What is a prediction market, and how does a price become a probability?
In 1503 people were already betting on who would be the next pope. Today the same instinct runs on apps where a price is read as a probability.
Guess first
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A prediction market is a market where people trade on whether something will happen, and the price they settle on is read as a probability. Traders buy and sell whose payoff depends on a future event, such as an election result, and the market prices are treated as the crowd's combined belief about how likely that event is.
The most common contract is a simple yes-or-no one that ends at either 0 or 100 percent. It trades at a price between one cent and one dollar. If the event happens, each winning contract pays one dollar and all the others . So if a "Yes" contract trades at 93 cents, the market is implying a 93 percent chance, and the matching "No" contract sits at 7 cents.
Why would a price tell you anything? Traders with different beliefs, and different information, put money behind them. When the optimists bet more than the pessimists, the price rises, so it ends up reflecting the balance of everyone's money and views. The result is not a perfect probability. Economists Justin Wolfers and Eric Zitzewitz showed that prices are often close to the average trader's belief but may also be , and a study of Kalshi found that cheap lost money on average.
The idea is old. As far back as 1503, people were betting on who would become the next pope, and even then it was called an old practice. As of October 2026, many governments treat prediction markets as gambling, and some places ban them.
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Recap
In a yes-or-no market, a price between 0 and 1 dollar is read as a probability, but it can be biased, especially for long shots.
💡 A trick to remember it · Think of a price tag on tomorrow: 93 cents for a dollar prize means the crowd is nearly sure, and a price tag can still be wrong.
Surprising fact · A "Yes" price of 93 cents implies a 93 percent chance, and the "No" side then sits at 7 cents.
Connects to
- 🐂 Can a crowd of ordinary people guess better than the experts?
- 📈 How do you measure how steep a curve is at a single point?
- ⚖️ Who decides what a share costs right now?
- 🎲 How did an unfinished game of chance give birth to probability theory?
- ⚖️ Who actually decides what things cost?
- ✂️ Why is rock paper scissors a serious piece of mathematics?
Sources (1)
No source, no claim. Every fact in this lesson (16 claims) cites at least one of these.