The Mental Capacity Act (MCA) 2005 requires you to use a two-stage test to assess capacity. You must only assess capacity for a specific decision at a specific time.
Do not assume someone lacks capacity based on their age, appearance, or medical condition.
First, you must establish if the person has an impairment of the mind or brain. You must also check if they have a disturbance affecting how their mind or brain functions.
This condition can be permanent or temporary. Examples include dementia, significant learning disabilities, brain injuries, or the effects of drugs and alcohol.
Next, you must prove that the impairment stops the person from making the specific decision. To lack capacity, the person must fail at least one of four functional requirements.
You must check if the person can:
Understand the information relevant to the decision.
Retain that information long enough to make their choice.
Use or weigh the information to decide.
Communicate their decision through speech, sign language, or simple muscle movements.
If the person cannot do one or more of these four things, they lack capacity for this specific decision. You must then act in their best interests.
The 'causal nexus' is the vital link between the two stages of the capacity test. It connects the diagnostic test directly to the functional test.
You must prove that the impairment of the mind or brain actually causes the person to fail the functional test. The person must lack capacity specifically because of their mental impairment.
You cannot decide someone lacks capacity just because they make an unwise choice. You must also rule out other reasons for their inability to decide. For example, check if a lack of education or family pressure stops them from deciding rather than their mental impairment.
If the mental impairment does not directly cause the failure, the causal nexus does not exist. In this case, the law states the person still has capacity for that decision.