Soon forthcoming in the top-ranked Quarterly Journal of Economics is a very well- received paper by four economists with convincing evidence of what many believe was the primary cause of the subprime boom and bust: That securitization took away the incentive for lenders to properly vet borrowers.

But there's some new evidence questioning the paper's findings. To understand the how and why, we have to get into the nitty-gritty of the empirics.

If you plot FICO (i.e., credit) scores against the number of mortgages outstanding, you'll notice a peculiar pattern:

Instead of a smooth curve, at certain FICO scores there are big jumps in the number of people with mortgages.