This study re-examines the empirical validity of the Capital Asset Pricing Model (CAPM) by asking whether the Security Market Line (SML) remains stable when markets experience asymmetric volatility and regime transitions. Using high-frequency data from 2015 to 2024 for four major U.S. large-cap firms, we implement a unified empirical framework that integrates quantile regression, rolling-window CAPM estimation, and a two-state Markov-Switching AR(1) model.
The results reveal that the beta–return relationship is non-linear, time-varying, and state-dependent. Beta significance concentrates around the median of the return distribution but vanishes in the tails, confirming distributional asymmetry. Rolling estimations indicate persistent yet gradual beta drift, while the Markov-Switching model uncovers regime-contingent SML slopes—positive in stable phases and inverted during stress periods. These findings demonstrate that systematic risk is not uniformly priced and that the classical CAPM’s assumptions of linearity, stationarity, and rational expectations fail under dynamic market conditions.
Beyond U.S. equities, the framework offers a diagnostic tool for risk monitoring and portfolio management in environments characterized by volatility shocks and behavioral frictions—conditions typical of many emerging markets. By integrating dynamic, distributional, and regime dimensions, the paper contributes to a more adaptive understanding of asset pricing under uncertainty.
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