Momentum is Not an Anomaly Robert F. Dittmar, Gautam Kaul, and Qin Lei October 2007 Dittmar is at the Ross School of Business, University of Michigan (email:
[email protected]). Kaul is at the Ross School of Business, University of Michigan (email:
[email protected]). Lei is at the Cox School of Business, Southern Methodist University (email:
[email protected]). We thank Jennifer Conrad, Rex Thompson, and seminar participants at Southern Methodist University for thoughtful comments. We retain the responsibility for all remaining errors. Abstract In this paper, we develop a new approach to test whether momentum is indeed an anomaly in that it re‡ects delayed reactions, or continued overreactions, to …rm speci…c news. Our methodology does not depend on a speci…c model of expected returns and, more importantly, does not require a decomposition of momentum pro…ts. Yet we provide distinct testable predictions that can discrim- inate between the two diametrically opposed causes for the pro…tability of momentum strategies: time-series continuation in the …rm-speci…c component of returns, and cross-sectional di¤erences in expected returns and systematic risks of individual securities. Our results show that, contrary to the common belief in the profession, momentum is not an anomaly; we …nd no evidence of continuation in the idiosyncratic component of individual-security returns. The evidence is instead consistent with momentum being driven entirely by cross-sectional di¤erences in expected returns and risks of individual securities. 1 Introduction The proposition that the idiosyncratic component of individual …rms’returns displays continuation now appears to be widely accepted among academics and practitioners.