Adverse Selection

Also: pick-off risk · toxic flow

When your resting limit order gets filled instantly, the counterparty is often better-informed than you. Adverse selection is the systematic loss makers absorb on those picked-off fills.

You post a passive bid. It fills in milliseconds. Feel good? You shouldn’t — the fastest fills are the ones where someone with better information hit you right before the price moved against you. That’s adverse selection, formalized by Glosten-Milgrom: a spread exists even for a risk-neutral, cost-free market maker purely because some counterparties are informed.

The maker earns the spread on uninformed flow and loses on informed flow. Profitability is the balance of the two. This is why “just quote tighter to win more fills” is a trap — tighter quotes win more of the wrong fills.

An autonomous market-making agent has to detect toxicity in real time (see VPIN and OFI) and defend: widen, skew away from the pressure, or pull quotes entirely. Encoding that defense as policy — max inventory, toxicity kill-switch — is what keeps an agent from slowly bleeding to informed traders.

  • microstructure
  • risk
  • market making

Research source: rSwarm research library →

Related concepts

Maker vs. Taker

A maker posts a resting limit order and provides liquidity; a taker crosses the spread to fill immediately and removes it. The distinction drives fees, fill quality, and who bears adverse selection.

Order Flow Imbalance (OFI)

OFI measures net buying-versus-selling pressure over a window, normalized to compare across markets. Cont, Kukanov & Stoikov (2014) tied it directly to short-horizon price moves.

VPIN

Volume-Synchronized Probability of Informed Trading — a real-time gauge of order-flow toxicity. When one side consistently absorbs the other, VPIN rises and market makers are at risk.