As discussed in a former post, “So Who Can Create a Will?” a testator is required to have capacity in order to create a valid will. As long as the testator is recognized to be capable of understanding (1)the nature and extent of his/her property, (2) the effect of the disposition they are making, (3) the nature of their relationships with apparent heirs, and how the above three criterion work together to dispose of their property, they generally are able to act as testator to a will.
There is one interesting exception to this rule. Though a person may be considered to have capacity to execute a will according to the criterion set out above, if the testator can be shown to be suffering from insane delusions, parts of the will affected by the delusions or the entire will may be set aside as invalid. If the testator can be shown to have a mistaken belief in a fact held against all argument and reason; and the mistaken belief actually affects the provisions in the will, the will may be invalid.
So if your great aunt Betty believes that you are trying to poison her in order to take her property, and she removes you from her will for this reason, you may be able to have that provision in the will made invalid as the effect of an insane delusion. However, if our same beloved great aunt Betty believes that the government is coming to take her away, or aliens are listening to all her conversations, and she for no clear reason has only left you an unproportionate share of her estate, you most likely would not be able to have that provision set aside as the result of an insane delusion because the delusion most likely did not materially affect the disposition in the will.
Image by: U.S. National Archives