When would a married couple actually save money by filing separately?

Married filing separately usually costs more because it disqualifies you from several credits and shrinks some thresholds, but it wins in specific cases. It can help when one spouse has high medical bills, since the AGI floor for deducting them is based on that spouse's lower separate income. It can also lower payments on income-driven student loan plans, which may key off individual income. Couples wanting to keep tax liabilities legally separate, or protect one spouse from the other's issues, may choose it too. The catch: if one itemizes, both must, and you lose credits like the Earned Income Tax Credit. Always compute both ways, because the IRS rules make separate filing a case-by-case call.