A will-maker is presumed to be sane, and to have sufficient mental capacity to make a will, unless the contrary is proved. A lack of mental capacity may derive from a congenital intellectual difficulty, trauma affecting the brain, or cognitive processes and disease, including those more commonly occurring with old age. The Dickensian language of Lord Chief Justice Alexander Cockburn, delivering a judgment in 1870, provides a quaint and accurate summary—
It is essential to the exercise of such power that the testator [will-maker] shall understand the nature of the act and its effects; shall understand the extent of the property of which he is disposing; shall be able to comprehend and appreciate the claims to which he ought to give effect; and, with a view to the latter object, that no disorder of the mind shall poison his affections, pervert his sense of right, or prevent the exercise of his natural faculties—that no insane delusion shall influence his will in disposing of his property and bring about a disposal of it which, if the mind had been sound, would not have been made.
According to another case decided in the 1870s, the highest degree of mental capacity is required to make a will. Compare that with a 1953 case, which held that a lower degree is sufficient to contract a valid marriage. The distinction seems to be based on the premise that, if the same test were applied to marriages as is applied to wills, then the fact of getting married might of itself be sufficient to make the marriage invalid. Think about that if you’re still reading.
And while a will-maker ‘is bound to look all around, to consider his family, if he has one, his wife, his brothers and sisters, uncles and aunts, poor relations, possibly charities he has helped in his life …’ the capacity to make a will ‘is not reserved for people who are wise, or fair, or reasonable, or whose values confirm to generally accepted community standards. A person may disinherit a child for reasons that would shock the conscience of most ordinary members of the community, but that does not make the will invalid’.
Some examples of things which have proved lack of capacity include: belief in things impossible; belief in things possible but so improbable that no sane person could believe them; a belief, influencing the drafting of the will, invoked by trickery or misunderstanding; disease; injury; congenital disability; and the effects of certain medications. Neither a belief in God nor a belief in a happy marriage is considered to be a belief in a thing impossible; and suicide, not being evidence of a lack of sanity, is apparently rational.