This article compares the bid‐ask spread for New York Stock Exchange (NYSE)‐listed securities before and after a major third market broker‐dealer, Bernard L. Madoff Investment Securities (Madoff), begins to selectively purchase and execute orders in those securities. Tests reveal the quoted bid‐ask spread tightens when Madoff enters the market. Furthermore, trading costs as measured by the difference between the transaction price and the midpoint of the contemporaneous bid‐ask spread do not increase. Together, these results suggest that the adverse selection problem associated with allowing agents to selectively execute orders in exchange‐listed securities may be economically insignificant.

The National Association of Security Dealers alleges that professional-trader use of the Small Order Execution System (SOES) causes greater security price volatility. We docu? ment bidirectional Granger causality between a proxy for professional SOES trading (the frequency of maximum-sized SOES trades) and a measure of stock price volatility. We find that high levels of volatility precede high levels of maximum-sized SOES trades, suggesting that volatility causes more frequent large SOES trades. Likewise, over a one-minute time interval, high levels of maximum-sized SOES trades cause high volatility. Over longer periods, however, intense maximum-sized SOES trading causes lower volatility. Inter? preted in conjunction with Harris and Schultz (1997), these results suggest that high levels of maximum-sized SOES trades lead to more efficient price discovery. In light of these results, we believe that efforts to eliminate SOES based on volatility considerations are unwarranted.