Common Factors in Return Seasonalities


Matti Keloharju, Ph.D., Juhani T. Linnainmaa, Ph.D., and Peter Nyberg, D.Sc.

A strategy that selects stocks based on their historical same-calendar-month returns earns an average return of 13% per year. We document similar return seasonalities in anomalies, commodities, international stock market indices, and at the daily frequency. The seasonalities overwhelm unconditional differences in expected returns. The correlations between different seasonality strategies are modest, suggesting that they emanate from different common factors. Our results suggest that seasonalities are not a distinct class of anomalies that requires an explanation of its own — rather, they are intertwined with other return anomalies through shared common factors. A theory that is able to explain the risks behind any common factor is thus likely able to explain a part of the seasonalities.


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