if the relaxation of collateral constraints had been widespread, it should have resulted in a surge of mortgage debt relative to the value of real estate.
In the data, however, household debt and real estate values rose in tandem, leaving their ratio roughly unchanged over the first half of the 2000s, as shown in Figure 3. In fact, this ratio only spiked when home prices tumbled starting in 2006.
Pointer from Mark Thoma.
Suppose that back when lenders asked for 20 percent down, three families bought houses for $100,000 each and put $20,000 down each. Total mortgage debt is $240,000 and total home values are $300,000. The ratio of household to real estate debt is 80 percent.
Next, lenders allow someone to buy a house with no money down. As a result, home prices rise to $130,000. Adding $130,000 debt to the debt of the other three households (ignoring any equity they may have built up through paying down mortgage principal), we have total mortgage debt of $370,000. But total home values are $520,000, so that the average ratio of debt to equity has actually fallen, to just over 70 percent.
As long as home prices are rising, the last thing you should expect is for the average debt to equity ratio to rise. The fact that it did not fall is an indication of how powerful the boom in credit was. Only if you use a silly representative-agent model, in which there is no difference between average and marginal borrowers, would you predict something different. I have not read the paper, but I suspect that is what the authors did.