This article examines how the extent of short-term trading relates to the efficiency of stock prices. We employ a new duration measure based on quarterly institutional investors’ portfolio holdings, next to existing proxies such as trading volume, the percentage of transient institutions, and fund turnover. Momentum returns and subsequent returns reversal are generally much stronger for stocks held primarily by short-term investors, especially if these investors recently had superior recent performance which could make them overconfident. Our results point toward the behavioral theory in Daniel, Hirshleifer and Subrahmanyam (1998) and seem inconsistent with short-term institutions improving efficiency.

We examine the pricing of both aggregate jump and volatility risk in the cross‐section of stock returns by constructing investable option trading strategies that load on one factor but are orthogonal to the other. Both aggregate jump and volatility risk help explain variation in expected returns. Consistent with theory, stocks with high sensitivities to jump and volatility risk have low expected returns. Both can be measured separately and are important economically, with a two‐standard‐deviation increase in jump (volatility) factor loadings associated with a 3.5% to 5.1% (2.7% to 2.9%) drop in expected annual stock returns.