The Mental Capacity Act 2005

The Statutory Framework Governing All Lawful Capacity Assessments in England and Wales

When concerns arise about whether a person can make a specific decision, one statute governs the assessment:

The Mental Capacity Act 2005 (MCA 2005).

The Act applies across England and Wales and provides the legal foundation for:

Decision-specific mental capacity assessments
Best interests decision-making
Deputyship applications (COP3)
Lasting Powers of Attorney (LPA)
Litigation capacity under CPR Part 21 & 35
Retrospective capacity disputes
Protection of vulnerable adults

Every lawful capacity assessment must follow the statutory structure set out in the Act.

Failure to apply the framework correctly can:

Invalidate decisions
Undermine court proceedings
Expose professionals to challenge
Result in expert reports being rejected

This page explains how the Act operates in practice, how the legal test is applied, and where capacity assessments commonly fail.

If you require an independent mental capacity assessment compliant with the Mental Capacity Act 2005, explore our specialist services below.

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The Five Statutory Principles

At the heart of the MCA are five mandatory principles.
They are not guidance. They are law.

Every assessment and decision must demonstrate compliance.

5️⃣ Least Restrictive Option

Any intervention must restrict the person’s rights and freedom as little as possible.

4️⃣ Best Interests

If a person lacks capacity for a specific decision, any act or decision must be made in their best interests, following a structured statutory process.

3️⃣ Right to Make Unwise Decisions

A decision others consider imprudent does not equal incapacity.

The Act protects autonomy - including risky or unconventional choices.

2️⃣ Support to Make a Decision

A person must be given all practicable help before being treated as unable to decide.

This may include:

Simplified explanations
Visual aids
Adjusted timing
Communication support
Use of interpreters or advocates

Capacity cannot lawfully be concluded without demonstrating that support has been attempted.

1️⃣ Presumption of Capacity

Every adult is presumed to have capacity unless proven otherwise.

Capacity is not removed simply because a person:

Has a diagnosis
Appears vulnerable
Makes an unwise decision
Disagrees with professionals

The Legal Test for Capacity

A Functional-First, Decision-Specific Approach

Capacity is always:

Decision-specific
Time-specific

A person may have capacity for one decision but not another.

Capacity may also fluctuate.

The legal test is structured and two-stage.

Stage One: The Functional Test

The question is:

Can the person make this specific decision?

A person lacks capacity if they are unable to:

Understand the relevant information
Retain that information long enough to decide
Use or weigh that information as part of the decision-making process
Communicate their decision (by any means)

Each element must be considered separately.

Stage Two: The Causative Test

Is that difficulty because of an impairment of, or disturbance in, the functioning of the mind or brain?

Examples may include:

Dementia
Delirium
Brain injury
Learning disability
Severe mental illness
Intoxication

Both stages must be satisfied.

It is not enough to identify a diagnosis.

It is not enough to show risk.

The issue is whether the impairment causes the inability.

The Causative Nexus

The Required Link Between Impairment and Inability

This is one of the most misunderstood aspects of the MCA.

To lawfully conclude incapacity, an assessment must clearly demonstrate:

What the impairment is
What the functional difficulty is
How the impairment causes that difficulty

Without this link, there is a real risk of confusing:

Risk-taking behaviour
Family disagreement
Professional frustration
Vulnerability

with legal incapacity.

Diagnosis does not equal incapacity.

Disagreement does not equal incapacity.

Risk does not equal incapacity.

The causative nexus is often where reports succeed or fail under scrutiny.

How the Mental Capacity Act Applies in Practice

The MCA 2005 underpins a wide range of legal and practical situations.

These include:

Each assessment must apply the same statutory structure - adapted to the specific decision in question.

*specific case law applies to Testamentary Capacity learn more in our guide

Decision-Specific and Time-Specific Capacity

Capacity is:

Decision-specific - a person may have capacity for one decision but not another.
Time-specific - capacity can fluctuate.

For example:

A person may lack capacity to manage complex financial investments but retain capacity to decide where they wish to live. This principle prevents over-broad conclusions about a person’s abilities.

Best Interests Decision-Making

If a person lacks capacity for a specific decision, the Act requires a structured best interests process. Decision-makers must consider:

The person’s past and present wishes
Beliefs and values
Views of family and others involved
Less restrictive alternatives

Best interests decisions must never be based solely on age, appearance, condition, or behaviour.

Key Case Law Shaping Interpretation

Although the Mental Capacity Act is statutory, its interpretation has been shaped by case law.

Notable decisions include:

A Local Authority v JB [2021] UKSC 52 — clarified the functional test and “use or weigh” element.
PC v City of York Council [2013] EWCA Civ 478 - confirmed decision-specific analysis.
Re B (Capacity: Social Media) [2019] EWCOP - demonstrated the importance of decision-focused assessment.

Testamentary capacity remains governed by the common law test in Banks v Goodfellow (1870), which operates alongside — but distinct from — the MCA framework.

Where Capacity Assessments Go Wrong

We regularly see errors including:

Confusing diagnosis with incapacity
Failing to identify the specific decision
Skipping the causative nexus analysis
Inadequate exploration of “use or weigh”
Over-reliance on risk
Insufficient evidence gathering

In court proceedings, such errors can lead to reports being challenged or rejected.

Who This Page Is For

This guidance is relevant to:

Solicitors and legal professionals
Court of Protection practitioners
Deputies and Attorneys
Families concerned about decision-making ability
Professionals seeking independent expert assessment

Understanding the statutory framework is essential before commissioning or challenging a capacity assessment.

Frequently Asked Questions

No. The Act governs decision-making capacity, not funding.
No. Diagnosis alone is insufficient. The functional test must be applied.
Yes. Capacity can vary depending on time, health, and decision complexity.
Any suitably trained professional may assess capacity, but court proceedings often require experienced assessors and CPR Part 35 compliance.
Not directly. Testamentary capacity follows the Banks v Goodfellow test, though MCA principles often inform modern practice.