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.