Is illiquidity a risk factor? A critical look at commission costs

A quarterly time series of the aggregate commission rate for NYSE trading over 1980-2003 allowed an investigation of the information conveyed by this liquidity risk metric and analysis of its critical role in the generation of stock returns. The aggregate commission rate was found to be highly correlated with other illiquidity metrics, such as the bid-ask spread. The rate is significantly and positively related to the excess returns of the stock market portfolio and has significant explanatory power for the cross-sectional variation in stock returns. An analysis of size-based portfolios indicates that returns become more sensitive to the aggregate commission rate with declining market capitalization.