I live abroad in South Korea/Japan, and got married to a South Korean last year (2016). We plan to move back to the US after I've finished graduate school. We are both self-employed: I'm a software developer, and she did a little English tutoring work. The vast majority of our income comes from my work (her ~$4,000 to my $55,000).
I file taxes with the US and pay self-employment tax. I've done the math in Turbo Tax for both filing separately and filing jointly, and the later gives me a higher return. To do that, though, I have to treat her as a US Resident Alien even though she isn't one (see https://www.irs.gov/individuals/international-taxpayers/nonresident-spouse-treated-as-a-resident). This means I have to report her world-wide income, which, I think, makes her $4,000 subject to US self-employment tax, too (see https://www.irs.gov/individuals/international-taxpayers/self-employment-tax-for-businesses-abroad).
And so that is where the problem is: she doesn't have a SSN, and, to my understanding, doesn't qualify for one. So, if she pays this self-employment tax, will that not be applied towards her social security benefits? I feel like I've misunderstood something.