The federal income tax system treats married couples as if each spouse earned approximately one-half of the couple's combined income through a mechanism called "income splitting. " For many one-earner and unequal-earner couples, income splitting produces a significant advantage, a "marriage bonus," by shifting income from higher to lower rate brackets. Marriage-based income splitting relies on a presumption that marriage is a good indicator of economic unity between two taxpayers. It is not. Marriage does not require spousal sharing, and many unmarried couples share everything they earn. As a result, the current system extends the benefit of income splitting to some taxpayers who do not deserve it while withholding it from others who do. Because marriage is a poor proxy for economic unity, this Article proposes a new eligibility criterion for income-splitting: only couples legally committed to sharing their income, regardless of marital status, would be permitted to file jointly.
Shari Motro, A New I Do: Towards a Marriage-Neutral Income Tax 91 Iowa L. Rev. 1509 (2006)