I am often asked by family members about the grounds for contesting a last will. This is especially so for offspring of the decedent who expected to be, but were not, beneficiaries under the will presented for probate. The grounds for contesting a will are limited and, in most cases, expensive to pursue. So get out your checkbook before picking up the phone to call your lawyer.
The easy case is one where there are errors in the required formalities of execution such as the failure to have to two witnesses to the execution of the last will. But those are unusual cases. The more typical grounds for contesting a will are either undue influence or lack of required mental capacity. "Courts require substantial evidence to upset a testator's written will. [Factors of undue influence listed by courts are as follows]: (1) unnatural disposition, (2) opportunity to exert the undue influence, (3) susceptibility, and (4) activity of beneficiaries in procuring the will."
Link. "Unnatural disposition" is a case where an individual who is not a relative of the deceased becomes a substantial beneficiary. Susceptibility involves cases where the decedent was more easily influenced at the time the will was executed due to age or health. Activities supporting a claim of undue influence can be such things as the beneficiary selecting the attorney to draw up the new will and / or physically taking the decedent to the attorney's office to execute the new will. No one factor controls undue influence cases. The courts look at the totality of the circumstances in making the determination.
Another common ground for contesting a last will is the decedent lacked
testamentary capacity at the time the will was executed. All adults are presumed to possess testamentary capacity so the burden is on the party challenging the will. "To have testamentary capacity, the author of the will must understand the nature of making a will, have a general idea of what he/she possesses, and know who are members of the immediate family or other natural objects of his/her bounty."
Link. As determining mental capacity is a medical issue, this is the type of case is susceptible to expert testimony. See
example of an psychiatrist specializing in these cases. See also
attorney article on generally on challenging last wills.
Can last wills be successfully challenged? Yes but it is a very difficult case to make.