What is mental capacity, and who decides whether someone has it?
Capacity is the ability to make a particular decision at a particular time — and the most misunderstood point is that it is decision-specific and time-specific, not a status a person holds or loses in general.
The principles that govern it, set out in capacity legislation in many jurisdictions:
Assume capacity unless it is established otherwise. The burden is on whoever says a person lacks it.
Support the decision first. A person is not to be treated as lacking capacity until all practicable steps to help them decide have been taken — different language, a better time of day, a familiar setting, visual aids, an interpreter, or involving someone they trust.
An unwise decision is not incapacity. People are entitled to make choices others consider foolish, and this is the principle most frequently breached in practice — capacity is questioned precisely when someone disagrees with the decision.
Anything done must be in the person's best interests, considering their past and present wishes, beliefs and values, and consulting those close to them.
The least restrictive option that achieves the purpose.
The two-stage test: is there an impairment or disturbance of the functioning of the mind or brain; and if so, does it mean the person cannot understand the relevant information, retain it long enough to decide, use or weigh it, or communicate the decision. All four abilities are required, and failing any one means lacking capacity for that decision.
Who assesses it. Whoever needs the decision made — a clinician for treatment, a solicitor for a will or power of attorney, a social worker for care arrangements, a bank for a transaction. There is no single official assessor, and complex or disputed cases go to a specialist court.
Why it is decision-specific. Someone may have capacity to decide what to eat and not to manage complex finances, or capacity in the morning and not in the evening. Capacity must be assessed for each decision as it arises.
Planning ahead — lasting powers of attorney and advance decisions — must be done while capacity exists, which is why leaving it is so costly.
General information, not legal advice.