Testamentary incapacity and undue influence are common grounds to contest the will of a deceased testator. The former refers to the testator’s lack of the necessary mental ability to execute the will, while the latter describes the influence of another person in making the will such that it does not genuinely reflect the testator’s own wishes.
The Supreme Court of Canada decision of Vout v. Hay,  2 S.C.R. 876, remains a leading case in this regard.
With respect to testamentary capacity, Vout stands for the principle that if the will was signed with the requisite formalities (by a testator who appeared to understand it) it will generally be presumed that she knew and approved of the will’s contents and had the necessary testamentary capacity.
This presumption is rebuttable if it can be shown on a balance of probabilities that there were “suspicious circumstances” leading to execution of the will. These include circumstances surrounding the preparation of the will (e.g. a beneficiary was in the room when the testator signed the will), circumstances that question the capacity of the testator (e.g. she left her property to a stranger rather than family), and circumstances suggesting that the free will of the testator was overborne by acts of coercion or fraud.
If suspicious circumstances are established, the onus reverses to the proponents of the will to prove testamentary capacity. This can be accomplished through evidence such as medical records (e.g. capacity assessment reports from medical professionals that examined the deceased around the time the will was signed) and statements from the deceased’s family, friends and neighbours that the testator was of sound mind.
The onus is always on those attacking the will to prove undue influence. Though the standard remains the balance of probabilities, recent case law in Alberta indicates that the evidentiary standard is rising. Suspicion and speculation is not enough; direct evidence is required. An example of such evidence is a statement from a witness that he saw someone threaten the deceased financially or physically to change her will.