In California, anyone who is at least 18 years old and of sound mind can make a will. However, the term “sound mind” means having testamentary capacity, and the law describes this as being “mentally competent”.
Mental competency means that, at the time the will was signed, the “testator” (person signing the will) was able to do all of the following:
(1) Understand the nature of the testamentary act. That is, they knew that they were giving specific instructions on who their beneficiaries would be and what property (real or personal) the beneficiaries would receive.
(2) Understand and recollect the nature and situation of their property. Did they know what their estate consisted of? A house? A bank account? Some stock certificates? They need not know, for example, the actual bank account numbers, or even the total number of accounts they have. But they must know that they have bank accounts, and a general understanding of the assets that make up their estate at the time the will is signed.
(3) Understand who their close relatives are and how they will be affected by the will. If the person cannot remember that s/he is married, or has children, then the will is likely to be determined invalid, because only a mentally incompetent testator could have forgotten that such relationships existed.
(4) The testator must not suffer from a mental disorder. This includes hallucinations or delusions that result in the testator leaving property to individuals or entities that, absent any such delusions, s/he would never have done.
An example would be where the testator suffered from a delusion that he was being visited by ghosts every night and believed that the only way to stop the haunting would be to name a paranormal group (like the ghostbusters) as a major beneficiary to the will.
Other circumstances exist, however, that would allow a will to be made even though a person has been declared mentally incompetent. In a conservatorship case, where the court has appointed a conservotor, a will can be executed on behalf of the conservatee. However, for the will to be valid, the conservator would have to first petition the court and obtain a court order for such authorization.
From the above rules, you can see that testamentary capacity is not a high bar to hurdle. When issues such as hallucinations and delusions appear, then a will may be challenged through a procedure known as a will contest.