Mental capacity: what it means and how it is assessed

Mental capacity determines whether a person can make a particular decision for themselves. It underlies wills, powers of attorney, advance directives, medical consent and a great deal else.

Frameworks differ by jurisdiction, and several principles are broadly common.

The core principles

Capacity is presumed. An adult is assumed to have capacity unless it is established otherwise. The burden is on whoever asserts its absence.

It is decision-specific. A person may lack capacity for a complex financial decision and have it for a decision about where to live, or what to eat, or whether to accept a particular treatment.

Blanket statements that someone lacks capacity are generally wrong.

It is time-specific. Capacity fluctuates — with illness, medication, delirium, time of day, and fatigue. Someone may lack capacity in the evening and have it in the morning.

Unwise decisions are not evidence of incapacity. A person is entitled to make choices others consider foolish. This is an important protection and it is frequently overlooked by families who conclude that a relative lacks capacity because they disagree with them.

Support must be provided first. Before concluding that someone cannot decide, reasonable steps should be taken to help them — explaining differently, using visual aids, choosing a better time, involving someone they trust.

The general test

Most frameworks ask whether the person can:

Understand the information relevant to the decision.

Retain it long enough to make the decision — briefly is sufficient.

Use or weigh it as part of deciding.

Communicate the decision, by any means.

Failure on any of these, caused by an impairment of the mind or brain, generally establishes lack of capacity for that decision.

The third element is frequently the one in question. Someone can repeat information back accurately and still be unable to apply it to themselves — a person with dementia who understands what a fall risk is and cannot connect it to their own situation, for instance.

Testamentary capacity

Capacity to make a will has its own established test in many common law jurisdictions, generally requiring that the person understands the nature of making a will and its effects, the extent of their property, and the claims of those they might be expected to provide for, and is not affected by a disorder of mind that distorts their judgement.

This is a different test from the general capacity test in some jurisdictions, and the relationship between them has been the subject of litigation.

Practically: where there is any doubt, a contemporaneous assessment by a suitably qualified professional, recorded and retained, is the best protection against later challenge.

Who assesses

Depends on the decision.

For routine matters, whoever needs the decision — a clinician for treatment, a solicitor for a legal document.

For significant decisions, a formal assessment by a doctor, psychiatrist or psychologist.

Where there is dispute, a court.

Assessment is a judgement rather than a test score. Cognitive screening tools contribute and do not determine the outcome — someone can score poorly on a screening test and retain capacity for a specific decision, and vice versa.

What happens when capacity is lost

If documents were made in advance — a power of attorney, an advance directive — they take effect.

If not, the alternatives are a court-appointed representative, or decisions made by others under a best interests framework.

The court route is slower, more expensive and more restrictive. This is the argument for making documents early, and it is the single most consequential practical point in this area.

Best interests

Where a decision must be made for someone lacking capacity, most frameworks require it to be made in their best interests, considering their past and present wishes, their beliefs and values, and the views of those close to them.

Best interests is not what the decision-maker would choose, and it is not purely medical. It is an attempt to reach the decision the person would have reached.

Which is why recorded wishes matter so much even where they are not legally binding — they are direct evidence of what the person valued.

Fluctuating capacity

Common and frequently mishandled.

Where capacity fluctuates, decisions should be made during periods of capacity where possible, and non-urgent decisions deferred to a better moment.

Delirium in particular — common in hospital, especially in older people with infection — causes temporary and reversible loss of capacity, and it is frequently mistaken for permanent decline.

A person judged to lack capacity during an acute admission should be reassessed once the acute problem resolves.

What families get wrong

Assuming a diagnosis equals incapacity. A dementia diagnosis does not mean a person cannot make decisions. Many people with early dementia have capacity for most decisions for a long period.

Assuming capacity is all or nothing.

Treating disagreement as evidence of incapacity. This is the most common error and the most damaging, because it removes autonomy from someone who retains it.

Waiting too long. Believing there is time to arrange documents, and finding there is not.

The practical advice

Make the documents while capacity is unquestionable, which for most people means considerably earlier than they think.

If a diagnosis has been made, act quickly — capacity for making a power of attorney is frequently still present in the early stages, and it disappears.

Where there is any doubt, obtain and retain a contemporaneous professional assessment.

And involve the person in decisions for as long as they can participate, in whatever way they can, which is both a legal requirement in many systems and the right thing to do.

This is general information, not legal or medical advice. Capacity frameworks and tests differ by jurisdiction. Seek professional assessment and legal advice where capacity is in question.