Prediction Markets: How Yes/No Contracts Work

Prediction markets let you buy and sell contracts that pay out based on whether a specific event happens, with yes/no contracts being the simplest form: a “yes” contract pays $1 if the event occurs and $0 if it doesn’t, while a “no” contract pays the opposite. You enter by choosing a side and a price, and the market price of these contracts reflects the crowd’s estimated probability of the event. This article explains the mechanics, pricing, risks, and common questions about these event contracts.

Formula

The core math is straightforward. For a yes/no contract priced at t (in dollars, between 0 and 1), and your own probability estimate p (as a percentage), the expected value of buying a yes contract is:

EV = (p/100) × $1 + (1 – p/100) × $0 – t

Which simplifies to EV = p/100 – t. If EV is positive, the contract is underpriced relative to your belief; if negative, it’s overpriced. The market price itself is often interpreted as the market’s implied probability: a price of $0.60 suggests a 60% chance of the event occurring.

Worked Example

Suppose a contract on “Will it rain in New York on July 4?” is trading at $0.60. You believe there’s a 70% chance of rain.

1. Your probability estimate: p = 70%, so p/100 = 0.70.
2. Market price: t = $0.60.
3. Expected value per contract = 0.70 – 0.60 = $0.10.
4. Since EV is positive, buying the yes contract is theoretically favorable.

If you buy 100 contracts at $0.60, you pay $60. If it rains, you receive $100 (each contract pays $1), netting $40 profit. If it doesn’t rain, you lose your $60. The calculator above shows the same result: EV = $0.10 per contract, and edge = 10%.

How Yes/No Contracts Actually Work

In a prediction market, each event has two types of contracts: yes and no. They are complementary: a yes contract pays $1 if the event happens, $0 otherwise; a no contract pays $1 if the event does not happen. The two prices always sum to $1 (ignoring fees), so if yes is $0.60, no is $0.40. You can buy or sell either side, and you can also sell a contract you don’t own (short selling) in some markets, which effectively lets you bet against an outcome.

When the event resolves, the market pays out automatically. Most platforms handle this digitally: your account is credited or debited based on the final outcome. Some markets allow early trading, so you can exit before resolution by selling your contracts at the current market price, realizing a profit or loss based on price movement.

Choosing a Market and a Side

To participate, you first pick an event with a clear, binary outcome—for example, “Will the Federal Reserve raise interest rates in March?” (yes/no). Then decide which side you believe is more likely than the market price suggests. The key is not just predicting the outcome, but finding a price that undervalues your belief. A common strategy is to compare the market price to your own assessment, and only buy when the expected value is positive.

• If you think the event is more likely than the price implies, buy yes.
• If you think it’s less likely, buy no (or sell yes).
• Consider the spread (difference between buy and sell prices) and fees, which reduce your edge.
• Check the market’s liquidity—wider spreads and thin order books make entry/exit more costly.

Common Problems and Misconceptions

One frequent misunderstanding is that the market price is a “true” probability. It’s actually an aggregation of opinions, influenced by liquidity, manipulation, and information asymmetry. Another issue is that event definitions can be ambiguous—for example, “Will the president win re-election?” depends on the exact candidate and timing. Always read the market’s rules carefully to avoid disputes.

Another problem is overconfidence in your own probability estimates. Even if you’re right, the market may already reflect that, leaving no edge. Also, beware of markets with low liquidity: you might not be able to sell at a fair price when you want to exit. Finally, some platforms have caps on how much you can invest per contract, limiting potential returns.

Costs, Fees, and Taxes

Most prediction markets charge a trading fee, often a small percentage of profits or a flat fee per transaction. Some also charge a withdrawal fee when you move money out. These costs directly reduce your net return, so factor them into your expected value calculation. For example, if the fee is 2% of profits, a $0.10 edge per contract might shrink to $0.098.

Tax treatment varies by jurisdiction. In the U.S., gains from prediction markets may be considered taxable income, and losses may be deductible, but rules are complex and change. Always consult a tax professional for your specific situation. Some platforms are regulated as financial exchanges, while others operate in legal gray areas; check the legality in your country before participating.

Safety and Responsible Participation

Prediction markets involve real money and risk of loss. Never invest more than you can afford to lose. The markets are not a get-rich-quick scheme; most participants lose money over time due to fees and mispricing. Be aware that some platforms may restrict access in certain jurisdictions, and using unregulated sites carries additional risks like fraud or hacking.

Also, be cautious about markets with vague outcomes—if the event isn’t defined precisely, resolution can be contested, leading to delays or losses. Stick to well-established markets with clear rules and a track record of fair resolution. If you’re unsure about the legal status in your area, err on the side of caution and avoid participating.

Alternatives to Prediction Markets

If you’re interested in forecasting but don’t want to risk money, consider these alternatives:

• Opinion polls and survey aggregators (e.g., FiveThirtyEight’s forecast models) give probability estimates without financial stakes.
• Play-money prediction markets let you practice with virtual currency.
• Betting exchanges for sports or politics offer similar mechanics but are often regulated differently.
• Statistical models and machine learning can help you form your own probabilities.

Frequently Asked Questions

How is the price of a yes contract determined? The price is set by supply and demand from traders, just like a stock. If more people buy yes than sell, the price rises; if more sell, it falls. The equilibrium price reflects the market’s collective belief about the probability of the event.

What happens if the event is a tie or ambiguous? Most platforms have predefined resolution rules. If the event is a tie, some markets pay both sides partially or refund the contracts. Always read the market’s terms before trading to understand how edge cases are handled.

Can I lose more than I invest? In most prediction markets, no—your maximum loss is the amount you paid for the contracts. However, if you short sell (sell without owning), you could theoretically lose more if the price rises sharply. Stick to buying contracts if you want limited risk.

Are prediction markets legal in my country? Legality varies widely. In the U.S., some platforms are regulated by the CFTC, while others operate offshore. In the EU, rules differ by member state. Check your local laws and the platform’s terms before depositing money.

How do prediction markets compare to traditional betting? Prediction markets are similar to betting exchanges but often focus on non-sporting events like elections, economic indicators, or movie box office. They typically have lower fees and more flexible trading, but the core concept of pricing probabilities is the same.

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in t | Contract price ($) | $ | 0.60 | 0 | 1
in p | Your probability estimate | % | 70 | 0 | 100
out ev | Expected value per contract | $ | (t * 1 + (1 - t) * 0) - (p/100 * 1 + (1 - p/100) * 0) | 2
out edge | Edge vs. market price | $ | (p/100) - t | 2