Testamentary capacity is the legal and mental ability of a person making a will (the testator) to make a legally binding disposition of their assets and to understand the nature and effect of the act of making a will—including disposing of their assets at death.

Proof of testamentary capacity is a required element of a will being admitted to probate and recognized by the court as having legal effect.

Adults are generally presumed to have the legal capacity to make a will, but minors (persons under a certain age) are not—with some exceptions in some states, such as if the minor has served in the military or has been married.

Persons who suffer from dementia, Alzheimer’s disease, or insanity, for example, may lack testamentary capacity to make a will.

In legal jargon, the historical terms of art for testamentary capacity were that the testator was “of sound mind and memory” or “of sound mind and disposing memory,” and this terminology is sometimes recited in a will as a statement of the testator regarding the testator’s mental capacity when making the will.

Laws vary from state to state and the requirements related to testamentary capacity are often located in a state’s statutes—usually in the estates code or probate code—and interpreted and applied by the courts in specific factual circumstances or cases that are reported in court opinions involving will contests, for example.

In California, testamentary capacity is defined as the legal and mental ability of a person to create a will, with the understanding of the nature of the act, the extent of their property, and the claims of those who might naturally expect to benefit from the estate (such as family members). The standard for testamentary capacity in California is relatively low; the person making the will (testator) must simply be able to understand the nature of the testamentary act, know and understand the nature and situation of their property, and remember and understand their relations to living descendants, spouse, and parents who might be affected by the will. Adults are presumed to have testamentary capacity, but minors typically do not, except in certain circumstances such as marriage or military service. Individuals with mental impairments, such as dementia or Alzheimer's disease, may lack testamentary capacity if they do not meet the aforementioned criteria. The relevant statutes can be found in the California Probate Code, and case law provides further guidance on how courts interpret and apply these rules in specific situations.