This is what it says in answer to that question on SSA's website http://www.socialsecurity.gov/pubs/10024.html:
Some people who get Social Security will have to pay taxes on their benefits. Less than one-third of our current beneficiaries pay taxes on their benefits.
You will have to pay taxes on your benefits if you file a federal tax return as an "individual" and your total income is more than $25,000. If you file a joint return, you will have to pay taxes if you and your spouse have a total income that is more than $32,000.
For more information call the Internal Revenue Service's toll-free number, 1-800-829-3676.
Publication 915 is also available on the IRS Web site http://www.irs.gov/formspubs/index.html?portlet=3
If you wish to have federal taxes withheld from your check, see "Can I have federal taxes withheld from my Social Security check?"
The Social Security Administration has no authority to withhold state or local taxes from your benefit. Many states and local authorities do not tax Social Security benefits. You should contact your state or local taxing authority for more information.
Tax law is very complex. Please talk to a tax specialist if you have any questions about taxes on your social security benefits.
Answered on Aug 24th, 2011 at 8:57 AM