Aggregate idiosyncratic volatility

"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"--National Bureau of Economic Research web site.