Unmarried taxpayers who can claim a dependent should check to see if they qualify for the Head of Household filing status. Head of Household status will provide more tax benefits than claiming single.
The IRS says, "Your filing status is single if, on the last day of the year, you are unmarried or legally separated from your spouse under a divorce or separate maintenance decree, and you do not qualify for another filing status" (from IRS Publication 501, "Single").
Considered Unmarried
The IRS usually follows state law for determining marital status. You are "considered unmarried" for tax purposes if on the last of the year you are:- unmarried or
- legally separated from your spouse under a divorce or separate maintenance decree.
Domestic Partners are Considered Unmarried
The IRS does not follow state law for recognizing same-sex marriages. The Federal Defense of Marriage Act of 1996 defined marriage as "a legal union between one man and one woman as husband and wife, and the word 'spouse' refers only to a person of the opposite sex who is a husband or a wife."Some states, such as California, are requiring domestic partners to file tax returns as if they were married. Domestic partners should consult with an experienced tax professional for advice on filing their federal and state tax returns.

