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Concept · mechanism-design

Market Manipulation

Deliberately trading to distort prices away from true probabilities for strategic or financial gain. In prediction markets, manipulation has unusually high stakes because prices double as public information signals, so distorted prices contaminate journalism, policy, and downstream financial decisions.

Key insights

In their words

We find that prediction markets can be manipulated: the effects of our trades are visible even 60 days after they have occurred. However, as predicted by our model, the effects of the manipulations somewhat fade over time. Markets with more traders, greater trading volume, and an external source of probability estimates are harder to manipulate.· Rasooly & Rozzi, *How Manipulable Are Prediction Markets?*
Wash traders trade with other wash traders in their collusive clique, while market makers neither know nor care who their counterparties are. In fact, market makers seldom trade with other market makers.· Rajiv Sethi, *The Detection of Wash Trading*
These wallets collectively traded over 116 million shares and generated more than 113 million in dollar volume, but ended up with an aggregate loss of just $57.86.· Sethi, *ibid.*
Speculative markets are actually far more resistant to manipulation than other info institutions. When traders expect more efforts to manipulate a price, they respond so that prices on average become MORE accurate.· Robin Hanson, *On Prediction Market Regulation*
The entire squadron is on Polymarket. The entire air force is betting.· IDF Air Force major, quoted in Gunitsky, *Priced to Kill*
Polymarket settled daily temperature bets for Paris using an unguarded sensor near the Charles de Gaulle airport. On April 6, someone bought the long-shot contract for abnormally high temperatures. Later that evening, the sensor saw a four-degree spike before dropping back to normal. No neighboring station registered anything similar.· Gunitsky, *ibid.*
While 93% of PredictIt markets correctly predicted outcomes better than chance, accuracy fell to 78% on Kalshi and 67% on Polymarket.· Clinton & Huang, summarized by Sides, *The Perils of Election Prediction Markets*
Prices for identical contracts diverged across exchanges, daily price changes were weakly correlated or negatively autocorrelated, and arbitrage opportunities peaked in the final two weeks before Election Day.· Clinton & Huang, quoted in Sides, *ibid.*
Markets that resolve on whether someone stays in power, speaks publicly, shows up somewhere, or holds a certain job carry implicit incentives connected to their continued existence. Nearly every such market dealing with control, decision, or public appearance is technically an assassination market.· Guillory & Zimmermann, quoted in Sethi, *Trading on Violence*

Where it matters

Manipulation resistance is the gating constraint for prediction markets being treated as public-good information infrastructure rather than gambling. The same property that makes them useful (prices are read as truth) makes them attractive to attack, and most current architectures (binary order books, optimistic oracles, pseudonymous wallets) have weak structural defenses. The 2024-election data shows that high volume does not automatically buy accuracy · Polymarket's largest US-election market was less accurate than PredictIt's much smaller market, and arbitrage opportunities grew into the resolution window. For continuous markets like Dekant, the manipulation surface shifts from single-tick attacks to curve-shape distortion · relevant when designing how much capital is needed to move a kernel.

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