parliamentary process but AP and MH have previously expressed concerns about mental capacity as a safeguard in assisted dying legislation in part due to a concern about the potential for subjectivity and normativity in the process and outcome of clinical assessment . One of the authors (MH) has undertaken a review of reliability in mental capacity assessment and found that this is good when rigorous assessment procedures are applied but less so for less structured clinical assessments . MH has also commented previously on the difficulties of clearly defining mental capacity due to its varying conceptualisation as a legal, clinical or social construct and differing definitions across jurisdictions .