A Columbia University study has uncovered significant artificial trading activity on the prediction market platform Polymarket. The research, detailed in an 80-page paper titled "Network-Based Detection of Wash-Trading," found that wash trades constituted nearly 60% of Polymarket's trading volume in July 2024. This activity persisted until April 2025, before decreasing and then rising again to 20% in October 2025. Over the past three years, 25% of the platform's total trading volume was attributed to wash trading. The study, which has not yet been peer-reviewed, suggests that Polymarket's operational structure may have facilitated this artificial trading. Wash trading, which involves the same trader buying and selling an asset to create a misleading impression of market activity, is illegal in the United States. The findings come as Polymarket prepares to re-enter the US market following a no-action letter from the Commodity Futures Trading Commission (CFTC). Polymarket has not yet responded to requests for comment on the study.