Asset Price Bubbles: A Selective Survey. By Anna Scherbina
IMF Working Paper No. 13/45
Feb 21, 2013
Summary: Why do asset price bubbles continue to appear in various markets? This paper provides an overview of recent literature on bubbles, with significant attention given to behavioral models and rational models with frictions. Unlike the standard rational models, the new literature is able to model the common characteristics of historical bubble episodes and offer insights for how bubbles are initiated and sustained, the reasons they burst, and why arbitrage forces do not routinely step in to squash them. The latest U.S. real estate bubble is described in the context of this literature.
The persistent failure of present-value models to explain asset price levels led academic research to introduce the concept of bubbles as a tool to model price deviations from presentvalue relations. The early literature was dominated by models in which all agents were assumed to be rational and yet a bubble could exist. In many of the more recent papers, the perfect rationality assumption was relaxed, allowing the models to shift the focus to explaining how a bubble may be initiated, under which conditions it would burst, and why arbitrage forces may fail to ensure that prices reflect fundamentals at all times. In light of the recent U.S. real estate bubble, the question of why bubbles are so prevalent is once again a matter of concern of academics and policy makers. This paper surveys the recent literature on asset price bubbles, with significant attention given to behavioral models as well as rational models with incentive problems, market frictions, and non-traditional preferences. For surveys of the earlier literature, see, e.g., Camerer (1989) and Stiglitz (1990).
There are a number of ways to define a bubble. A very straightforward definition is that a bubble is a deviation of the market price from the asset’s fundamental value. Value investors specialize in finding and investing in undervalued assets. In contrast, short sellers, who search the market for overvalued assets in order to sell them short, are routinely vilified by governments, the popular press, and, not surprisingly, by the overvalued firms themselves.1 Trading against an overvaluation involves the additional costs and risks of maintaining a short position, such as the potentially unlimited loss, the risk that the borrowed asset will be called back prematurely, and a commonly charged fee that manifests itself as a low interest rate paid on the margin account; for this reason, a persistent overvaluation is more common than a persistent undervaluation.
A positive or negative mispricing may arise when initial news about a firm’s fundamentals moves the stock price up or down and feedback traders buy or sell additional shares in response to past price movement without regard for current valuation, thus continuing the price trend beyond the value justified by fundamentals.2 However, because of the potentially nontrivial costs of short selling an overvaluation will be less readily eliminated, making positive bubbles more common. The paper will, therefore, focus predominately on positive price bubbles. We can define a positive bubble occurring when an asset’s trading price exceeds the discounted value of expected future cash flows (CF):
where r is the appropriate discount rate.3 Since it may be difficult to estimate the required compensation for risk, an alternative definition may be used that replaces the discount rate with the risk-free rate, r sub f:
[same formula with r sub f instead of r].
When the asset’s cash flows are positively correlated with market risk, as is the case for most firms, the required rate of return is strictly greater than the risk-free rate and the discountedcash- flow formula represents an upper limit of the justifiable range of fair values. Likewise, when it is difficult to forecast future cash flows for a particular asset or firm, an upper bound of forecasted cash flows for other firms in the same industry or asset class may be used.
Over the years, the academic study of bubbles has expanded to explore the effects of perverse incentives and of bounded rationality. The new generation of rational models identifies the incentive to herd and the limited liability compensation structure as pervasive problems that encourage professional money managers to invest in bubbles. Another problem contributing to bubbles is that information intermediaries are not paid directly by investors, and their incentives are not always compatible with reporting negative information. And rather than merely trying to answer under what conditions bubbles may exist in asset prices, behavioral models offer new insights for how a bubble may be initiated, under which conditions it would burst, and why arbitrage forces may fail to ensure that prices reflect fundamentals at all times. Moreover, some models offer the explanation for why many bubble episodes are accompanied by high trading volume. The behavioral view of bubbles finds support in experimental studies.