Mental capacity
Supposing a person is unable to make decisions for themselves, what happens then? In the UK (and many other countries) there is a legal framework to make sure no-one takes advantage of the said person. It often consists of a physician or psychiatrist dealing with the person in question, a solicitor representing the person, and a solicitor representing the "public interest" (the state). Meetings are held at agreed times, say once every three months. The purpose is to make sure that decisions taken are in the person's interest.