Let me see if I can make a succinct restatement of my point re lying on mortgage applications. I think I meant this: let's stipulate that there are a lot of false statements on mortgage applications. Does this fact in itself mean that debtors are "responsible" for default? It does not. The question of ultimate responsibility a different issue--one that will depend, inter alia, on the principles of the law of misrepresentation.
And here's my point: this issue of "ultimate responsibility" is not a matter of "economics." It is a matter of (for lack of better name) moral philosophy. It may be that a truly hairy-chested economist will demonstrate that all questions of "moral philosophy" resolve themselves into questions of economics. But it hasn't been done yet.