Very few wills are challenged in court—one law professor who looked at the records estimated that it happens only once in a hundred cases. But if a will doesn't fulfill certain legal requirements, or the maker of the will was not of sound mind, a would-be heir or beneficiary can challenge it in probate court after the will-maker's death.
It's often tough to prove that a will, if it appears valid and was properly witnessed, should be ruled invalid. Here are the arguments that can be made by someone who stands to benefit from getting the will thrown out.
To be valid, a will must have been by someone who was of "sound mind" when the will was made. The legal term for the mental ability needed to execute a valid will is "testamentary capacity." This is not a rigorous requirement. Forgetfulness or even the inability to recognize friends doesn't, by itself, establish incapacity.
Usually, a court faced with resolving a question of mental capacity requires only that the person who made the will:
In a lawsuit over testamentary capacity, the testimony of people who saw the will-maker at or very near the time the will was signed—the witnesses who also signed the will, a doctor who saw the will-maker the week before, the lawyer in whose office the will was signed—is typically very important. For example, in one case challenging the capacity of a will-maker, the court ruled that the opinion of an attorney who barely knew the will-maker but saw him the day the will was signed was more relevant than the testimony of a doctor who had formed an opinion only from medical records. (In re Bosley, Pa. Super. Ct. 2011)
A will can also be declared invalid if someone proves in court that it was procured by "undue influence." This usually involves some evil-doer who occupies a position of trust -- for example, a caregiver or adult child -- manipulating a vulnerable person to leave all, or most, of his property to the manipulator instead of to the people who would have been expected to receive it. In other words, the will accomplishes the wrongdoer's goals, not the goals of the will-maker.
Of course, a will contest can be based on fraud or forgery. For example, someone could claim that a will-maker was tricked into signing a document or that a signature was forged. Typically, these claims go along with an allegation of undue influence and lack of testamentary capacity.
A will must have been dated and signed in the presence of at least two adult witnesses, who also signed the will. In most states, the witnesses cannot be people who are named to inherit property under the will.
About half the states allow handwritten, unwitnessed wills to be admitted to probate. These documents are called "holographic wills" and must be written and signed entirely in the handwriting of the person making the will. (Some states also require that they be dated.) Because there are no witnesses, holographic wills are more easily to challenge than standard typewritten wills. The probate court must be satisfied that the document is actually in the deceased person's handwriting and was intended to serve as a will.