Mich. Comp. Laws § 700.2514: Contracts concerning succession.

Sec. 2514.

If executed after July 1, 1979, a contract to make a will or devise, not to revoke a will or devise, or to die intestate may be established only by 1 or more of the following:

Provisions of a will stating material provisions of the contract.

An express reference in a will to a contract and extrinsic evidence proving the terms of the contract.

A writing signed by the decedent evidencing the contract.

The execution of a joint will or mutual wills does not create a presumption of a contract not to revoke the will or wills.