Why file single on your federal taxes when you are married and does it effect your spouse if one of you dies?

If you are married, you cannot (and should not) file single. Your choices are Married Filing Jointly or Married Filing Separately. The only time I usually see a Married Filing Separate return is either if the spouses, as a rule, just keep all of their finances separate, or if one of them owes taxes. Remember if you owe taxes the IRS will keep your refunds to apply to that balance due, so if only one of you owes taxes you can file Married Filing Separately and the one of you that does not owe taxes can still get their refunds. Also if you are going to owe on a tax return and file that return as Married Filing Separate, and then later die, your widow will not be responsible for the taxes.