Although private financial institutions played a key role in the booms and busts of both Japan and the U.S., monetary policy was a critical root cause. In both cases, the central bank helped set off a boom in asset prices by expanding credit and driving interest rates to artificially low levels. This encouraged individuals and businesses to take on debt they otherwise would not have accepted and make investments they otherwise would not have considered.
When a central bank inflates the money supply and drives interest rates below those that would exist in a free marketplace, it sends a false signal to businesses to borrow and invest more in capital projects and goods than they otherwise might. Similarly, consumers respond to the signal by taking on higher mortgage and/or credit card debt, saving less, and spending more. Credit binges cannot last forever; when interest rates increase again, the bad investments are revealed, and it becomes painfully clear that much of the outstanding credit cannot be paid back.
Between January 1986 and February 1987, the Bank of Japan cut its discount rate — the interest rate charged by the central bank on loans to its member banks — from 5 percent to 2.5 percent, leading to an increase in real estate and stock market prices. Realizing a bubble was forming, the central bank then raised rates five times in 1989 and 1990, to a high of 6 percent. This increase revealed that many investments were built on extensive, unsustainable debt.