Marriage tax brackets reward single-earner households and penalize dual earners.

Evidence

Let’s look at the tax rates from 1975, six years into tax reform: For a single wage earner with 45,000 of taxable income, black or white, her first 500 of taxable income was subject to a 14 percent tax, while her last 500 of income (income greater than 44,000) was taxed at 60 percent. Her marginal tax rate (the highest tax rate she is subject to) was 60 percent, the highest a single wage earner could be subjected to under the new law. (Yes, significantly higher than our current top marginal rate of 37 percent.) … A single wage earner making half that income—22,500 (half of 45,000)—had a marginal tax rate of 40 percent. But if two singles each earning 22,500 got married—and statistically, if they were a black couple, kept earning two incomes—their last 1,000 of income (income greater than 44,000) was taxed at 50 percent, an increase of 10 percentage points from what they paid as unmarried individuals. A household with one wage earner making 45,000 a year and one earning no income—statistically speaking, most likely a white couple—had a marginal tax rate of 50 percent, too. But this represents a decrease of 10 percentage points from the single wage earner’s marginal tax rate when unmarried. When the two 22,500 wage earners get married, their marginal tax rates increase from 40 to 50 percent, but when the 45,000 wage earner marries a nonworking spouse, his marginal tax rate decreases from 60 to 50 percent. That is how married white couples got a tax cut and married black couples paid higher taxes for equal household income. — Dorothy A. Brown, The Whiteness of Wealth, 121

Categorization

• XREF: This is the concrete, falsifiable mechanism the reader needs — the exact rate arithmetic showing how the penalty shifts with the 50-50 vs 0-100 split.
• DEFINE: Defines ‘marginal tax rate’ concretely with worked numbers.