Dr Ryan's comparison of UK and Commonwealth jurisdictions versus American jurisdictions highlights important differences in the letter of the law. He rightly states that the ‘four abilities model’ arises from American case law.Reference Berg, Appelbaum and Grisso¹ The UK and Commonwealth nations have independently developed tests of capacity. Fortunately for medical practitioners who must navigate the difficult waters of comparative jurisprudence, the underlying concepts remain essentially the same.

In the ‘four abilities model,’ understanding is the ability to ‘grasp the fundamental meaning of information communicated by [the] physician’.Reference Appelbaum² It is analogous to the Mental Capacity Act's test of ‘comprehend[ing] and retain[ing] the information’ that is material to the decision.³ Indeed, the Oxford English Dictionary gives ‘to comprehend’ as one definition of grasp.⁴

Appreciation is the ability to ‘acknowledge [the] medical condition and likely consequences of treatment options’.Reference Appelbaum² Another common description of appreciation is that the person must be able to apply information meaningfully to his or her own situation.Reference Appelbaum and Grisso⁵ Although it is true that the UK Law Reform Commission specifically rejected the word ‘appreciation’, the Commission went on to say that a person lacks capacity if ‘he or she is unable to make a decision based on the information relevant to the decision, including information about the reasonably foreseeable consequences of deciding one way or another’.³ Making decisions based on relevant information and foreseeable consequences is equivalent to acknowledging a condition and the consequences of treatment and applying that information to oneself when making a choice. Whether this ability is termed ‘appreciation’ or ‘knowing’ or some other synonym is less significant.

Reasoning or rational manipulation refers not to the idea that decisions must appear rational to outside observers but that the patient has the ability ‘to compare treatment options and consequences and to offer reasons for selection of [an] option’.Reference Appelbaum² Furthermore, ‘this criterion focuses on the process by which a decision is reached, not the outcome of the patient's choice, since patients have the right to make ‘unreasonable’ choices’.Reference Appelbaum² In short, individuals who exhibit the ability to reason in this way are using and weighing information as part of the process of making the decision.

We agree with Dr Ryan, who, together with colleagues, has rightly argued that ‘Decision-making capacity is decision- and time-specific’.Reference Ryan, Callaghan and Peisah⁶ Their example was a person with mania who simultaneously has capacity to choose between different mood stabilisers but lacks the capacity to decline mood stabilisers altogether.Reference Ryan, Callaghan and Peisah⁶ The decision-specific nature of capacity gives rise to a sliding scale because without a variable threshold, the standard for capacity would be the same for all decisions (and not decision-specific).

Finally, although the US government has not ratified the United Nations Convention on the Rights of Persons with Disabilities, American physicians certainly agree that their ethical duty when assessing capacity is to assess the patient's abilities and, where possible, assist incapacitated patients in regaining capacity. The American psychiatric literature is replete with exhortations to restore capacity or enhance decision-making abilities following a finding of incapacity.Reference Kim⁷ We hope that our editorial provides guidance on one aspect of that process of assessment and assistance.