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Author and Publication
Author: Nellie Supports Ltd
Publication Date: 15/05/2026
Citation
Aintree University Hospitals NHS Foundation Trust v James [2013] UKSC 67.
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Copyright © 2026 Nellie Supports Ltd. All rights reserved.
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Mental Capacity Act 2005: Key Principles Explained
This guide explains the five statutory principles of the Mental Capacity Act 2005 and how they shape every capacity decision in England and Wales. It is written for families, carers, professionals and anyone supporting someone who may lack capacity. It sets out each principle, what it means in practice, and how the principles work together. It does not replace legal advice on a particular case.
The five principles at the heart of the Act
Section 1 of the Mental Capacity Act 2005 sets out five principles that govern all decisions made under it. They are the presumption of capacity, the duty to support decision-making, the right to make unwise decisions, the best interests duty, and the least restrictive principle. Every assessment and decision must be read through these principles.
The presumption of capacity
A person must be assumed to have capacity unless it is established that they lack it. The burden is on the person asserting incapacity, and the standard is the balance of probabilities. This means capacity is the starting point, and an assessment exists to test that presumption, not to confirm an assumption that someone cannot decide.
Supporting the person to decide
A person is not to be treated as unable to make a decision unless all practicable steps to help them have been taken without success. This includes giving information in an accessible way, choosing the right time and place, using communication aids, and involving people who can help. Support must come before any conclusion about capacity.
The right to make unwise decisions
A person is not to be treated as lacking capacity merely because they make a decision others consider unwise. Capacitous adults are entitled to make choices that seem eccentric, risky or wrong to others. The assessment must distinguish a genuinely unwise but capacitous decision from an inability to use or weigh the relevant information.
Acting in a person's best interests
Any act or decision taken for someone who lacks capacity must be in their best interests. This is a structured process under section 4, taking account of the person's past and present wishes, feelings, beliefs and values, and the views of those close to them. Best interests is not simply what others think is best for the person.
The least restrictive option
Before any act is done for a person who lacks capacity, regard must be had to whether the same outcome can be achieved in a way that is less restrictive of their rights and freedom of action. Where two options would meet the need, the one that interferes least with the person's liberty should be preferred.
How the principles work together
The principles are not a menu to choose from. A decision must presume capacity, offer support, respect unwise choices, and, only where capacity is genuinely lacking, move to a best interests decision taken in the least restrictive way. Skipping any step, particularly the duty to support, undermines the lawfulness of what follows.
Why the principles matter in assessments
A capacity assessment that ignores the principles is open to challenge. Failing to provide support, treating an unwise decision as proof of incapacity, or moving to best interests without establishing a lack of capacity are all errors that the principles are designed to prevent. A sound assessment shows the principles being applied.
Key takeaway
The five principles protect autonomy while providing a lawful route to act for those who genuinely cannot decide. They require capacity to be presumed, decision-making to be supported, unwise choices to be respected, and any intervention to be in the person's best interests and as unrestrictive as possible.
Frequently asked questions
What are the five principles in short?
Presume capacity, support the person to decide, allow unwise decisions, act in the person's best interests if they lack capacity, and choose the least restrictive option. They are set out in section 1 of the Mental Capacity Act 2005 and apply to every decision made under the Act.
Does the Act apply to everyone over sixteen?
The Mental Capacity Act generally applies to people aged sixteen and over in England and Wales, with some provisions differing for younger people and for specific decisions such as making a will. Decisions for children are usually governed by other frameworks, and specialist advice should be taken where age is relevant.
What is the difference between capacity and best interests?
Capacity is whether a person can make a particular decision themselves. Best interests only arises once it is established that they cannot. The two are separate stages: you do not move to a best interests decision until a lack of capacity for that decision has been properly established.
Related mental capacity assessment pages
These internal links help readers move from this guide to the most relevant Nellie Supports service page, assessment option or legal framework page.
Need help applying the Act to a real decision?
Nellie Supports provides independent capacity assessments and social work support across England and Wales, grounded in the principles of the Mental Capacity Act 2005. For a decision involving capacity, care, funding or the Court of Protection, see the relevant service page or the Mental Capacity Assessments hub.