Robert Hodrick

Aggregate Idiosyncratic Volatility

Coauthor(s): Geert Bekaert, Xiaoyan Zhang.

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Abstract:
We examine aggregate idiosyncratic volatility in 23 developed equity markets, measured using various methodologies, and we find no evidence of upward trends. Instead, idiosyncratic volatility appears to be well described by a stationary autoregressive process that occasionally switches into a higher-variance regime that has relatively short duration. We also document that idiosyncratic volatility is highly correlated across countries. Finally, we examine the determinants of the time-variation in idiosyncratic volatility. In most specifications, the bulk of idiosyncratic volatility can be explained by a growth opportunity proxy, total (US) market volatility, and in most but not all specifications, the variance premium, a business cycle sensitive risk indicator. Our results have important implications for studies of portfolio diversification, return volatility and contagion.

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