On the basis of extensive evidence indicating that large numbers of Indian children were being separated from their families and tribes and were being placed in non-Indian homes through state adoption, foster care, and parental rights termination proceedings, and that this practice caused serious problems for the children, their parents, and their tribes, Congress enacted the Indian Child Welfare Act of 1978 (ICWA), which, inter alia, gives tribal courts exclusive jurisdiction over custody proceedings involving an Indian child "who resides or is domiciled within" a tribes reservation. This case involves the status of twin illegitimate babies, whose parents were enrolled members of appellant Tribe and residents and domiciliaries of its reservation in Neshoba County, Mississippi. After the twins births in Harrison County, some 200 miles from the reservation, and their parents execution of consent-to-adoption forms, they were adopted in that countys Chancery Court by the appellees Holyfield, who were non-Indian. That court subsequently overruled appellants motion to vacate the adoption decree, which was based on the assertion that under the ICWA exclusive jurisdiction was vested in appellants tribal court. The Supreme Court of Mississippi affirmed, holding, among other things, that the twins were not "domiciled" on the reservation under state law, in light of the Chancery Courts findings (1) that they had never been physically present there, and (2) that they were "voluntarily surrendered" by their parents, who went to some efforts to see that they were born outside the reservation and promptly arranged for their adoption. Therefore, the court said, the twins domicile was in Harrison County, and the Chancery Court properly exercised jurisdiction over the adoption proceedings.

Held:

The twins were "domiciled" on the Tribes reservation within the meaning of the ICWAs exclusive tribal jurisdiction provision, and the Chancery Court was, accordingly, without jurisdiction to enter the adoption decree. Pp. 42-54.

(a) Although the ICWA does not define "domicile," Congress clearly intended a uniform federal law of domicile for the ICWA and did not consider the definition of the word to be a matter of state law. The ICWAs purpose was, in part, to make clear that in certain situations the state courts did not have jurisdiction over child custody proceedings. In fact, [490 U.S. 30, 31] the statutory congressional findings demonstrate that Congress perceived the States and their courts as partly responsible for the child separation problem it intended to correct. Thus, it is most improbable that Congress would have intended to make the scope of the statutes key jurisdictional provision subject to definition by state courts as a matter of state law. Moreover, Congress could hardly have intended the lack of nationwide uniformity that would result from state-law definitions of "domicile," whereby different rules could apply from time to time to the same Indian child, simply as a result of his or her being moved across state lines. Pp. 43-47.

(b) The generally accepted meaning of the term "domicile" applies under the ICWA to the extent it is not inconsistent with the objectives of the statute. In the absence of a statutory definition, it is generally assumed that the legislative purpose is expressed by the ordinary meaning of the words used, in light of the statutes object and policy. Well-settled common-law principles provide that the domicile of minors, who generally are legally incapable of forming the requisite intent to establish a domicile, is determined by that of their parents, which has traditionally meant the domicile of the mother in the case of illegitimate children. Thus, since the domicile of the twins mother (as well as their father) has been, at all relevant times, on appella