We investigate the relative importance of various factors in explaining the volatility skew observed in the prices of stock options traded on the Chicago Board Options Exchange. The skewness of the risk-neutral density implied by individual stock option prices tends to be more negative for stocks that have larger betas, suggesting that market risk is important in pricing individual stock options. Also, implied skewness tends to be more negative in periods of high market volatility, and when the risk-neutral density for index options is more negatively skewed. Other firm-specific factors, including firm size and trading volume a so help explain cross-sectional variation in skewness. However, we find no robust relationship between skewness and the firm's leverage. Nor do we find evidence that skewness is related to the put/call ratio, which may be viewed as a proxy for trading pressure or market sentiment. Overall, firm-specific factors seem to be more important than systematic factors in explaining the variation in the skew for individual firms.

We investigate the relationship between the ownership structure and returns of firms on days when the absolute value of the market's return is two percent or more. We find that a firm's abnormal return on these days is related to the percentage of institutional ownership, that there is abnormally high turnover in the firm's shares on these days, and that this abnormal turnover is significantly related to the percentage of institutional ownership in the firm. Taken together, these results are consistent with positive feedback herding behavior on the part of some institutions, particularly mutual and pension funds.