Worse yet, bank runs can be contagious. If depositors at one bank lose their money, depositors at other banks are likely to get nervous, too, setting off a chain reaction. And there can be wider economic effects: as the surviving banks try to raise cash by calling in loans, there can be a vicious circle in which bank runs cause a credit crunch, which leads to more business failures, which leads to more financial troubles at banks, and so on.

That, in brief, is what happened in 1930-1931, making the Great Depression the disaster it was. So Congress tried to make sure it would never happen again by creating a system of regulations and guarantees that provided a safety net for the financial system.

And we all lived happily for a while  but not for ever after.

Wall Street chafed at regulations that limited risk, but also limited potential profits. And little by little it wriggled free  partly by persuading politicians to relax the rules, but mainly by creating a “shadow banking system” that relied on complex financial arrangements to bypass regulations designed to ensure that banking was safe.

For example, in the old system, savers had federally insured deposits in tightly regulated savings banks, and banks used that money to make home loans. Over time, however, this was partly replaced by a system in which savers put their money in funds that bought asset-backed commercial paper from special investment vehicles that bought collateralized debt obligations created from securitized mortgages  with nary a regulator in sight.

As the years went by, the shadow banking system took over more and more of the banking business, because the unregulated players in this system seemed to offer better deals than conventional banks. Meanwhile, those who worried about the fact that this brave new world of finance lacked a safety net were dismissed as hopelessly old-fashioned.

In fact, however, we were partying like it was 1929  and now it’s 1930.

The financial crisis currently under way is basically an updated version of the wave of bank runs that swept the nation three generations ago. People aren’t pulling cash out of banks to put it in their mattresses  but they’re doing the modern equivalent, pulling their money out of the shadow banking system and putting it into Treasury bills. And the result, now as then, is a vicious circle of financial contraction.