It seems to me that if I'm paying current expenses from my "stake", that I'll need to make up the $$ I don't draw from SocSec. So in your example, defering 1 year would not result in a 0 in the NPV calc, but rather, in a negative $18,000. This, in effect, looks at SocSec in the context of my entire stake, rather than in isolation. I think that this approach would change the outcome. What say you??