Zero-Intelligence Order Book Models

A striking finding in market microstructure research is that many features of a real limit order book, the shape of the price impact curve, the bid-ask spread, roughly how volume clusters near the top of the book, can be reproduced by agents that submit orders at entirely random prices and sizes, with no strategy, no information, and no intelligence at all. These are called zero-intelligence models, and the name is meant literally.

In a typical setup, "traders" arrive at random times and place either a limit order at a random price (relative to the current best bid/ask) or a market order, with no view on where the price should go. Run enough of these random orders through a standard matching engine and a surprisingly realistic-looking order book emerges, complete with a sensible bid-ask spread and a price impact curve that resembles real markets. This tells researchers something important: some stylized facts of market microstructure come from the mechanics of order matching itself, not from trader sophistication, and any theory claiming to explain a feature of real markets should first check whether zero-intelligence agents already produce it.

The models aren't meant to be realistic descriptions of actual trader behavior, no real market maker submits orders uniformly at random, they're a null hypothesis. If a real market's behavior differs from the zero-intelligence baseline, that difference is where genuine strategic or informational effects live.