Wait there is more. To qualify for a mortgage, consumers must have enough income and assets to repay the loan, and lenders must determine that includes both principal and interest over the long term, not a discounted introductory period.
I would not call this "unintended consequences". Low documentation loans were definately part of the problem. Now the pendulum has swung to the other extreme. During the depths of the financial crisis I had to get a mortgage and the banks required reams of documentation, which they scrutinized, despite having a very good credit score. I even had to provide yet another pay stub just before closing to prove I still had a job.