It actually gets more complicated.  You may find one person making the mortgage payments but the other pays utilities, food, child care, etc.  So an argument could be made that one is gifting the other.  Sometimes you can do the math and see who would benefit most by claiming the deduction(s).  The Tax Courts originally always went with whom paid the payment, but that was mainly before two unmarried people would own a home together.  Now we're seeing some more leniency in their decisions.