Mental capacity is the ability to make specific decisions when necessary. Someone will be considered as having mental capacity if they understand a decision they need to make, why they need to make it and the likely outcome of the decision.
In England and Wales, mental capacity is governed by the Mental Capacity Act 2005. Under the Mental Capacity Act, people are presumed to have capacity unless proven otherwise. The Mental Capacity Act sets out a 2-stage test to establish whether someone lacks mental capacity:
Does the person have an impairment of their mind or brain (whether as a result of an illness, like dementia, or external factors, like alcohol or drugs or trauma)?
Does this impairment mean the person is unable to make a specific decision when they need to?
In Scotland, mental capacity is govered by the Adults with Incapacity (Scotland) Act 2000. Under this Act, someone lacks capacity if they are unable to make decisions for themselves because they:
It’s important to note that people can lack capacity to make some decisions, but have capacity to make others. Further, someone’s mental capacity can fluctuate and change with time, and people may lack capacity at one point, before being able to make the same decision later.
Generally speaking, someone will lack mental capacity and be unable to decide for themselves if they cannot:
understand information relevant to the decision
retain and remember the information
use (eg evaluate) the information as part of the decision-making process
For more information on mental capacity, see the NHS website.