Asymmetric Information Risk (Glosten-Milgrom)
The Formal Definition
A pioneering market microstructure model demonstrating that market makers set bid-ask spreads wider than operational processing costs to protect their inventory against losses incurred when trading with informed counterparties possessing superior private data.
Bid-Ask Spread = Order Processing Cost + Inventory Holding Cost + Adverse Selection Markup (Information Asymmetry Premium)
Cole Barrett's Reality Check
The Unvarnished Bottom Line"Why does a bid-ask spread exist? It isn't just the market maker charging you for their time. The Glosten-Milgrom model proved that a huge chunk of the spread is an insurance premium against getting run over by people who know more than the broker. If an insider or high-speed fund is trading, the market maker loses. The spread is the tax the market maker charges everyone else to pay for those losses."
Interactive Simulator: Test the Math
Real-World Example: Scenario Breakdown
Examining the real numbers for: Market maker quoting a volatile biotechnology equity 10 minutes prior to a surprise FDA phase-3 trial approval print
| Execution Metric | Uninformed Retail Momentum Trader | Algorithmic Market Maker (Glosten-Milgrom Framework) |
|---|---|---|
| Fee / Rate | $0.00 'free' | Institutional clearing pass-through |
| Spread / Buffer | Quoted spread: $10.00 Bid / $10.50 Ask ($0.50 spread reflects massive information risk) | Detected incoming order size from informed institutional conduits |
| Execution / Status | Crossed the spread to buy at $10.50; paid the embedded information asymmetry premium | Widened quoted spread from $0.05 to $0.50 to offset inventory adverse selection |
| Total Cost / Result | Subsidized dealer adverse selection risk | Protected dealer balance sheet against information asymmetry |
How Brokers Weaponize This Term
Wholesale internalizers isolate 'uninformed' retail flow via PFOF precisely to remove Glosten-Milgrom information risk, capturing wide spreads with near-zero inventory loss.
Broker Evaluation Matrix
Cole Approves
Interactive Brokers: SmartRouting provides direct access to lit public exchanges and dark liquidity pools where competitive multi-party bidding narrows adverse selection markups.
Read Audit →Cole Flags / Avoids
Zero-Fee PFOF Desks: Sells uninformed retail orders to wholesalers who profit from the information asymmetry differential without passing price improvement to retail.
View Trap Details →Frequently Asked Questions
What happens to the bid-ask spread when information asymmetry increases?
Spreads widen immediately; market makers expand quotes to protect themselves against trading with counterparties who hold superior near-term directional information.
Why do liquid large-cap stocks have tiny bid-ask spreads?
Because high trading volume from millions of uncorrelated, uninformed retail and passive investors reduces the probability that any single trade represents toxic informed insider flow.