There is a special rule if you are filing as Married Filing Separately and you lived with your spouse during the year.
That special rule requires you to include your Social Security benefit in your gross income when determining whether to file. If your gross income, including Social Security is more than $3,700, then you have to file, and then 85% of your Social Security benefit is taxable. The $25,000 exclusion does not apply if you are filing MFS and live with your spouse.
You should carefully read the text in Table 1 of Page 3, IRS Pub. 501
http://www.irs.gov/pub/irs-pdf/p501.pdf**Gross income means all income you received in the form of money, goods, property,
and services that is not exempt from tax, including any income from
sources outside the United States or from the sale of your main home (even if you
can exclude part or all of it). Do not include any social security benefits unless (a)
you are married filing a separate return and you lived with your spouse at any time
during 2011 or (b) one-half of your social security benefits plus your other gross
income and any tax-exempt interest is more than $25,000 ($32,000 if married filing
jointly). If (a) or (b) applies, see the Form 1040 instructions to figure the taxable part
of social security benefits you must include in gross income. Gross income includes
gains, but not losses, reported on Form 8949. Gross income from a business means,
for example, the amount on Schedule C, line 7, or Schedule F, line 9. But in figuring
gross income, do not reduce your income by any losses, including any loss on
Schedule C, line 7, or Schedule F, line 9.
And then, in the same publication, carefully read all of the information about Married Filing Separately.
Each year you can choose to file as Married Filing Separately. However, that may not provide the benefit that you expect, and you will almost always end up paying more in tax than if you file jointly.
The Married Filing Separately filing status is very different than the Single filing status. There are a number of severe restrictions on deductions and credits, and on the amount of IRA contributions that you can deduct, especially if you live together with your spouse.
You can not take the EIC,
You can not take the credit for Child and Dependent Care, in most cases,
You can not take the Education credits/deductions, and there are many other restrictions.
If either of you receive Social Security benefits and you live with your spouse, more of the SS benefit will be taxable, and the person receiving it will have to include the SS benefit in their gross income when determining whether they have to file. If one of you itemizes deductions, the other must also itemize even if they have nothing to itemize.
Before you decide, you should carefully read the restrictions that go with MFS in IRS Pub. 501, at this link:
http://www.irs.gov/pub/irs-pdf/p501.pdfYou should carefully read the limits on IRA deductions in IRS Pub. 590-A at this link:
http://www.irs.gov/pub/irs-pdf/p590a.pdfIn addition, if you live in a Community Property state, there are special rules you must follow for reporting income and expense. For further information on that, see IRS Pub. 555, at this link:
http://www.irs.gov/pub/irs-pdf/p555.pdfand/or the Turbotax FAQ at this link:
https://ttlc.intuit.com/questions/1901162-married-filing-separately-in-community-property-states
