California Code, Probate Code - PROB § 810

(a) For purposes of this part, there shall exist a rebuttable presumption affecting the burden of proof that all persons have the capacity to make decisions and to be responsible for their acts or decisions.

(b) A person who has a mental or physical disorder may still be capable of contracting, conveying, marrying, making medical decisions, executing wills or trusts, and performing other actions.

(c) A judicial determination that a person is totally without understanding, or is of unsound mind, or suffers from one or more mental deficits so substantial that, under the circumstances, the person should be deemed to lack the legal capacity to perform a specific act, should be based on evidence of a deficit in one or more of the person's mental functions rather than on a diagnosis of a person's mental or physical disorder.