Joint Return Test : <a rel="nofollow" target="_blank" href="
https://www.irs.gov/publications/p17/ch03.html">https://www.irs.gov/publications/p17/ch03.html</a>
You generally cannot claim a married person as a dependent if he or she files a joint return.
Exception.... You can claim an exemption for a person who files a joint return if that person and his or her spouse file the joint return only to claim a refund of income tax withheld or estimated tax paid.
This means they can file a joint return if they are not otherwise required to file a return and they file ONLY to get back withheld or estimated taxes ... that they will not and do not have a tax liability or any other credits to claim.
