For the purposes of taking a deduction for an IRA contribution, the IRS provides special allowances for individuals who are married, file separate returns and lived apart for the entire year by treating these individuals as "single" filers.
1. If you and your spouse did not live together at any time during the year, then you are considered "single" for tax filing purposes and should use the guidelines for a single taxpayer.
2. If you and your spouse did not live together at any time during the year, you are each allowed a full deduction.
A person who was married during the tax year, regardless of whether he or she lived with the spouse, has a filing status of married filing jointly or married filing separately unless he or she qualifies for the exception that requires a separate household maintained for a dependent/child for a certain number of months of the year. The IRS provides that certain married individuals who lived apart from their spouses for the last six months of the year and are eligible to claim someone as a dependent may use the head of household filing status for tax filing purposes.
This question was answered by Denise Appleby