On the Rationality of the Post-Announcement Drift

This paper demonstrates that a post-announcement earnings drift, which is often advanced as an example of market irrationality, can arise even if traders act rationally on their information. Specifically, we show that in the presence of share supply variations which are unrelated to information, there is a positive correlation between the unexpected component of current public signals and future price changes. Such a correlation arises from the fact that while prices reveal private information that cannot be found in public signals, non-information based trading distorts the information content of prices relative to the implications of both private and public information. Under these circumstances, markets may appear semi-strong inefficient and slow to respond to earnings announcements even though information is processed in a timely and efficient manner. Our findings correspond well with previously documented empirical evidence and suggest that the robustness of earnings-based “anomalies” may be rational outcomes of varying uncertain share supply.