By Robin Greenwood, Andrei Shleifer, and Yang You from HBS.
Nobel laureate Eugene F. Fama has famously claimed that there is no such thing as a bubble, which he defines as a large price run-up that predictably crashes. Analyzing industry data for the US and internationally, the authors find that Fama is mostly right that a sharp price increase of an industry portfolio does not, on average, predict unusually low returns going forward. Yet the authors show that there is much more to a bubble than merely increases in prices; they show a number of characteristics that predict an end to the bubble.
We evaluate Eugene Fama’s claim that stock prices do not exhibit price bubbles. Based on U.S. industry returns 1926–2014 and international sector returns 1985–2014, we present four findings: (1) Fama is correct in that a sharp price increase of an industry portfolio does not, on average, predict unusually low returns going forward; (2) such sharp price increases predict a substantially heightened probability of a crash; (3) attributes of the price run-up, including volatility, turnover, issuance, and the price path of the run-up, can all help forecast an eventual crash and future returns; and (4) some of these characteristics can help investors earn superior returns by timing the bubble. Results hold similarly in U.S. and international samples.
Source: Bubbles for Fama