In my experience, there is a lot of confusion amongst the public as to the difference between a mutual will and a mirror will, with the latter often being mistaken for the former.
Mutual wills are actually very rare – I have seen two in almost 50 years of practice and they were both identified as a mutual will and both testator signed the same document wherein they contracted with the other that the survivor would not be able to vary the bequests in the future.
A mirror will typically contains gifts by each to the other of their respective estates, with the same contingent gifts over of the residue of the estate of the survivor of them. Typically, this is a husband and a wife, leaving everything to each other, and in the event that the other fails to survive for a certain period of time, such as 30 days, then to alternate beneficiaries who are usually their children.
A mutual will is much more than a mirror will, it is an agreement that the wills not be changed and is a constraint of testamentary freedom. It contains a contract between the parties to the mutual will that the will not be changed after the death of one of the parties.
The testamentary intentions of will makers expressed in their mirror wills is not enough to establish that they had agreed that the survivor would not be able to vary the bequests in the future.
A true mutual will therefore is a binding contract between typically spouses not to revoke or change or replace their wills. It is an agreement between the parties to dispose of their estate in a particular way that equity and forces through the mechanism of a constructive trust after the first of the spouses of died, if the survivor does not abide by their agreement. Oosteroff On Wills 8th Ed at pp127-28.
The most fundamental prerequisite for an application of the doctrine of mutual wills is that there be an agreement (contract) between the individuals who made the wills.
The mutual wills agreement must satisfy:
1) the requirements for a binding contract and not be just some loose understanding or sense of moral obligation;
2) it must be proven by clear and satisfactory evidence;
3) it must include an agreement not to revoke the wills. Edell v Sitzer (2001) 55 O.R. 198 at para.73
In Bellinger v Nuytenn Estate 2002 BCSC 571 .the court held that honor is not a sufficient foundation on its own, and that a mutual will agreement will not be found to exist for the evidence is more consistent with some loose understanding or moral obligation rather than a binding, enforceable agreement.
The agreement may be proven either from the words of the will itself or from extrinsic evidence. The extrinsic evidence does not necessarily have to come from documents and it may be hearsay testimony from interested parties, but the courts have held that mere assertions from which inferences should be drawn are not acceptable as reliable evidence, to prove the existence of a mutual will agreement. Trotman v Thompson 2006 OJ No. 681
The burden of proof rests with the party that alleges the existence of a mutual will agreement, and that onus is heavy in that there must be clear evidence of the mutual will agreement. Cassin v Cassin (2007) 30 ETR 289 at para. 37