X-CAPM: An Extrapolative Capital Asset Pricing Model

by Nicholas Barberis, Robin Greenwood, Lawrence Jin & Andrei Shleifer

Overview — Many investors assume that stock prices will continue rising after they have previously risen, and will continue falling after they have previous fallen. This evidence, however, does not mesh with the predictions of many of the models used to account for other facts about aggregate stock market prices. Indeed, in most traditional models, expected returns are low when stock prices are high: in these models, stock prices are high when investors are less risk averse or perceive less risk. In this paper, the authors present a new model of aggregate stock market prices which attempts to both incorporate expectations held by a significant subset of investors, and address the evidence that other models have sought to explain. The authors' model captures many features of actual returns and prices. Importantly, however, it is also consistent with survey evidence on investor expectations. This suggests that the survey evidence is consistent with the facts about prices and returns and may be the key to understanding them. Key concepts include:

The model in this study can reconcile evidence on expectations with the evidence on volatility and predictability that has animated recent work in this area.
A new model like this one is needed. Traditional models of financial markets have been able to address pieces of the existing evidence, but not the data on investor expectations. The same holds true for preference-based behavioral finance models as well as for the first generation belief-based behavioral models.

Author Abstract

Survey evidence suggests that many investors form beliefs about future stock market returns by extrapolating past returns: they expect the stock market to perform well (poorly) in the near future if it performed well (poorly) in the recent past. Such beliefs are hard to reconcile with existing models of the aggregate stock market. We study a consumption-based asset pricing model in which some investors form beliefs about future price changes in the stock market by extrapolating past price changes, while other investors hold fully rational beliefs. We find that the model captures many features of actual prices and returns, but is also consistent with the survey evidence on investor expectations. This suggests that the survey evidence does not need to be seen as an inconvenient obstacle to understanding the stock market; on the contrary, it is consistent with the facts about prices and returns and may be the key to understanding them.