Abstract We examine stock returns, order flow, and market conditions in the minutes before, during, and after recent short sales on the NYSE and Nasdaq. We find two very distinct types of short sales: those that provide liquidity, and those that demand it. Shorts that supply liquidity do so when spreads are unusually wide. These short sellers are also strongly contrarian, stepping in to initiate or increase a short position after fairly sharp share price rises over the past hour or so, and they tend to face greater adverse selection than other liquidity suppliers. In contrast, shorts that demand liquidity tend to be shortterm momentum traders. However, there is no evidence that liquidity-demanding short sellers are any different from other liquidity demanders. Overall, liquidity-providing short sales are important contributors to stock market quality, and regulators and policymakers should keep these salutary effects in mind. JEL classification: G14, G1
