Those that had doubts about the importance of the innovation or the persistence of the gain in asset prices lose confidence in their opinions as they underperform and lose business and market share. Those who believed that the large gains in asset prices were justified by the innovation and who benefited from those beliefs become more dominant. Casual investors see the large rise in prices and jump in. This shift is important because, in markets, prices are driven by the marginal investor. As new and often less well-informed investors plunge in to participate in the boom, they can overwhelm the so-called “smart money” that gets frustrated after having lost repeatedly trying to take the other side. In this respect, a bias toward optimism may also play an important role. Studies have found that most people believe that they are above average in terms of their acumen, be it as investors, car drivers or in other activities. 5 This overconfidence may cause some people to keep investing in the asset, even when they are skeptical about its valuation because they are overly confident that they will anticipate the end of the bubble and be able to get out in time. Fifth, asset bubbles occur more easily when it is difficult to short the assets.
Despite the fact that it is hard to discern bubbles, especially in their early stages, I conclude that uncertainty is not grounds for inaction. Instead, the decision whether to act depends on whether appropriate tools can be deployed to limit the size of a bubble and whether the benefits of acting and deploying such tools are likely to exceed the costs. That cost-benefit calculus, in turn, depends crucially on the tools we can deploy to limit the growth of bubbles and the consequences when they burst. In this respect, I will argue that, in most cases, use of the bully pulpit and macroprudential tools, such as rules limiting loan-tovalue ratios or leverage, are likely to prove superior to monetary policy. Turning to the first issue of whether there are asset bubbles, I am going to be a bit of a heretic and argue that there is little doubt that asset bubbles exist and that they occur fairly frequently. By an asset bubble, I mean price increases (or declines) that become unmoored from fundamental valuations. I want to be clear that I am distinguishing this from price movements that are tied to changes in fundamentals. I know this runs afoul of the efficient markets hypothesis – which in this context would argue that if a bubble were obvious, then people would take the other side and the bubble would not occur in the first place. There are several reasons why this argument does not hold in practice.