Financial planners often grumble about what they call the "marriage tax." There's no such tax in real life, they just mean that in some respects single people get to enjoy deeper discounts on their taxes than married people. You may be tempted to file as single to take advantage of those deductions, but it's illegal and the repercussions can be serious.
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It's Really Black and White
If you look at the fine print that runs above the signature line on your 1040, you'll see that the IRS spells things out pretty clearly: You're declaring, under penalty of perjury, that the information you've included on your return is accurate. To put it even more bluntly, if you file as single when you're married under the IRS definition of the term, you're committing a crime with penalties that can range as high as a $250,000 fine and three years in jail. Whatever advantage you think you'll gain by filing as single, it's probably not enough to make those penalties look like a good risk.
Defining "Married" for Tax Purposes
Even if you don't think of yourself as married, or only "technically" married, it's what the IRS thinks that counts. Before you file as single, you can save yourself a lot of headaches by finding out whether the IRS considers you to be married. The basic criterion is simple: If you were legally married on December 31st, even if you've been living apart or only just got married at one minute to midnight on New Year's Eve, you're married for tax purposes. You're also married if you meet the definition of a common-law marriage in a state that recognizes those, or — and this part's a bit tricky — if you no longer live in a common-law state but met the definition of a common-law marriage before you moved from that state. You're also considered married if you have separated, or are in the process of divorce, but your divorce decree is not yet final.
A Few Exceptions
There are a few exceptions to that rule, but you'll need to be sure they apply to you before you take advantage of one. For example, some states consider you to be unmarried if you aren't yet divorced, but are legally separated and have a maintenance agreement in place. You may also be able to file as "head of household" if your spouse didn't live with you for at least the last half of the tax year, if you paid at least half the costs of maintaining your household for the year, and if you had a dependent person living with you — a child, handicapped adult or dependent parent — for at least half the year. Head-of-household status can be more advantageous than filing as single, but you'll need to be sure you fit the criteria. Otherwise, you'll eventually have to pay as married anyway, and may be liable for penalties on top of the difference in your taxes. It may be worth your while to pay a tax professional to make sure you get it right.