26 U.S.C. § 2043

(a) In general If any one of the transfers, trusts, interests, rights, or powers enumerated and described in sections 2035 to 2038, inclusive, and section 2041 is made, created, exercised, or relinquished for a consideration in money or money’s worth, but is not a bona fide sale for an adequate and full consideration in money or money’s worth, there shall be included in the gross estate only the excess of the fair market value at the time of death of the property otherwise to be included on account of such transaction, over the value of the consideration received therefor by the decedent. (b) Marital rights not treated as consideration (1) In general For purposes of this chapter, a relinquishment or promised relinquishment of dower or curtesy, or of a statutory estate created in lieu of dower or curtesy, or of other marital rights in the decedent’s property or estate, shall not be considered to any extent a consideration “in money or money’s worth”. (2) Exception For purposes of section 2053 (relating to expenses, indebtedness, and taxes), a transfer of property which satisfies the requirements of paragraph (1) of section 2516 (relating to certain property settlements) shall be considered to be made for an adequate and full consideration in money or money’s worth.