This week, the court will hear a case challenging the Defense of Marriage Act, a 1996 law that prevents same-sex couples from receiving more than 1,000 federal benefits that opposite-sex married couples receive.
This includes the right to file federal taxes jointly -- which, depending on income, gives some married filers a "bonus" of thousands of dollars, while penalizing others.
A same-sex couple with combined income of $100,000, in which one person earns $70,000 and the other makes $30,000, currently pays an extra $1,625 per year by filing separately rather than jointly, according to an analysis H&R Block conducted for CNNMoney. The calculations assume a standard deduction, no children and no tax credits.
The extra tax liability jumps to nearly $8,000 when one spouse earns all $100,000 and the other reports no income. In this case, couples filing jointly owe tax of $11,858, while a same-sex couple filing separately owes $19,585 -- a 65% difference.
"[There's] a myth that any time married people file jointly they are worse off than filing singly, and that's just not correct at all -- sometimes they get a marriage bonus," said Jackie Perlman, a principal analyst at H&R Block.
That's because filing jointly merges the two incomes, shifting some of the higher-earning spouse's income into a lower tax bracket. In some scenarios, couples would even cut their tax bills in half by filing jointly -- typically when incomes are low, Perlman said.
As the gap between incomes shrinks, however, the difference in tax liability is less pronounced. In H&R Block's scenario, no extra tax is owed when each spouse earns an income of $50,000 and they file jointly instead of individually.
Other couples would end up owing more by filing jointly, especially if they miss out on deductions or credits like the Earned Income Tax Credit and the Child Tax Credit because, when combined, their income is no longer low enough to qualify or receive the full benefit.
Another tax issue at stake in the DOMA case is the estate tax. Currently, surviving spouses in federally-recognized marriages don't have to pay taxes on their deceased spouse's estate, while same-sex widows pay a 35% estate tax on anything in excess of a $5 million exemption.
The case challenging DOMA was filed by New Yorker Edith Windsor, who sued to get back the $363,000 in estate taxes she paid when her partner of more than 40 years died.
Her arguments are being presented on Wednesday. Meanwhile, opposition to the law is growing -- with the Obama Administration, a coalition of big businesses and even a group of prominent Republicans all signing legal briefs in support of gay marriage.
If the court decides to overturn DOMA, it could significantly impact the financial lives of same-sex couples married at the state level. But it's up in the air whether federal benefits would be extended to domestic partnerships and civil unions. Currently, same-sex marriage is legal in nine states and Washington, D.C.
In addition to not being able to file jointly and owing extra estate tax in certain cases, many same-sex couples owe tax on medical benefits received through a partner's employer-sponsored health insurance plan, are denied thousands of dollars in spousal Social Security benefits or don't qualify for survivors benefits if a spouse or partner passes away.
Mikey Rox and Earl Morrow, from New York City, would boost their refund by nearly $2,000 per year by filing jointly.
And the $2,500 in tax they currently pay on the health insurance benefits Mikey receives from Earl's plan would vanish.
Some couples could even get refunded for the extra tax they paid in the past three years as well, if they file protective refund claims with the IRS and amend their returns to file jointly. Adele and Jennifer Hoppe-House, from Los Angeles, expect to get more than $13,000 back by doing this if DOMA is struck down.
Even for those who would owe more tax if they were allowed to file jointly, the extra money is often a small price to pay to see DOMA overturned.
"I'm sure more people are going to get financial wins than losses, whether it's taxes or Social Security," said Nanette Miller, head of the LGBT practice at accounting firm Marcum LLP. "But it's not just an economic issue -- it's that they want that equality."
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