Domicile

Residency and domicile are not the same thing, and states apply both. You can be a statutory resident of a state you are not domiciled in — typically by keeping a home there and spending more than a set number of days in it — which means two states can both treat you as a resident and both tax your worldwide income.

Changing domicile requires abandoning the old one and establishing a new one, and revenue departments examine the whole pattern of a life to decide whether that happened: where the family lives, where the belongings went, where you are registered to vote and licensed to drive, where the professional and social ties are, where you spend your time.

The states that audit this most aggressively are the high-tax ones people leave. A move that is real in intention but thin in evidence is the common failure, and it is expensive: a rejected change of domicile means the old state taxes the entire year.