Unlike the first three articles of the Constitution which create the three branches of the federal government, Article IV establishes a set of rules to police the actions of states and knit them together into a single union. Notably absent from Article IV is any mention of the tribal sovereign. Concomitantly, there has been no comprehensive academic discussion thinking about how the tribal sovereign complicates the purposes of Article IV. This piece advances a completely new understanding of Article IV and its implications in federal Indian law. It suggests that where Article IV advances rights to individual citizens (i.e. a citizen’s right to enforce a court judgment or their claim to the protection of the Privileges and Immunities Clause) then states may not use their connection to any tribal sovereign as an excise to deny them the protections of those rights. In contrast, where Articles IV speaks to rules designed to ensure states treat each other respectfully (i.e. requests for extradition, claims under the Equal Footing Doctrine, or any attempt to enforce the Guarantee Clause) then Article IV’s rules do not permit states to abridge, abrogate, modify, or erode the inherent rights of tribal nations. As the Court has recently opined, tribal governments themselves were absent from the Constitutional Convention and so constitutional limitations on the inherent powers of sovereigns do not extend to tribal governments.

The prevalence of post-pandemic remote working arrangements and increased interstate migration have upended existing personal income taxation regimes. For decades, the current paradigm proved to be an imperfect but workable means to determine which state has the prevailing claim to impose tax on a particular item of income. The individual’s state of residence has a residual claim to all the individual’s worldwide income, but defers to the state in which the income is derived if such a state is determinable. To that end, the state of residence typically provides a credit for income taxes paid on a source basis to other states. Fundamentally, source trumps residence in the context of personal income taxation.

The problem is that taxing jurisdictions can no longer readily determine the source of income or the individual’s state of residence within the existing legal constructs. Existing legal structures designed to tax employment income cannot cope with widespread remote working arrangements. The rise of the digital economy and independent contractor “gig work” allows individuals to shift the source of their income. At the same time, individuals are also shifting their state of residence from high-tax to low-tax states at historic rates. Increased interstate migration, particularly of high-net-worth individuals and profitable closely-held businesses, has allowed income to migrate with individuals. The result is a genuine threat of multiple taxation for individuals and significant revenue losses for taxing jurisdictions. The stakes are enormous, as personal income tax regimes account for approximately one-quarter of all state and local tax revenues.

The solution—formulary apportionment—is a concept with which states are very familiar. Although formulary apportionment has been the prevailing paradigm for multistate corporate income taxation for decades, state legislatures and the existing literature have largely and surprisingly failed to recognize the promise of formulary apportionment for multistate personal income taxation. This Article remedies that oversight.