Testator's Mental Capacity
Heirs may challenge a will if the deceased was not mentally sharp at the time she made her will. In most states, “sound mind” means that the testator understood the nature of the document, the worth of her assets and her relationship with whoever she was bequeathing them to. This is one of the most difficult reasons to prove in court, however, because the mental capacity required to make a will is much less than most other documents, and the testator need only be lucid during the period of time she is making the will, not necessarily the day before or after, notes TheCommonLawyer.com.
Duress by a Beneficiary
Sometimes the elderly can find themselves at the mercy of an adult child, especially if that child is also their caretaker. Wills are often contested when beneficiaries or heirs have reason to believe that one of them coerced or threatened the testator into writing his will in a disproportionate way. Duress can happen through threats of actual violence, or more subtle manipulation such as promises to do something for the testator if he changes the will.
A will is open to a legal challenge if it does not meet all of the requirements of the state where it was made. Though criteria vary from state to state, a will must generally be signed, dated and witnessed by the required number of people who might or might not also be beneficiaries. Generally, however, the probate court declares the will invalid and bars it from being probated if it does not meet requirements.
An heir who has been written out of a will might destroy it and attempt to enter an older one -- one that still included him -- into probate, causing others heirs to contest the matter. If the will is a fake and the testator’s signature is a forgery, this is also grounds to contest. Both of these actions are criminal offenses in most states.
The easiest will contest to win is one where a more recent will can be produced, invalidating an older one. Generally, in instances where there is more than one will made by the decedent, the one dated closest to the date of death is the one that is accepted.