Domestic partners don't get unique treatment from the Internal Revenue Service when it comes to dependent deductions. They qualify only if they meet the same criteria imposed on all adult dependents. In some respects, your partnership can count against you, but for the most part, it's treated as a neutral circumstance. In and of itself, it doesn't have much effect on your taxes one way or the other.
IRS rules for a qualifying adult dependent require that your partner either live in your home through the entire tax year or be your relative. Your domestic partnership doesn't count as being related, so this means you and your partner must reside together from January 1 through December 31 if you're going to claim her as your dependent.
Your partner must also rely on you for her support, and the IRS has two rules that apply to this qualifying factor. First, her income from all sources can't exceed the amount of the year's dependent deduction -- $3,800 as of 2012. The deduction goes up periodically to keep pace with inflation. You must also pay for more than half her living expenses, such as rent or mortgage, utilities and groceries. If she contributes her entire $3,800 to your household, the portion of the expenses attributable to her must be at least $7,601, and you must pay $3,801 of that. The support rule is based on what she actually contributes, not her earnings, so if she saves her entire income and doesn't contribute anything, you've met the test.
The IRS rarely makes things easy, and this is true with respect to qualifying adult dependents as well. Assuming your partner meets the residence, income and support tests, the IRS still imposes some additional criteria. She must be either a U.S. citizen or a U.S. national, or a resident of the United States, Mexico, or Canada. She can't qualify as anyone else's dependent, such as her parents. This is true even if they don't claim her – the fact that they could if they wanted to disqualifies your partner as your own dependent. Even if your partner meets all the IRS rules, claiming her doesn't allow you to file as head of household unless another dependent lives with you, so you miss out on this tax advantage.
Registered domestic partnerships are not always limited to same-sex couples. For example, New Jersey recognizes partnerships between opposite-sex partners if both parties are 62 years of age or older. If your partner is 62 or older and she's collecting Social Security benefits, this typically doesn't count toward her income for purposes of determining if she earns more than the dependent deduction.
- Liberty Tax Service: Tax Guide for Dependents
- IRS: Personal Exemptions and Dependents
- IRS: Answers to Frequently Asked Questions for Same Sex Couples
- IRS: Questions and Answers for Registered Domestic Partners and Same Sex Spouses in Community Property States
- IRS: In 2012, Many Tax Benefits Increase Due to Inflation Adjustments
- New Jersey Department of Health: Registering a Domestic Partnership in New Jersey (PDF)