Because bubbles are essentially social-psychological phenomena, they are, by their very nature, difficult to control. Regulatory action since the financial crisis might diminish bubbles in the future. But public fear of bubbles may also enhance psychological contagion, fueling even more self-fulfilling prophecies.
One problem with the word bubble is that it creates a mental picture of an expanding soap bubble, which is destined to pop suddenly and irrevocably. But speculative bubbles are not so easily ended; indeed, they may deflate somewhat, as the story changes, and then reflate.
Trying to understand the mechanism, and the such, is indeed important for policy.
If you look closely, the parallels to the Fed’s dramatic QE policies and current financial stability concerns are uncanny. In both stories, the recession was identified as the result of speculative excess. In response to the crash, both times the Federal Reserve embarked on a program of monetary easing. However, in both instances excess reserves failed to budge, and this was interpreted as a sign that banks just didn’t want to lend — the Fed was pushing on a string. Finally, as excess reserves persisted, the threat of “speculative purposes” was used to bully the Fed into tightening. The key difference between now and then is that we have a Fed that recognizes its role in supporting the real recovery. Those in 1936 were not as lucky.
Why did the Fed go on such a destructive path in the 1930’s? Rotemberg identifies the tightness of policy as a consequence of something called the “real bills doctrine”. Under the real bills doctrine, the Fed saw its role as providing credit so that there was enough, and no more, credit to invest in “productive uses”. Since the Great Depression was preceded by a speculative stock bubble, then Fed officials put a premium on making sure credit was put to “productive uses”; The real bills doctrine was the result. According to this doctrine, monetary policy should tighten in recessions when demand for credit falls so as to make sure what credit remains is put towards productive uses. Conversely, monetary policy should ease in booms because firms are looking to find credit to fund their projects. In other words, the real bills doctrine prescribed a procyclical monetary policy.
This goes to show that we need to avoid framing effects when thinking about monetary policy. Because the Great Depression was the result of an equity bubble, then the economists of the day were so concerned about bubbles that they pursued destructive monetary policy. It is just as important to not make the same mistake today. As the real bills doctrine shows, using the tools of financial economics to solve monetary problems can be very destructive.
I would note that the GD likely wasn’t caused by an equity bubble – it was believed to have been caused by an equity bubble. The fallacy of composition was alive and well in the interpretation of macroeconomics!