Capacity is measured decision by decision, not person by person

The wrong question produces the wrong answer

Families discussing an elderly or unwell relative often ask whether the person still has capacity, as though it were a single attribute that a person either possesses or has lost. Legal systems have largely moved away from that framing, and for good reason: it is both inaccurate and, in practice, an efficient way of stripping people of decisions they were perfectly able to make.

The question most systems now ask is narrower. Can this person, at this moment, make this particular decision? Someone may be unable to manage a complex property transaction while remaining entirely able to decide what to eat, where to live, or who should visit. Treating those as one question flattens a person into a status.

What a functional test looks for

Where a functional test is used, it tends to examine a sequence of abilities: whether the person can understand the information relevant to the decision, retain it long enough to use it, weigh it in reaching a choice, and communicate the result by any means available to them. Failure at any stage, caused by an impairment of the mind or brain, may mean the decision cannot be made.

Two features of that test are easy to miss and both matter. The relevant information is information about this decision, not general knowledge, so a person need not understand everything to decide something. And the requirement to retain is only for long enough to make the choice — an inability to remember it afterwards does not by itself mean it could not be made.

The presumption runs in favour of the person

Most systems begin from a presumption that an adult can decide for themselves, placing the burden on whoever asserts otherwise. That presumption is not a formality; it determines who has to do the work and what happens when the evidence is genuinely unclear.

A further principle commonly sits alongside it: that a person is not to be treated as unable to decide merely because the decision looks unwise. People are entitled to make choices that their families consider foolish, and the discomfort that causes is the price of the principle. It also means an eccentric or unpopular will is not automatically evidence of anything, though it may sharpen the scrutiny applied to how it came about.

Capacity fluctuates, so timing is part of the assessment

Many conditions produce ability that varies through the day, or between weeks, or with medication, infection, pain and fatigue. A functional test that fixes on a particular decision at a particular time handles this better than a status approach, and it carries a practical implication: where a decision is not urgent, it may be right to wait for a better moment rather than to conclude that it cannot be made at all.

It also means assessments have limited shelf life. An opinion recorded some time ago says something about that moment and less about today. Where a significant document is being signed, contemporaneous evidence of the person’s understanding is worth far more than a historical report, which is why practitioners in many jurisdictions arrange an assessment on the day.

When someone genuinely cannot decide

The modern approach in many systems is to support the decision rather than replace the decision-maker: explaining in simpler terms, using different formats, involving someone the person trusts, choosing a better time. Only where support has failed does the question of deciding for them arise.

At that point the standards diverge sharply between jurisdictions. Some require the decision to be made in the person’s best interests, taking their past and present wishes into account. Others give greater weight to what the person would themselves have chosen, and several have moved towards supported decision-making models that resist substitution altogether. This is a genuinely contested area of law and policy, and the direction of travel is not the same everywhere.

Why this needs a professional, not an article

Capacity is where medicine, law and family feeling meet, and disagreement is common in all three registers at once. Whether a formal assessment is required, who may carry it out, what test applies, what happens when it is failed, and how a decision may be challenged afterwards all differ substantially by jurisdiction.

Nothing here is advice about a particular person, and the answer would depend on clinical and factual detail an article cannot have. Where a significant document is to be signed, or where a family disagrees about someone’s ability to decide, involve a qualified lawyer in the relevant jurisdiction and an appropriate clinician early. Delay is costly in both directions: capacity can be lost while a decision is postponed, and a challenge to a decision already made will usually carry a time limit.