Still wondering who pays more than half the household expenses here.  But also, does either parent have more than $16,530 in Social Security?  Because as MFS they would have to pay tax on 85% of anything above that amount.  And, the other spouse might have to file MFS also.  Even if it's less than that, how does the gross income test work in a situation like this?  If Mom gets $6,000 Social Security and files MFS, doesn't that make her AGI $5,100?