This paper merges two independent projects, Campbell and Lettau (1999) and Malkiel and Zu (1999). Campbell and Lettau are grateful to Sangjoon Kim for his contributions to the first version of their paper, Campbell, Kim and Lettau (1994). We thank two anonymous referees and René Stulz for useful comments. Jung-Wook Kim and Matt Van Vlack provided able research assistance. The views are those of the authors and do not necessarily reflect those of the Federal Reserve Bank of New York, the Federal Reserve System or the National Bureau of Economic Research. Any errors and omissions are the responsibility of the authors.

This paper uses a disaggregated approach to study the volatility of common stocks at the market, industry, and firm levels. Over the period 1962-97 there has been a noticeable increase in firm-level volatility relative to market volatility. Accordingly, correlations among individual stocks and the explanatory power of the market model for a typical stock have declined, while the number of stocks needed to achieve a given level of diversification has increased. All the volatility measures move together countercyclically and help to predict GDP growth. Market volatility tends to lead the other volatility series. Factors that may be responsible for these findings are suggested.

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This paper studies the behavior of idiosyncratic volatility for the post war period. Using aggregate idiosyncratic volatility statistics constructed from the Fama and French (1993) three-factor model, we find that the volatility of individual stocks appears to have increased over time. This trend is not solely attributed to the increasing prominence of the NASDAQ market. We go on to suggest that the idiosyncratic volatility of individual stocks is associated with the degree to which their shares are owned by financial institutions. Finally, we show that idiosyncratic volatility is also positively related to expected earning growth. * This work was supported by the Princeton University Center for Economic Policy Studies Investigating the Behavior of Idiosyncratic Volatility AbstractThis paper studies the behavior of idiosyncratic volatility for the post war period. Using aggregate idiosyncratic volatility statistics constructed from the Fama and French (1993) three-factor model, we find that the volatility of individual stocks appears to have increased over time. This trend is not solely attributed to the increasing prominence of the NASDAQ market. We go on to suggest that the idiosyncratic volatility of individual stocks is associated with the degree to which their shares are owned by financial institutions. Finally, we show that idiosyncratic volatility is also positively related to expected earning growth.

This paper uses a disaggregated approach to study the volatility of common stocks at the market, industry, and firm levels. Over the period from 1962 to 1997 there has been a noticeable increase in firm-level volatility relative to market volatility. Accordingly, correlations among individual stocks and the explanatory power of the market model for a typical stock have declined, whereas the number of stocks needed to achieve a given level of diversification has increased. All the volatility measures move together countercyclically and help to predict GDP growth. Market volatility tends to lead the other volatility series. Factors that may be responsible for these findings are suggested.