What are some of the common grounds upon which testamentary capacity may be challenged, and what must be shown?

To make a valid will, a testator must be of sound mind and memory.  A will may be challenged for lack of testamentary capacity where the decedent was insane, infirm, lost his or her memory as a result of the aging process, or was senile.  Another challenge to a will involves undue influence or manipulation of the testator by an individual occupying a position of trust.  Legally-speaking, a testator is presumed competent unless proven otherwise.  In general, individuals contesting a last will testament must show that decedent's mental unsoundness caused him or her to be insanely delusional concerning them or incapable of remembering members of their family.