The states' income tax systems are important repositories of experience which confirm the administrative benefits of citizenship-based taxation. Domicile today plays an important role in state tax systems as a gap-filler when more objective statutory residence laws fail to assign any state of residence to the taxpayer. Citizenship is an administrable proxy for domicile and serves a similar gap-filling role in the federal taxation of individuals whose income and activities straddle across national boundaries.

The states' difficulties enforcing domicile-based taxation highlight the administrative benefits of citizenship-based taxation. As long as residence is understood for tax purposes in terms of domicile, citizenship is an efficient proxy for such domicile. The states' experience defining residence supports the United States' citizenship-based approach to federal income taxation. Under the Internal Revenue Code, citizenship serves as an administrable proxy for domicile and fulfills the same gap-filling function played by domicile under the states' income taxes.