Community property states gave residents an unfair federal tax advantage.

Evidence

The Supreme Court thus created a situation where married couples with identical income and expenses but who lived in different states would pay different federal income tax bills. However, this was a violation of the horizontal equity principle underpinning the progressive tax system—and still only the 5 percent wealthiest Americans were paying in. They were determined to make a change. — Dorothy A. Brown, The Whiteness of Wealth, 89

Categorization

• DEFINE: Introduces the horizontal equity principle, a key concept for explaining why this tax rule is unfair.
• QUESTION: Worth chasing: how do later married-couple tax cases (Windsor) interact with the horizontal equity argument she establishes here?