Concept · liquidity-and-trading
Bid-Ask Spread
Quick definition. The difference between the highest price a buyer will pay and the lowest price a seller will accept · the cost of immediacy. In prediction markets the spread reflects adverse-selection risk, gap risk, and inventory holding costs in addition to the usual MM compensation, which is why headline spreads can be 10–50% on long-tail contracts.
Key insights
- XO Labs's classical Avellaneda–Stoikov adaptation broke because PM prices are bounded (0–1) probabilities with non-constant volatility and guaranteed terminal convergence. Logit-space transformation lost $1,114 in backtest; probability-space engine with inventory-skewed spreads, regime detection, and multi-outcome coordination was profitable at $453.
- @allquantor's "semantic tick size" thesis: on 600M Polymarket datapoints, ~70% of one-cent price moves do not continue in the same direction. Because each penny reads as a 1-percentage-point probability revision, PMs narrativize microstructure noise. A contrarian fade strategy harvests this overreaction.
- Sam Schneider on Kalshi: fee formula fee = 0.07 × C × P × (1-P) peaks at 50/50 · fees incentivize trading near the meaningful middle, and effectively widen the "true" spread retail pays.
- Rose-Berman makes the spread-as-tax economics explicit on Kalshi: every trade has a maker and a taker, fees apply both directions, and retail is structurally the taker because they don't have the systems to MM. A 50/50 coin-flip becomes a 3.4% expected loss purely from crossing the maker spread + fees, regardless of who wins.
- Marinson ("Seeing Like a Market"): the cost differential between PMs and derivatives is "apparatus rent" paid for constructed dealer infrastructure. FOMC compressed from a 12pp gap (March 2024) to <2pp (Feb 2026) at $3M · driven by liquidity compression, not vol expansion. High-VRP categories (BTC 4.83%, elections) have crossed the threshold; silver (VRP −10.19%) has not and never will under current structure. The 87-contract / 2.89M-trade dataset found 30 PM wins / 12 at threshold / 45 losses.
- Marinson's BTC natural experiment: 5 BTC contracts at identical 4.08% VRP · PM wins at every strike where volume is sufficient ($100k/$13.3M, $105k/$7.1M, $110k/$4.9M, $150k/$32.8M) and loses only at the thin one ($125k/$2.2M). Liquidity is the only variable.
- Della Vedova: automated traders profit by paying 2.52 cents less per contract than retail. The spread is the wealth-transfer mechanism.
- Becker on the spread-as-wealth-transfer category gradient: Finance gap 0.17pp, Sports 2.23pp, Crypto 2.69pp, Weather 2.57pp, Entertainment 4.79pp, Media 7.28pp, World Events 7.32pp. Category-level differences in spread economics swamp any platform-level differences.
- Human Invariant: FCFS order matching forces defensive spread widening; priority batch auctions (cancels → makers → takers) let MMs tighten quotes. The mechanism: in FCFS the MM's quote is fillable until they cancel; if news arrives faster than they can cancel, they're filled at stale prices. Priority batch auctions let cancels execute before takers, so stale quotes get pulled rather than picked off.
- sybilpm on dynamic spread economics: MM expected profit E[π] = (s/2 · V(s)) − P_news · L_snipe. In news-sensitive markets the sniping term dominates and spreads must widen to compensate; for "Will Israel strike Lebanon today?" type contracts the equilibrium spread is wide enough that retail can't trade economically.
- Jay Malavia: peer-to-peer exchange overround is ~3% (Betfair) vs. ~12% for traditional bookmakers · the spread differential is the exchange-vs-house-edge model difference made numerical.
- Taetaehoho: PM prices are 100–300 bps better than sportsbooks on liquid markets, but long-tail markets have 10–50% spreads (where pro MM competition is absent).
- Ranger Global: finance markets have the tightest spreads (0.17%); crypto the widest (2.69%) · efficiency correlates strongly with informed-trader presence.
- Lotus: the same contract trades at 58–67 cents on different venues simultaneously · cross-venue spread fragmentation is its own asset.
- Hall & Paschal complement: bid-ask spreads exceed 20% on most political contracts; only 1.3% are liquid enough to be manipulation-resistant. The spread is the most diagnostic stat for a market's status.
In their words
70% of one-cent price moves do not continue in the same direction.· @allquantor
Each penny reads as a one-percentage-point probability change, creating overreactions that a contrarian fade strategy can profitably harvest.· @allquantor, "Prediction Markets Have a Semantic Tick Size"
Automated traders profit by paying 2.52 cents less per contract.· Della Vedova
The cost differential is not a quality discount. It is apparatus rent. Same distributional content. Different transmission cost.· Lauris Marinson
FCFS creates wider spreads and other negative externalities.· Human Invariant
Where it matters
The spread is the single best summary statistic for a market's health: it bakes in liquidity depth, MM competition, adverse-selection risk, gap risk, and fee structure. Headline averages mask huge cross-section variation · finance markets at 17 bps next to entertainment at 700+ bps · which is why category mix is a strategic decision for every platform. The semantic-tick-size finding from @allquantor is particularly load-bearing: the same 1-cent move that traders read as new information is, 70% of the time, noise the MM can fade.
Connections
- Market making · spread is the MM's revenue stream.
- Adverse selection · wide spreads compensate for it.
- Semantic tick size · explains why PM spreads carry narrative weight beyond the price.
- Execution quality · measured against the spread mid.
- Batched auctions / continuous double auction · design choices that shape spread width.
- Liquidity provision / order book · the venue and capital that produce spreads.
- Gap risk / hedging · risks the spread must price in.
Platforms linked to this concept
- Kalshi · studies · Kalshi spreads referenced
- Polymarket · studies · Polymarket spread data referenced
- XO Market · affected-by · Cited as facing/exposed to Bid-Ask Spread
Related concepts
- Market Making
- Adverse Selection
- Semantic Tick Size
- Execution Quality
- Batched Auctions
- Continuous Double Auction
- Liquidity Provision
- Order Book
- Gap Risk
- Hedging
Sources
- Market Making for Prediction Markets: A Probability-Space Approach · XO Labs · Apr 17, 2026
- Prediction Markets Have a Semantic Tick Size · allquantor · Mar 19, 2026
- What's Kalshi's Revenue? Analyzing All 203 Million Trades on Kalshi. · Sam Schneider · Mar 12, 2026 (paywalled intro only)
- Seeing Like a Market: Event Contracts and Market Topology · Lauris Marinson · Mar 1, 2026
- Who Profits from Prediction Markets? Execution, Not Information · Joshua Della Vedova · Feb 7, 2026
- The Case For Alternative Ordering Mechanisms in Prediction Markets · Human Invariant · Nov 12, 2025
- Who Are You Really Playing Against? · Jay Malavia · Sep 18, 2025