Testamentary capacity is a legal term that describes a person's legal and mental ability to make a valid will or trust. The basic requirements are that a person be of legal age to execute a will, typically 18 years, and that a person be of sound mind.
In some states, a statute governs a person's capacity to make a will and in others case law that has developed over time. While each state has a different definition of testamentary capacity the requirement that someone be of sound mind generally involves three inquiries.
Whether a person has testamentary capacity is determined at the time he or she signed the will or trust. Some states have a presumption that a person has the necessary capacity to execute a will. Because mental competency is determined at the time a will is executed, it may be possible for a person with Alzheimer's, dementia, or another mental disorder to execute a valid will so long as the person is executing during a lucid interval.
A person may possess testamentary capacity when he or she executed the will, but a court may still determine that the will is not valid. If the will was executed at a time when the person is suffering from a delusion and the will was executed as a result of that delusion, that will may be determined to be invalid. For example, where a parent executes a will and disinherits a child because the parent believes that child stole property from him or her when in fact the parent had sold the property to the child.
A will may also be determined to be invalid on the basis of fraud or where someone exercised undue influence with respect to execution of the will. In these cases, it is typical that an unnatural disposition of the property has been made by a person susceptible to undue influence to the advantage of someone who had the opportunity to exercise undue influence and used that opportunity to procure the contested disposition through improper means. For example, a caretaker taking advantage of a relationship with an elderly client and convincing the client to leave the caretaker all of their property instead of the client's family members, friends, or long-favored charities.