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Understanding mental capacity 

What is mental capacity?

Having mental capacity is the ability to make a decision, whether it is an everyday decision such as what to wear, or a more important decision such as choosing where to live. 

What is the Mental Capacity Act?

The Mental Capacity Act 2005 applies to both England and Wales today. It provides the legal framework for making decisions on behalf of people who lack the mental capacity to make decisions themselves.

The Mental Capacity Act 2005 states: 

  • a person must be assumed to have capacity unless it is established that they lack capacity 
  • a person is not to be treated as unable to make a decision unless all practicable steps to help them to do so have been taken without success 
  • a person is not to be treated as unable to make a decision merely because they make an unwise decision 
  • an act done, or decision made, under the Act for or on behalf of a person who lacks capacity must be done, or made, in their best interests 
  • before the act is done, or the decisions is made, regard must be had to whether the purpose for which it is needed can be as effectively achieved in a way that is less restrictive of the person’s rights and freedom of action. 

There is a  Mental Capacity Act Code of Practice  which explains the Mental Capacity Act 2005 in detail. 


There is a two-stage test to work out if someone lacks mental capacity.

Stage one: Does the person have an impairment of, or a disturbance in the functioning of, their mind or brain? This could include dementia, learning disabilities, mental illness, brain damage, etc. 

Stage two: Does the impairment or disturbance mean that the person is unable to make a specific decision when they need to?  
A person is unable to make a decision if they cannot: 

  • understand information about the decision to be made 
  • retain that information in their mind 
  • use or weigh that information as part of the decision-making process 
  • communicate their decision. 

If you disagree with a mental capacity assessment decision, for example if the person you look after is assessed as lacking mental capacity when you think they have mental capacity, there are various ways you can try and resolve this.

You can raise the matter with the person who carried out the assessment and ask them to give reasons why they believe the person lacks mental capacity to make the decision, and ask them to provide objective evidence to support that belief. 

You could try and get a second opinion from an independent professional. 

If you cannot resolve the disagreement then you may be able to apply to the Court of Protection, who can make a decision as to whether a person has mental capacity. 

If the person you look after is assessed as lacking mental capacity, any decision made on their behalf should be done in their ‘best interests’. This is the case whether the person making the decision on behalf of the person lacking capacity is a carer, a lasting power of attorney, a court appointed deputy, or a professional.

Who should actually make the best interest decision will depend on the type of decision in question and on whether there is anyone with the legal right to make the decision. 

If the decision is an everyday decision, the person most directly involved with the person should be the decision maker. For example, if you help the person you look after get dressed, you will make the decision as to what they wear. 

If the decision is about their health and wellbeing and you or have lasting power of attorney for health and welfare or are the court appointed deputy, you should be the decision maker. 

If the decision is about property or financial affairs and you have lasting power of attorney for property and financial affairs or are the court appointed deputy for property and financial affairs, you should be the decision maker.

If there is not a lasting power of attorney or court appointed deputy, the professional involved should be the decision maker. 

When making the best interest decision, if it is practical and appropriate to do so, the decision maker should consult any lasting power of attorney or court appointed deputy.  

This applies even if the power of attorney or court appointed deputy does not have the relevant authority to be the decision maker themselves. For example, a power of attorney for property and financial affairs should still be consulted about a decision relating to health and welfare.  

The decision maker should also consult anyone who is caring for the person, any close relatives and friends, and anyone else who is interested in the welfare of the person you look after. 

When making the best interest decision, the decision maker should also take into account the past and present wishes and feelings of the person you look after. 

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