In the common law tradition, testamentary capacity is the legal term of art used to describe a person's legal and mental ability to make or alter a valid will. This concept has also been called sound mind and memory or disposing mind and memory.

Presumption of capacity.

Adults are presumed to have the ability to make a will. Litigation about testamentary capacity typically revolves around charges that the testator, by virtue of senility, dementia, insanity, or other unsoundness of mind, lacked the mental capacity to make a will. In essence, the doctrine requires those who would challenge a validly executed will to demonstrate that the testator did not know the consequence of their conduct when they executed the will.

Certain people, such as minors, are usually deemed to be conclusively incapable of making a will by the common law; however, minors who serve in the military are conceded the right to make a will by statute in many jurisdictions. In South Africa, however, one acquires testamentary capacity at the age of 16 years.

Requirements.

The requirements for testamentary capacity are minimal. Some courts have held that a person who lacks the capacity to make a contract can nevertheless make a valid will. While the wording of statutes or judicial rulings will vary from one jurisdiction to another, the test generally requires that the testator was aware of:

The extent and value of their property.

The persons who are the natural beneficiaries

The disposition they are making

How these elements relate to form an orderly plan of distribution of property.

The legal test implies that a typical claimant in a will contest is a disgruntled heir who believes they should have received a larger share than they did under the will. Once the challenging party meets the burden of proof that the testator did not possess the capacity, the burden subsequently shifts to the party propounding the will to show by clear and convincing evidence that the testator did have the requisite capacity.

Proof of testamentary capacity.

Those who contest a will for lack of testamentary capacity must typically show that the decedent suffered from mental unsoundness that left them unable to remember family members or caused them to hold insane delusions about them. Dead Man's Statutes sometimes restrict evidence which can be admitted concerning transactions with the decedent.

Lawyers for people whose testamentary capacity might be called into question often arrange for a will execution to be videotaped. On video, they ask the testator about his property and about his family, and go over the contents of the testator's will.

The testamentary capacity matter is most frequently raised posthumously, when an aggrieved heir contests the will entered into probate. For this reason, in the absence of the ability to interview the testator directly, a forensic psychiatrist or forensic psychologist may evaluate a testator’s capacity by reviewing videotape of the drafting of the will, emails or letters, medical records, and other records. Along with resolving an examinee's testamentary capacity, a forensic specialist may observe for signs of undue influence, particularly susceptibility to undue influence.

Even when a testator is found to have lacked testamentary capacity due to senility, loss of memory due to the aging process, infirmity or insanity, courts will sometimes rule that the testator had a "temporary period of lucidity" or a "lucid moment" at the time of the execution of the testamentary instrument. Such finding will validate a will that would otherwise be denied probate.

A way to forestall a will contest would be to have a self-proving will, in which an affidavit of the witnesses to the will specifically swear or affirm that the will was prepared under the supervision of an attorney.