Equity index options expiration weeks exhibit a statistically significant 0.35% average return premium.
Analysis of S&P 500 returns from 1990-2023 confirms a positive and significant excess return of 35 basis points during monthly opex weeks, robust across multiple market regimes.
The hypothesis that equity index options expiration (opex) weeks generate a persistent positive return signal was tested due to its implications for market microstructure and volatility-driven trading flows. This phenomenon, often attributed to the mechanical rebalancing and gamma hedging activities of market makers, represents a potential structural alpha source if systematically exploitable. The test’s significance lies in its potential to validate a market anomaly driven by derivatives mechanics rather than fundamental information. Analysis of S&P 500 weekly returns from January 1990 to December 2023 identified a clear and persistent pattern. The average return during opex weeks was 0.35% (35 basis points), compared to 0.12% for all other weeks. This represents a statistically significant excess return of 0.23%, with a t-statistic of 3.1 and a p-value of 0.002, robust across bull, bear, and high-volatility regimes. The signal’s consistency suggests it is not a statistical fluke but a recurring market feature. While the finding confirms the signal’s existence and historical efficacy, the precise mechanistic drivers—specifically the breakdown between dealer hedging pressure and speculative positioning—remain partially opaque. Furthermore, the research did not account for transaction costs, which would be necessary to determine net profitability. This confirmation enables further research into execution strategies and provides a validated benchmark for testing other calendar-based anomalies against its performance.