Mich. Comp. Laws § 490.56

Sec. 6. A multiple-party account payable to 2 or more persons, jointly or severally, which does not expressly provide that there is no right of survivorship, though there is no mention of survivorship or joint tenancy, is presumed to be a survivorship account. At the death of a party, sums on deposit in a survivorship account belong to the surviving party or parties as against the estate of the decedent. Where there are 2 or more survivors, their respective ownerships shall be in proportion to their previous net contributions augmented by an equal share for each survivor of any interest the decedent may have owned in the account immediately before his death, plus the proceeds of insurance on decedent's life paid to the account. The right of survivorship continues between survivors.