Score contribution per author:
α: calibrated so average coauthorship-adjusted count equals average raw count
In August 1999, U.S. exchanges began to compete directly for order flow in many options that had been exclusively listed on another exchange, shifting 37% of option volume to multiple‐listing status by the end of September. Effective and quoted bid–ask spreads decrease significantly after multiple listings with spreads generally maintaining their initial lower levels 1 year later. These results hold for both time series and pooled regressions and are robust. We reject that economies of scale in market making cause the decrease in spreads and support the view that interexchange competition reduces option transaction costs.