Of Sound Mind and of Age
All states require that a testator be of sound mind when she makes the will. A testator is of sound mind -- what courts call "testamentary capacity" -- if she understands the nature and extent of her property, knows to whom she wishes to leave the property and knows that the document she is signing relates to the distribution of her property when she dies. Courts judge testamentary intent at the time the testator signed the will, not a later date. Most states also require that a testator have achieved a certain age, generally 18 years or older, although there are exceptions.
Intent Freely Exercised
Since a will is not effective until after the testator's death, she cannot answer questions during its probate. Given this, courts require that a testator have "testamentary intent" when she executes a will. This means that the testator must act in contemplation of death and also that she exercise her own intent. The first requirement is met by the testator acknowledging the document as her last will and testament to witnesses as she signs it. Courts presume the existence of the second element, freely exercised intent, absent allegations of undue influence or fraud proved in a will contest.
Signature and Witnesses
Probate laws require certain formalities during will-signing that are essential to the validity of the will. The precise procedures vary among types of wills and also between jurisdictions. Although some states permit handwritten "holographic" wills or oral "nuncupative" wills in some circumstances, most testators use prepared wills, typed or printed by themselves or someone else. All states require that the testator acknowledge and sign prepared wills before at least two witnesses, who then sign after her. The probate court calls the witnesses to testify that the testator both signed the will and had testamentary intent. In most states, witnesses must be disinterested; that is, they must not receive bequests under the will they are witnessing. A notarized signature cannot substitute for the witness requirement since it does not speak to testamentary intent.
Invalidating a Will
The probate court invalidates a will if evidence establishes failure of testamentary capacity, testamentary intent or proper execution. An improperly executed will is void on its face, as is a revoked will. Revocation can be express or implied at law. If a testator writes a superseding will, it automatically revokes the earlier will; this is an express revocation. In some states, marriage, divorce or the birth of a child -- occurring after a will is written -- automatically revokes the will; the revocation is implied at law.