Mental capacity is a multidimensional construct that is a central determinant of an individual's ability to make autonomous decisions. Its assessment has become increasingly important with the move away from the paternalistic role of healthcare professionals towards a greater emphasis on an individual's own treatment decisions (Reference SchneiderSchneider, 1998). The American Psychiatric Association has developed a model statute which uses a mental capacity test (Reference Stromberg and StoneStromberg & Stone, 1983). In many other jurisdictions mental capacity and mental health legislation have developed along different lines to deal with the specific needs of different groups of patients. In both England and Scotland, mental capacity legislation has developed with the aim of providing a framework for people with either severe communication difficulties or cognitive problems (intellectual disability, dementia and other organic brain syndromes). In contrast, mental health legislation has developed with the needs of patients with psychiatric disorders – in particular, although not exclusively, psychotic disorders – in mind. Mental health legislation that does not use capacity tests generally applies a ‘status’ approach, whereby a wide range of treatments can be given to the patient on the basis of certain general conditions being met (e.g. the presence of a mental disorder, or the presence of perceived risk to the patient or others).

The use of status approaches has numerous implications. Under mental capacity legislation treatments are only provided in the patient's best interests (with particular attention paid to previously expressed wishes, including advance directives, which have legal weight), whereas under mental health legislation best interests do not have to be considered, although in practice many psychiatrists effectively apply a best interests test (Reference PeayPeay, 2003). Further, the use of a ‘status’ approach means that the patient can be given a range of treatments, even if he or she might have capacity to refuse one or more of these. This has led some to suggest that current status-based approaches are anachronistic and unethical (Reference Szmukler and HollowaySzmukler & Holloway, 1998) and that mental capacity and mental health legislation could be fused (Reference Dawson and SzmuklerDawson & Szmukler, 2006).

A review of emergent case law literature in the USA (Reference Grisso, Applebaum and Hill-FotouhiGrisso et al, 1997) has resulted in a ‘four abilities’ model, namely the ability to express a choice about treatment; the ability to understand information relevant to the treatment decision; the ability to appreciate the significance of that treatment information for one's own situation; and the ability to reason with relevant information so as to engage in a logical process of weighing treatment options. Despite the influential work of the MacArthur Foundation (Grisso & Appelbaum, Reference Grisso and Appelbaum1995a ,Reference Grisso and Appelbaum b ; Reference Grisso, Appelbaum and MulveyGrisso et al, 1995), concern exists regarding the reliability of capacity assessments in individuals with a mental disorder, and the extent to which legislation that uses a capacity test covers the same or different groups of patients as mental health legislation which uses a status approach. Some have pointed to particular areas of perceived difficulty such as the area to appreciation, which may be difficult to operationalise (Reference Saks, Dunn and MarshallSaks et al, 2002; Reference Breden and VollmannBreden & Vollmann, 2004).

Our aim was to make a systematic review of empirical, quantitative studies of mental capacity in order to answer the following three questions:

1. (a) Can the mental capacity of a patient be reliably assessed by two or more raters?

2. (b) What is the proportion of patients with psychiatric disorders in in-patient settings who are judged to lack capacity?

3. (c) What factors are associated with lack of capacity in individuals with psychiatric disorders?

METHOD

We aimed to identify all studies relevant to the aims of this review. Inclusion criteria were that the papers should be in the English language, describe defined populations of patients with psychiatric disorders; report quantitative research (i.e. research that produces numerical summaries of results, as opposed to qualitative research), and describe how the assessment of mental capacity was performed; capacity had to be assessed in relation to a current treatment decision, as opposed to capacity to make advance directives, capacity to participate in research, testamentary capacity or capacity to stand trial. Studies were excluded if they were conducted on children or young people less than 18 years old; exclusively concerned organic psychiatric disorders (dementia or delirium) or intellectual disability; were case reports, commentaries or review articles; or were retrospective case-note reviews.

RESULTS

The searches identified 15 490 references, which were scanned by abstract and title. On the basis of the abstract or title, 367 papers were retrieved; 316 did not meet the inclusion criteria, leaving 51 identified from the electronic search, many of which were known to us already. The original review was broader than the aims of the present paper (including mental capacity in individuals with medical illness or dementia) and we finally identified 37 papers relevant to this review (see data supplement to the online version of this paper).

Reliability of capacity assessments

Seventeen studies reported interrater reliability of competency assessments. These had a median sample size of 56 participants (interquartile range 14–62). These studies could be categorised under three broad themes:

1. (a) binary decisions (capacity present or absent) using the same assessment tool and two or more raters;

2. (b) binary decisions comparing a clinician's assessment with an assessment made by a clinical researcher using a mental capacity tool;

3. (c) score on an individual dimension of capacity measured on an assessment tool.

Where available, we report agreement using Cohen's kappa, which is used as a measure of reliability taking into consideration the level of agreement expected by chance. Kappa takes a value between –1 and 1, and we define kappa scores as follows (Reference Landis and KochLandis & Koch, 1977): <0, poor; 0–0.2, slight; 0.2–0.4, fair; 0.4–0.6, moderate; 0.6–0.8, substantial; 0.8–1, almost perfect.

Reliability of binary assessment of mental capacity using interviews

Five studies (Table 1) assessed mental capacity using two or more raters administering the same structured or semi-structured interview (Reference Roth, Lidz and MeiselRoth et al, 1982; Reference Wong, Clare and HollandWong et al, 2000; Bellhouse et al, Reference Bellhouse, Holland and Clare2003a ,Reference Beckett and Chaplin b ; Reference Cairns, Maddock and BuchananCairns et al, 2005b ). Methods mainly involved raters assessing the same videotaped or transcribed interview performed by a single interviewer, although one paper described the results of two interviews performed by separate interviewers (Reference Cairns, Maddock and BuchananCairns et al, 2005b ). Assessments used a variety of methods: one (Reference Roth, Lidz and MeiselRoth et al, 1982) used a derivative of a 15-item questionnaire (Reference Roth, Meisel and LidzRoth et al, 1977); one used the MacArthur Competence Assessment Tool for Treatment (MacCAT–T) (Reference Cairns, Maddock and BuchananCairns et al, 2005b ) and one used a semi-structured interview adapted from the MacCAT–T (Reference Wong, Clare and HollandWong et al, 2000). Two papers described interrater reliability on two different decisions (admission and treatment) in similar samples (Bellhouse et al, Reference Bellhouse, Holland and Clare2003a ,Reference Bellhouse, Holland and Clare b ), using a checklist derived from English legal definitions. Kappa values ranged from ‘moderate’ to ‘almost perfect’ (median κ=0.81 IQR 0.75–0.82). These results suggest that when a consistent approach is taken to the assessment of mental capacity, two or more raters can make a binary assessment with a high level of agreement.

Table 1

Interrater reliability of mental capacity assessments using same assessment

Study	κ
Bellhouse et al (Reference Bellhouse, Holland and Clare2003a )	0.74
Bellhouse et al (Reference Bellhouse, Holland and Clare2003b )	0.75
Roth et al (Reference Roth, Lidz and Meisel1982)	0.81
Cairns et al (Reference Cairns, Maddock and Buchanan2005a )	0.82
Wong et al (Reference Wong, Clare and Holland2000)	0.87

Binary decisions comparing a clinician's assessment and that made by a clinical researcher using a mental capacity tool

Six studies (Reference Bean, Nishisato and RectorBean et al, 1996; Reference Tomoda, Yasumiya and SumiyamaTomoda et al, 1997; Reference Vollmann, Bauer and Danker-HopfeVollmann et al, 2003; Reference Vellinga, Smit and van LeeuwenVellinga et al, 2004; Reference Cairns, Maddock and BuchananCairns et al, 2005b ; Reference Beckett and ChaplinBeckett & Chaplin, 2006) assessed agreement between an interviewer performing a structured or semi-structured mental capacity assessment and a clinician's view of the patient's mental capacity. The kappa values ranged from ‘slight’ to ‘substantial’, (median κ=0.45, IQR 0.39–0.66). This suggests that when formal assessments are compared with clinical impressions, agreement is well above chance, but not as high as when two raters are using the same assessment tool. Clinicians universally reported fewer patients lacking mental capacity than did researchers.

Other studies comparing agreement using dimensional scales

Eleven studies addressed interrater agreement on dimensional scales (Reference Norko, Billick and McCarrickNorko et al, 1990; Reference Grisso and AppelbaumGrisso & Appelbaum, 1991; Reference Janofsky, McCarthy and FolsteinJanofsky et al, 1992; Reference Bean, Nishisato and RectorBean et al, 1994; Grisso et al, Reference Grisso, Appelbaum and Mulvey1995, Reference Grisso, Applebaum and Hill-Fotouhi1997; Reference Palmer, Nayak and DunnPalmer et al, 2002; Reference Vollmann, Bauer and Danker-HopfeVollmann et al, 2003; Reference Cairns, Maddock and BuchananCairns et al, 2005b ; Reference Wong, Cheung and ChenWong et al, 2005; Reference Appelbaum and RedlichAppelbaum & Redlich, 2006). These studies are difficult to summarise, since they tend to present correlation coefficients between raters on dimensional scales, or give kappa values for sub-scales of multidimensional scales. Deserving particular mention are the studies of Grisso and Appelbaum on the development of the MacCAT–T and related measures (Grisso et al, Reference Grisso, Appelbaum and Mulvey1995, Reference Grisso, Applebaum and Hill-Fotouhi1997), which present detailed analyses of interrater agreement for each of the dimensions of the MacCAT–T and show that high interrater correlations are the rule. Reliability indices were generally similar for each sub-scale of the MacCAT–T, suggesting that there is no single particularly hard-to-measure dimension (Reference Grisso, Applebaum and Hill-FotouhiGrisso et al, 1997; Reference Palmer, Nayak and DunnPalmer et al, 2002; Reference Vollmann, Bauer and Danker-HopfeVollmann et al, 2003; Reference Cairns, Maddock and BuchananCairns et al, 2005b ).

Frequency of mental incapacity in psychiatric patients

Admission to psychiatric units

We identified five studies that assessed mental capacity in relation to admission to a psychiatric unit (Appelbaum et al, Reference Appelbaum, Mirkin and Bateman1981, Reference Appelbaum, Appelbaum and Grisso1998; Reference Norko, Billick and McCarrickNorko et al, 1990; Reference Poythress, Cascardi and RitterbandPoythress et al, 1996; Reference Bellhouse, Holland and ClareBellhouse et al, 2003a ). One British study (Reference Bellhouse, Holland and ClareBellhouse et al, 2003a ), described a mixed clinical population of patients and found that 67% had mental capacity to make the decision. Three studies (Appelbaum et al, Reference Appelbaum, Mirkin and Bateman1981, Reference Appelbaum, Appelbaum and Grisso1998; Reference Norko, Billick and McCarrickNorko et al, 1990) described capacity to make this decision among voluntary patients admitted to psychiatric hospital. It is difficult to summarise the results of these studies since each presents more than one measure of incapacity; however, approximately 30–50% of participants scored in a range that suggests they were competent to make decisions, a sizeable minority scored in an intermediate range, and as many as 50% (Reference Norko, Billick and McCarrickNorko et al, 1990) had significant impairments of mental capacity despite accepting voluntary admission. One study (Reference Poythress, Cascardi and RitterbandPoythress et al, 1996) described patients who were brought to hospital on a court order (of whom half subsequently accepted informal admission), and found that 55% had an impairment of capacity on a stringent definition and 35% had impairment on a less stringent definition.

Psychiatric in-patients: other treatments

Of the remaining studies of psychiatric patients, most described treatment for diverse interventions (Reference Grisso and AppelbaumGrisso & Appelbaum, 1991; Reference Hoffman and SrinivasanHoffman & Srinivasan, 1992; Reference Janofsky, McCarthy and FolsteinJanofsky et al, 1992; Grisso et al, Reference Grisso, Appelbaum and Mulvey1995, Reference Grisso, Applebaum and Hill-Fotouhi1997; Reference Billick, Naylor and MajeskeBillick et al, 1996; Reference Melamed, Kimchi and ShnitMelamed et al, 1997; Reference Tomoda, Yasumiya and SumiyamaTomoda et al, 1997; Reference Kitamura, Tomoda and TsukadaKitamura et al, 1998; Reference Melamed, Kimchi and ShnitMelamed et al, 1999; Reference Bellhouse, Holland and ClareBellhouse et al, 2003b ; Reference Vollmann, Bauer and Danker-HopfeVollmann et al, 2003; Reference Vellinga, Smit and van LeeuwenVellinga et al, 2004; Reference Cairns, Maddock and BuchananCairns et al, 2005b ; Reference Jacob, Clare and HollandJacob et al, 2005; Reference Beckett and ChaplinBeckett & Chaplin, 2006), whereas a few focused on either antipsychotic medication (Reference Paul and OyebodePaul & Oyebode, 1999; Reference Wong, Cheung and ChenWong et al, 2005) or electroconvulsive therapy (Reference Roth, Lidz and MeiselRoth et al, 1982; Reference Bean, Nishisato and RectorBean et al, 1996). In some studies the population was well defined, and a true cross-sectional study of consecutive patients had been performed. In others the population under study was much less well characterised, and convenience samples were used. For those 12 studies that provided a binary (present/absent) rating of mental capacity in the various psychiatric in-patient groups, estimates ranged from 10% to 95% of the participants lacking capacity (Table 2). However, all but two studies estimated that less than half of psychiatric in-patients lacked capacity, and the median value was 29% (IQR 22–44).

Table 2

Frequency of mental capacity among psychiatric in-patients

Study	Participants (n)	Participants rated as having mental capacity % (95% CI)	Notes
Bean et al (Reference Bean, Nishisato and Rector1994, Reference Bean, Nishisato and Rector1996)	96	78 (70-86)	In-patients referred for ECT (13% detained)
Beckett & Chaplin (Reference Beckett and Chaplin2006)	50	38 (25-51)	In-patients with acute mania only; clinical assessment
Bellhouse et al (Reference Bellhouse, Holland and Clare2003b )	43	80 (73-87)	20% detained
Billick et al (Reference Billick, Naylor and Majeske1996)	20	75 (56-94)
Cairns et al (Reference Cairns, Maddock and Buchanan2005b )	112	56 (47-65)	Consecutive sample (32% detained)
Hoffman & Srinivasan (Reference Hoffman and Srinivasan1992)	60	35 (23-47)	47% of patients detained
Janofsky et al (Reference Janofsky, McCarthy and Folstein1992)	41	66 (51-81)	Included patients (n=16) admitted to general medical ward
Kitamura et al (Reference Kitamura, Tomoda and Tsukada1998)	48	76 (64-88)	Included 48% medical patients
Melamed et al (Reference Melamed, Kimchi and Shnit1997, Reference Melamed, Kimchi and Shnit1999)	113	66 (57-75)	All voluntary psychiatric patients
Paul & Oyebode (Reference Paul and Oyebode1999)	40	5 (0-13)	All voluntary patients; high standard of capacity based on presence of insight into disorder
Roth et al (Reference Roth, Lidz and Meisel1982)	57	71 (60-82)	In-patients referred for ECT; 7% detained
Vollman et al (2003)	109	78 (70-86)	Only voluntary patients
Wong et al (Reference Wong, Clare and Holland2000)	62	90 (82-98)	11% detained

Specific psychiatric diagnoses

Four studies (Reference Grisso and AppelbaumGrisso & Appelbaum, 1995c ; Reference Grisso, Applebaum and Hill-FotouhiGrisso et al, 1997; Reference Vollmann, Bauer and Danker-HopfeVollmann et al, 2003; Reference Appelbaum and RedlichAppelbaum & Redlich, 2006) presented the results of capacity assessments for patients with psychiatric diagnoses separately. Three used the MacCAT–T, and compared participants with schizophrenia or depression, all finding that impairments in mental capacity were much more common in the schizophrenia group. The MacArthur study (Reference Grisso, Applebaum and Hill-FotouhiGrisso et al, 1997) found that 52% of patients with schizophrenia had impaired capacity, as opposed to 24% of those with depression. This study gave a further detailed breakdown of areas of difficulty, indicating that when individuals with schizophrenia had difficulties in decision-making, their appreciation, understanding and reasoning could all be affected. In contrast, decision-making difficulties in depression were mainly related to difficulties in appreciation. The third study (Reference Vollmann, Bauer and Danker-HopfeVollmann et al, 2003) reported a remarkably consistent result: 53% of in-patients with schizophrenia were judged to lack capacity, as opposed to 20% of those with depression.

DISCUSSION

We identified a number of studies that have used a systematic approach to measure mental capacity in individuals with psychiatric disorders. Although the methods used to measure capacity varied considerably, we have been able to address the three aims of this review. Our first aim was to determine whether mental capacity could be assessed in a reliable manner. The answer to this question depends upon the study design. Studies that used a standardised assessment reported very high interrater reliabilities, with a median kappa of 0.81. Despite capacity being a complex, value-laden, multidimensional construct, this finding suggests that it can be assessed with greater reliability than cardiologists interpreting exercise electrocardiograms, radiologists interpreting mammograms or haematologists reading peripheral blood films (Reference Sackett, Haynes and GuyattSackett et al, 1991). When interrater reliability of single dimensions of capacity such as understanding is measured, results again suggest that these measures are highly reproducible. It is noteworthy that even dimensions such as reasoning and appreciation, which are hard to operationalise, are assessed with good interrater reliability. However, interviewers using standardised assessments agreed much less frequently with the clinicians who had been treating the study participants, and although this difference may be artefactual (under certain circumstances kappa values may be low despite good agreement) we think that it is probable that reliability is generally lower when a researcher's assessment is pitted against that of a clinician. In general, clinicians were much less likely to judge a patient to lack capacity, and it may be that if a patient is prepared to accept the treatment proposed the issue of incapacity does not arise – the clinician presumes it is present. Clinicians might have a tendency to equate treatment refusal with incapacity and treatment acceptance with capacity. Alternatively, it might be that clinicians lack training or the time in which to perform careful assessments. Finally, although there is no gold standard criterion of capacity, it may be that formal assessments, although reliable, lack specificity and tend to ‘overdiagnose’ incapacity compared with the clinicians’ assessments.

The second question related to the frequency of incapacity to make key treatment decisions among patients with psychiatric disorders. Taking the median values as an approximate estimate, the results of the reviewed studies indicate that of in-patients with psychiatric disorders, a sizeable proportion – usually the majority – are capable of making treatment decisions. Indeed, the frequency of incapacity in psychiatric in-patients found in the reviewed studies did not differ greatly from that in general hospital in-patients (Reference Raymont, Bingley and BuchananRaymont et al, 2004). The consistency of estimates of incapacity in psychiatric in-patients is striking, given the diverse nature of the populations studied. Half the studies estimated the frequency of participants’ lack of capacity to be within the range 22–44%. Similarly, the two studies to report on rates of incapacity in schizophrenia and depression found almost exactly the same rates, despite one being conducted in the USA and the other in Germany, where differences in healthcare systems might have led to differences in patient characteristics. This suggests that although diverse measures of mental capacity have been used, they are capable of making fairly consistent estimates.

The frequency of incapacity in voluntary patients when consenting to admission was remarkably high. This leads to a potential dilemma, as individuals lacking capacity may acquiesce to admission, but may lack protections that an admission under a legal framework would afford. Such patients may, to some extent, feel coerced into accepting admission, presumably since they felt that if they did not agree to an admission they would be detained anyway. Finally, the British studies of mental capacity (Reference Bellhouse, Holland and ClareBellhouse et al, 2003a ; Reference Cairns, Maddock and BuchananCairns et al, 2005a ) in those detained under the Mental Health Act 1983 indicated that a sizeable proportion have capacity to accept or refuse admission to hospital. Further work needs to be done to understand the implications of capacity-based mental health legislation for these individuals.

Mental capacity is not associated with any individual socio-demographic variable apart from advancing age. It is unclear why this should be, but it may be driven by cognitive decline or increased negative symptoms in older patients with psychotic illness. Given that mental capacity assessments are value-laden, it is reassuring that neither gender nor ethnic group has an effect; associations with educational level and social class are inconsistent. The clinical and legal variables associated with mental incapacity in the psychiatric groups should come as no surprise – psychosis, illness severity, involuntary admission and treatment refusal are all consistently reported as risk factors.