NSW Law Society guidelines for testamentary capacity. Consequently, she failed to take. In order for a will to be vali the will-maker must have had testamentary capacity at the time it was made. Dementia and testamentary capacity : definition and causes Dementia is a neurodegenerative syndrome characterised by the development of multiple, cognitive (intellectual) deficits, usually including memory impairment. According to the DSM- IV, dementia is diagnosed in the presence of memory impairment and one or more of the following. Testamentary capacity in England and Wales. The presumption of capacity Where a will is rational, professionally drawn, seemingly regular in form and is made by a person whose capacity is not in doubt, there is a presumption that the will is valid. This presumption can be rebutted by a challenger to the will showing that there is a real doubt. If the person making the will (testator) lacks testamentary capacity at the time that the will is execute the ...