[We empirically demonstrate that the opportunities the Boston Stock Exchange and the Cincinnati Stock Exchange offer members to take the other side of their customers' orders through affiliated market makers (to internalize orders) have little short-run effect on posted or effective bid-ask spreads. This is true despite substantial movement of order flow away from the New York Stock Exchange when trading under one of these regional stock exchange programs begins. These results contrast with the adverse effects of market fragmentation and internalization predicted by some theoretical market microstructure analyses and the popular financial press.]

We empirically demonstrate that the opportunities the Boston Stock Exchange and the Cincinnati Stock Exchange offer members to take the other side of their customers’ orders through affiliated market makers (to internalize orders) have little short-run effect on posted or effective bid-ask spreads. This is true despite substantial movement of order flow away from the New York Stock Exchange when trading under one of these regional stock exchange programs begins. These results contrast with the adverse effects of market fragmentation and internalization predicted by some theoretical market microstructure analyses and the popular financial press.

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.