Decomposing U.S. Stock Market Comovement into spillovers and common factors

• We identify spillovers and common factors in simultaneous heteroscedastic systems.
• Identification is achieved by ARCH processes of the structural shocks.
• The model is applied to Dow and Nasdaq stock returns.
• Spillovers from the Dow dominate, common factor exposure is rising.

This paper disentangles direct spillovers and common factors, the sources of correlations in simultaneous heteroscedastic systems. While these different components are not identifiable by standard means without restrictions, it is shown that they can be distinguished by specifying the variances of the latent idiosyncratic and common shocks as ARCH-type processes. By applying an adapted Kalman filter estimation method to Dow and Nasdaq stock returns, predominant spillovers from the Dow are found, as well as substantial rising factor exposure. While the latter is shown to prevail in the recent global financial crisis, volatility in the dot-com bubble period was driven by Nasdaq shocks.