A retirement account such as a 401k or IRA usually has what is called a designated beneficiary. This is a designation of a specific person by the owner of the account that they would like to be paid out on the policy at the time of their death. Unless there is a specific court order that a certain person be named as the designated beneficiary, such as per a Divorce Decree, the owner of the account can name whomever they would like. A prenuptial agreement usually would not legally bind an individual to name the spouse as the designated beneficiary. Upon death of the owner, the designated beneficary would receive the funds, and this would be considered a non-probate asset, not subject to being administered by the estate. If there is no person named as the designated beneficary then if there is a will controlling the distribution of assets, this would be considered an estate asset to be distributed per the instructions of a will if there was one. If there was no will it would be distributed according to the intestacy laws of the state the decedent resided in at the time of death.