The mental capacity to execute a trust or trust amendment is primarily based on California Probate Code §§810-812, and the mental capacity to execute a will or will codicil is primarily based on California Probate Code §6100.5; however, in the case of a trust or trust amendment §6100.5 may apply if the provisions are more simple in nature, and in the case of a will the provisions of §§810-812 may apply if the provisions are more complicated and in the nature of a trust. Andersen v. Hunt (2011) 196 Cal. App. 4th 722; Lintz v. Lintz (2014) 222 Cal. App. 4th 1346. However, various other rules and case law also apply including that it is the intent and natural wishes of the decedent testator that should prevail, extrinsic evidence and evidence of the surrounding circumstances might be admissible to determine that testator’s intent, presumptions for and against the validity of a document apply in certain circumstances, and the validity of the document can also be attacked in various other circumstances including for example, undue influence, fraud, forgery, elder abuse, fiduciary or confidential relationship and mistake.
California Probate Code §§810-812 provide:
Section 810. The Legislature finds and declares the following:
(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.
Section 811. (a) A determination that a person is of unsound mind or lacks the capacity to make a decision or do a certain act, including, but not limited to, the incapacity to contract, to make a conveyance, to marry, to make medical decisions, to execute wills, or to execute trusts, shall be supported by evidence of a deficit in at least one of the following mental functions, subject to subdivision (b), and evidence of a correlation between the deficit or deficits and the decision or acts in question:
(1) Alertness and attention, including, but not limited to, the following:
(A) Level of arousal or consciousness.
(B) Orientation to time, place, person, and situation.
(C) Ability to attend and concentrate.
(2) Information processing, including, but not limited to, the following:
(A) Short- and long-term memory, including immediate recall.
(B) Ability to understand or communicate with others, either verbally or otherwise.
(C) Recognition of familiar objects and familiar persons.
(D) Ability to understand and appreciate quantities.
(E) Ability to reason using abstract concepts.
(F) Ability to plan, organize, and carry out actions in one’s own rational self-interest.
(G) Ability to reason logically.
(3) Thought processes. Deficits in these functions may be demonstrated by the presence of the following:
(A) Severely disorganized thinking.
(D) Uncontrollable, repetitive, or intrusive thoughts.
(4) Ability to modulate mood and affect. Deficits in this ability may be demonstrated by the presence of a pervasive and persistent or recurrent state of euphoria, anger, anxiety, fear, panic, depression, hopelessness or despair, helplessness, apathy or indifference, that is inappropriate in degree to the individual’s circumstances.
(b) A deficit in the mental functions listed above may be considered only if the deficit, by itself or in combination with one or more other mental function deficits, significantly impairs the person’s ability to understand and appreciate the consequences of his or her actions with regard to the type of act or decision in question.
(c) In determining whether a person suffers from a deficit in mental function so substantial that the person lacks the capacity to do a certain act, the court may take into consideration the frequency, severity, and duration of periods of impairment.
(d) The mere diagnosis of a mental or physical disorder shall not be sufficient in and of itself to support a determination that a person is of unsound mind or lacks the capacity to do a certain act.
(e) This part applies only to the evidence that is presented to, and the findings that are made by, a court determining the capacity of a person to do a certain act or make a decision, including, but not limited to, making medical decisions. Nothing in this part shall affect the decisionmaking process set forth in Section 1418.8 of the Health and Safety Code, nor increase or decrease the burdens of documentation on, or potential liability of, health care providers who, outside the judicial context, determine the capacity of patients to make a medical decision.
Section 812. Except where otherwise provided by law, including, but not limited to, Section 813 and the statutory and decisional law of testamentary capacity, a person lacks the capacity to make a decision unless the person has the ability to communicate verbally, or by any other means, the decision, and to understand and appreciate, to the extent relevant, all of the following:
(a) The rights, duties, and responsibilities created by, or affected by the decision.
(b) The probable consequences for the decisionmaker and, where appropriate, the persons affected by the decision.
(c) The significant risks, benefits, and reasonable alternatives involved in the decision.
California Probate Code §6100.5 provides:
Section 6100.5. (a) An individual is not mentally competent to make a will if at the time of making the will either of the following is true:
(1) The individual does not have sufficient mental capacity to be able to (A) understand the nature of the testamentary act, (B) understand and recollect the nature and situation of the individual’s property, or (C) remember and understand the individual’s relations to living descendants, spouse, and parents, and those whose interests are affected by the will.
(2) The individual suffers from a mental disorder with symptoms including delusions or hallucinations, which delusions or hallucinations result in the individual’s devising property in a way which, except for the existence of the delusions or hallucinations, the individual would not have done.
(b) Nothing in this section supersedes existing law relating to the admissibility of evidence to prove the existence of mental incompetence or mental disorders.
(c) Notwithstanding subdivision (a), a conservator may make a will on behalf of a conservatee if the conservator has been so authorized by a court order pursuant to Section 2580.
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