The ecology of an order-driven market system

A securities market is order driven if the limit orders of some public participants establish the prices at which others can trade by market order at anp rime when the marker is open. The paper considers how, without market maker intermediation, a population of public investors separates into limit order and market order traders, and how the order driven marker achieves an ecological balance between the suppliers and the demanders of immediacy. It also explains why a bid-ask spread is a natural property of an order driven market, even when the market comprises a large number of participants irrespective of the technology sustaining the market. The paper considers a stylized environment that enables the optimal bid and ask quotes to be solved for, and the bid-ask spread to be analyzed. We summarize empirical evidence concerning price determination by the order driven marker, and discuss implications for marker structure that are suggested by the analysis.