Mental capacity

Mental capacity, what it means and what it doesn’t

Correct as of August 2026

Someone tells you your mother “hasn’t got capacity”. It is said quickly, often in a corridor, usually about a decision nobody has formally assessed.

Capacity is a legal test, not an opinion. This page sets out what the test is, what does not count as evidence and what to ask.

The law starts from the opposite end

The Mental Capacity Act 2005 begins with a presumption. Section 1(2) says: “A person must be assumed to have capacity unless it is established that he lacks capacity.”

Nobody has to prove your parent has capacity. Someone has to establish that she does not. And under section 2(4), that has to be decided “on the balance of probabilities”, not on an impression.

What cannot establish it

The Act rules some things out directly. Section 2(3) says a lack of capacity “cannot be established merely by reference to”:

“a person’s age or appearance”
“a condition of his, or an aspect of his behaviour, which might lead others to make unjustified assumptions about his capacity”

A diagnosis is a condition. Being frail, being old or being agitated on a ward are all covered by that sentence.

Six things that are not evidence

Each of these gets treated as proof in practice. None of them is.

Making a decision you disagree with. Section 1(4) is explicit: “A person is not to be treated as unable to make a decision merely because he makes an unwise decision.” Wanting to go home when you think she should not is a decision, not a symptom.
Having a diagnosis. Dementia, a stroke or an episode of delirium does not settle the question. See section 2(3)(b) above. Capacity is assessed for one decision at a time.
Being confused on one occasion. People are worse at 3am, worse with an infection, worse on a noisy ward. Section 2(1) asks about capacity “at the material time”, meaning the moment the decision has to be made.
Not explaining herself well. The test is whether she can use and weigh the information. It is not whether she can argue it as fluently as you would.
Forgetting afterwards. Section 3(3) says that being able to retain the information “for a short period only does not prevent him from being regarded as able to make the decision”.
Struggling to say it out loud. Section 3(1)(d) names talking, sign language “or any other means”. Speech is one route, not the only one.

What actually counts

Section 2(1) sets the threshold. A person lacks capacity “if at the material time he is unable to make a decision for himself in relation to the matter because of an impairment of, or a disturbance in the functioning of, the mind or brain”.

So two things have to be true at once. There must be that impairment or disturbance. And because of it, she must be unable to do at least one of four things, listed in section 3(1):

understand the information relevant to the decision
retain that information
use or weigh it as part of making the decision
communicate the decision, by any means

Relevant information includes the reasonably foreseeable consequences of deciding either way, or of not deciding at all. That is section 3(4).

An assessment that does not name the specific decision, or does not say which of these four she cannot do, has not applied the test.

One decision, at one moment

Capacity is decision-specific and time-specific. Someone can lack capacity to decide where to live and still have capacity to decide what to eat, who visits or whether to accept a particular treatment.

Section 2(2) adds that it “does not matter whether the impairment or disturbance is permanent or temporary”. That cuts both ways: a temporary state can remove capacity for a decision today, and it can pass by next week.

Before deciding she cannot

This is the step most often skipped. Section 1(3): “A person is not to be treated as unable to make a decision unless all practicable steps to help him to do so have been taken without success.”

Section 3(2) says she is not to be regarded as unable to understand something if she can understand an explanation given “in a way that is appropriate to his circumstances (using simple language, visual aids or any other means)”.

In practice that means a good time of day, a quiet room, hearing aids and glasses on, plain language and more than one attempt. A conclusion reached after one rushed bedside conversation has usually skipped this.

If a decision is made on her behalf

Where someone genuinely lacks capacity for a decision, it must be made in her best interests, not for anyone else’s convenience. Section 4(4) requires that she is permitted and encouraged to participate as fully as possible.

Section 4(6) requires regard to her past and present wishes and feelings, any relevant written statement made when she had capacity and the beliefs and values that would have influenced her decision.

Your views have to be taken into account

Section 4(7)(b) requires the decision-maker to take into account, so far as it is practicable and appropriate, the views of “anyone engaged in caring for the person or interested in his welfare”.

That is you. If a best interests decision is being made about your parent and nobody has asked you anything, say so, and ask for your views to be recorded.

Where there is no family member or friend appropriate to consult, an Independent Mental Capacity Advocate should be instructed for serious decisions about treatment or a change of accommodation.

Two questions to ask

Both are things the Act requires. Neither is usually asked.

“Which specific decision are we saying she cannot make?”

“What was done to help her make it before deciding that she could not?”

Ask for the answers, and for the assessment itself, to be recorded in her notes.

If you disagree with an assessment

Start with the person who carried it out, and ask what specifically led to the conclusion. An assessor should be able to give concrete examples.

If that does not resolve it, ask for a formal review or a second opinion, and put the request in writing. Beyond that, the routes are the organisation’s complaints process, advice from a local carers’ service or solicitor and, for serious disputes, the Court of Protection.

Sources

Mental Capacity Act 2005, section 1 The principles, including the presumption of capacity and the unwise decision provision.
Mental Capacity Act 2005, section 3 Inability to make decisions. The four-part functional test, short-term retention and communication by any means.
Mental Capacity Act 2005, section 2 People who lack capacity. The material time, the impairment or disturbance requirement, what cannot establish a lack of capacity and the balance of probabilities.
Mental Capacity Act 2005, section 4 Best interests. Participation, past and present wishes and the duty to consult anyone engaged in caring for the person.
NHS: Consent to treatment, assessing capacity Who assesses capacity in a healthcare setting, best interests decisions and when an IMCA is instructed.
Alzheimer’s Society Factsheet 460: Mental Capacity Act 2005 Charity guidance on the two-stage test, decision-specific assessment and challenging an assessment. Reviewed March 2024.

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