What is the house edge on Polymarket?

Polymarket does not have a traditional house edge. Instead, traders pay a bid-ask spread that functions like one. On liquid political markets, this spread is typically 1-3%. On thin or niche markets, it can reach 5-10%. Unlike a casino house edge, the Polymarket spread varies by market and can be avoided by placing limit orders. However, you also face adverse selection risk from trading against better-informed participants, which is an invisible cost that does not appear in the spread.