A friend of mine in Washington DC has just remortgaged his home, to take advantage of falling interest rates. Mortgages in the US tend to be fixed-rate, unlike the variable-rate loans we have in the UK. So if rates fall, you get a new loan at a lower rate and pay off the old one.
But my friend found a new line on the application form. He had to agree to give his lender access to his tax records. The reason is that in the boom, many borrowers simply made up their income figures when applying for loans. Often they would download the IRS tax form and just fill it with fanciful information. Sometimes, lenders would just ask them to scribble their income down on the loan application form, but it would never be checked.
Now you might think that if you were lending a large sum to someone, you'd want to be pretty sure that they could repay. Even if you could repossess their home should they default, that's a cost and nuisance you could do without. But in fact the lenders made a rational calculation. All their computer models showed that checking people's real financial circumstances was a waste of time. It didn't make much difference. Most people would actually make their mortgage payments. Delving into the details of their income and other outgoings was a waste of costly staff time.
They were undoubtedly right. But that was when everything was booming. While house prices were soaring, credit was easy, and jobs were plentiful, not many people defaulted, even those who fibbed about their real income. The trouble was that when everything went sour, people defaulted in legions.
The banks' error was the same as everyone's error. We actually believed that the boom was real. That we really had learned how to abolish slumps and run a continually expanding economy. I don't think you should blame the banks for the inevitable outcome of this hubris; you would be better blaming the politicians, monetary authorities, and regulators who created it in the first place.