In Robinson v. Mandell, the authenticity of a mutual will was in question.
Hetty Green (Robinson) claimed that her aunt (Howland) intended to disinherit the father of Robinson and requested that they were both to make a will, so that in case Howland predeceased Robinson but Robinson predeceased her father, the latter would not obtain anything derived from Howland's estate via Robinson.
At the centre of the case is the allegation that
it was at the same time mutually agreed
between the parties that the respective wills, so to
be made, were to be exchanged, and that each was
to have possession of the will of the other, and that
neither was to make any other will without notifying
the other, and returning the other's will so to be
held in exchange.
This is essentially a contract between (at the time) two living persons, and at least at the time, generally enforceable unless there exist motives to nullify the contract. Of course, the parties can also revoke the contracts according to the rules in the contract or provided by statutes.
The court made the following observation (among others):
persons agree each with the other to make mutual
wills, and both execute the agreement, it is held that
neither can properly revoke his will without giving
notice to the other of such revocation. The death of
one of the parties in such a case carries his part of
the contract into execution, and the better opinion
perhaps is, that the other party, after that event, if
the agreement was definite and satisfactory, cannot
rescind the contract.
The particular contract here would be irrevocably executed by one party due to her death and the contract is definitive and cannot be changed.
However, in this case, it was decided against Robinson because the existence of the mutual will was not proved.
By contrast, a traditional will is a unilateral declaration made by the testator, who in general cannot bind their future self from making another legally valid decision overriding the previous will, unless another party's interests are adversely impacted under certain conditions (e.g. under proprietary estoppel) or the law provides otherwise in that jurisdiction.
Nowadays in Massachusetts, the statutes provide for methods to revoke a will as well as contracts regarding the making or non-making of a will that could make certain wills irrevocable.
Section 2–507. [Revocation by Writing or by Act.]
(a) A will or any part thereof is revoked:
(1) by executing a subsequent will that revokes the previous will or
part expressly or by inconsistency; or
(2) by performing a revocatory act on the will, if the testator
performed the act with the intent and for the purpose of revoking the
will or part or if another individual performed the act in the
testator's conscious presence and by the testator's direction. For
purposes of this paragraph, ''revocatory act on the will'' includes
burning, tearing, canceling, obliterating, or destroying the will or
any part of it.
(b) If a subsequent will does not expressly revoke a previous will,
the execution of the subsequent will wholly revokes the previous will
by inconsistency if the testator intended the subsequent will to
replace rather than supplement the previous will.
(c) The testator is presumed to have intended a subsequent will to
replace rather than supplement a previous will if the subsequent will
makes a complete disposition of the testator's estate. If this
presumption arises and is not rebutted, the previous will is revoked;
only the subsequent will is operative on the testator's death.
(d) The testator is presumed to have intended a subsequent will to
supplement rather than replace a previous will if the subsequent will
does not make a complete disposition of the testator's estate. If this
presumption arises and is not rebutted, the subsequent will revokes
the previous will only to the extent the subsequent will is
inconsistent with the previous will; each will is fully operative on
the testator's death to the extent they are not inconsistent.
Section 2–514. [Contracts Concerning Succession.]
A contract to make or not to make a will or devise, or to revoke or not to revoke a will or devise, or to die intestate, if executed after the effective date of this article, may be established only by (i) provisions of a will stating material provisions of the contract, (ii) an express reference in a will to a contract and extrinsic evidence proving the terms of the contract, or (iii) a writing signed by the decedent evidencing the contract. The execution of a joint will or mutual wills shall not create a presumption of a contract not to revoke the will or wills.