A contract law
remedy developed under the auspices of equity
to vacate contracts
where a person has overwhelmed the free will of another by getting the other to sign a contract or a will
There have been more legal dissertations on what is and is not undue influence than there seem to be fish in the sea, rendering it difficult to sort out.
The reigning case in England is Barclays Bank v O'Brien although it remains to be seen if this dissection will catch on elsewhere:
"A person who has been induced to enter into a transaction by the undue influence of another ("the wrongdoer") is entitled to set that transaction aside as against the wrongdoer. Such undue influence is either actual or presumed....
"Actual undue influence: in these cases it is necessary for the claimant to prove affirmatively that the wrongdoer exerted undue influence on the complainant to enter into the particular transaction which is impugned.
"Presumed undue influence: in these cases the complainant only has to show, in the first instance, that there was a relationship of trust and confidence between the complainant and the wrongdoer of such a nature that it is fair to presume that the wrongdoer abused that relationship in procuring the complainant to enter into the impugned transaction. In presumed undue influence cases therefore there is no need to produce evidence that actual undue influence was exerted in relation to the particular transaction impugned: once a confidential relationship has been proved, the burden then shifts to the wrongdoer to prove that the complainant entered into the impugned transaction freely, for example by showing that the complainant had independent advice. Such a confidential relationship can be established in two ways:
a: certain relationships (for example solicitor and client, medical advisor and patient) as a matter of law raise the presumption that undue influence has been exercised.
b: even if there is no relationship falling within (a), if the complainant proves the de facto existence of a relationship under which the complainant generally reposed trust and confidence in the wrongdoer, the existence of such relationship raises the presumption of undue influence. In a (b) case therefore, in the absence of evidence disproving undue influence, the complainant will succeed in setting aside the impugned transaction merely by proof that the complainant reposed trust and confidence in the wrongdoer without having to prove that the wrongdoer exerted actual undue influence or otherwise abused such trust and confidence in relation to the particular transaction impugned."
Similarly, Canadian author S. Waddams:
"Another well-established class of cases is that granting relief from agreements because of undue influence.
"There are two kinds of cases.
"The first is when the relationship between the contracting parties falls into an established category; a parent taking advantage of a child, a trustee of a beneficiary, a solicitor of a client, a priest of a penitent, a doctor of a patient. In these cases undue influence is said to be 'presumed' and a transaction - a gift or contract - will be set aside at the instance of the weaker party, unless the presumption is rebutted.
"In the second kind of case none of these special relationships exists, but it is still open to the weaker party to prove that the relationship between the parties was such that the stronger was enabled to take unfair advantage. This may be shown in either of two quite different ways: by actual pressure relating to the particular transaction, as in a case where the stronger party threatened to prosecute the weaker's son, or by a general relationship of a trust between the parties of which the stronger party took advantage. It is plain that the undue influence umbrella covers two quite separate concerns, the first akin to duress - abuse of adversary power, the second akin to fiduciary duty - the abuse of trust."
In regards to the context of wills, the Saskatchewan Court of Queen's Bench in Royal Trust v Ritchie adopted these words to describe undue influence:
"A will may be set aside on the ground that it has been obtained by undue influence.
"To constitute undue influence in the eyes of the law there must be coercion; pressure if exerted so as to overpower the volition without convincing the judgment is a species of undue influence which will invalidate a will.
"To establish undue influence, it is necessary to prove that power was exercised over the testator, and it was the exercise of that power that resulted in the will being made. Undue influence can usually only be discovered by examining the circumstances leading up to the preparation of the will or by looking at the relationship that existed between the person benefiting from the will and the testator."