If I am filing a separate return why do I have to list my spouse’s information on my return?
Even if you file separate returns (the worst way to file) you each have to list each other's SSN's and some other information on your own tax return. The IRS can then cross check to make sure you are not "double dipping" for itemized deductions, dependents, etc.
If you choose to file married filing separately, both spouses have to file the same way—either you both itemize or you both use standard deduction. Your tax rate will be higher than on a joint return. Some of the special rules for filing separately include: you cannot get earned income credit, education credits, or deductions for student loan interest. A higher percent of your Social Security benefits may be taxable. In many cases you will not be able to take the child and dependent care credit. If you live in a community property state, you will be required to provide additional information regarding your spouse’s income. If you are using online TurboTax to prepare your returns, you will need to prepare two separate returns and pay twice.