An individual is entitled to Medicaid if he fulfills the criteria established by the State in which he lives. State Medicaid plans must comply with requirements imposed both by the Act itself and by D. Eligibility decisions could 'not take into account the financial responsibility of any individual for any applicant or recipient of assistance . . . unless such applicant or recipient is such individual's spouse' or minor, blind, or disabled child. Some States adopted plans that considered the spouse's income in determining Medicaid eligibility and benefits. These States calculated an amount considered necessary to pay the basic living expenses of the spouse and 'deemed' any of the spouse's remaining income to be 'available' to the applicant, even where the applicant was institutionalized and thus no longer living with the spouse. Congress eventually created an SSI program that nearly bankrupted some of the states but allowed States to provide Medicaid assistance only to those individuals who would have been eligible under the state Medicaid plan in effect on January 1, 1972. D promulgated regulations governing the administration of Medicaid benefits in both SSI States and § 209 (b) States. In SSI States, 'deeming' is conducted in the following manner: When the applicant and his spouse live in the same household, the spouse's income and resources always are considered in determining eligibility, 'whether or not they are actually contributed.' When the applicant and spouse cease to share the same household, the spouse's income is disregarded the next month, unless both are eligible for assistance. In the latter case, the income of both is considered for six months after their separation. Greater 'deeming' is authorized in § 209 (b) States. The regulations require such States to 'deem' income at least to the extent required in SSI States. And, if they choose, § 209 (b) States may 'deem' to the full extent that they did before 1972. P, an organization dedicated to helping the Nation's elderly, filed this suit attacking some of D's regulations applicable in § 209 (b) States. P claims that before a State may take into account the income of a spouse in calculating the benefits of any institutionalized applicant, the State must make a factual determination that the spouse's income actually is contributed to that applicant. The District Court agreed with P and declared the regulations invalid. The Court of Appeals for the District of Columbia Circuit affirmed. The appeals court held that the regulations were invalid because the Secretary, in authorizing 'deeming' of income between non-cohabiting spouses, had failed to '[take] . . . into account' two 'relevant factors.' First, where spouses are separated they maintain two households rather than one. For those already put to this additional expense, it is unfair to continue to treat the couple as a 'single economic unit' jointly responsible for the medical expenses of each. Second, the requirement of support carries with it the potential to interject 'disruptive forces' into people's lives. The court reasoned that the 'deeming' requirement creates an incentive for couples to divorce. D appealed.