[Contracts Concerning Succession.]\nA 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.
Massachusetts Legal Code