The estate tax marital deduction, section 2056 of the Internal Revenue Code, was enacted in 1948, along with the split-income provisions of the income tax law and the marital deduction and split-gift provisions of the gift tax law. The purpose was to give married residents of common law states approximately the same federal tax advantages that were available to married residents of community property states. Ordinarily, upon the death of a married resident of a community property state, only one-half of the community property is taxed in the decedent's estate. Section 2056 achieves approximately the same result for married residents of common law states by providing a deduction - limited, in general, to the greater of $250,000 or one-half of the estate - for the value of property interests included in the decdent's gross estate that pass from the decedent to the surviving spouse.
Kahn, Douglas A. "The Estate Tax Marital Deduction." H. Dubroff, co-author. Prac. Law. 26, no. 7 (1980): 23-42.