# Gambler's Fallacy > The gambler's fallacy is believing past random outcomes change future odds. Learn how it shows up after streaks and in prediction markets, and how to avoid it. Source: https://learn.tradelabsai.com/psychology/gamblers-fallacy/ Track: Trading Psychology · Level: Intermediate · Updated: 2026-10-03 Publisher: TradeLabs AI (https://tradelabsai.com). Education, not financial advice. Cite as: TradeLabs Learn, "Gambler's Fallacy", https://learn.tradelabsai.com/psychology/gamblers-fallacy/ The gambler's fallacy is the mistaken belief that if something has happened more often than expected recently, it is less likely to happen next, or that the opposite is "due". After five heads in a row, many people feel tails is more likely. It is not: a fair coin has no memory, and the chance of tails is still 50%. In trading, the fallacy pushes people to bet on reversals, increase size after losses or expect a win simply because they have lost several times. ## A famous example On 18 August 1913, at the Monte Carlo Casino, the ball on a roulette wheel landed on black 26 times in a row. As the streak grew, gamblers bet heavily on red, convinced it was overdue. Each spin was independent, so the odds of red did not improve, and gamblers reportedly lost millions of francs. The fallacy is sometimes called the Monte Carlo fallacy because of this night. ## How it shows up in trading | Belief | Why it is a fallacy | |---|---| | "I've lost five trades in a row, the next one has to win" | If trades are independent, the next trade has the same odds as any other | | "This stock is up eight days in a row, it must fall" | Strong trends can and do continue; streaks alone are not reversal signals | | "Bitcoin finished up in the last six 5 minute rounds, so down is due" | Short term rounds are close to independent; past rounds do not lower the chance of up | | "I'm due for a big winner, I'll size up" | Increasing size on this belief increases risk without increasing edge | **Example: Doubling after losses** A trader with a 50% win rate decides to double their stake after each loss, believing a win must be coming. Starting at $100, five losses in a row require stakes of $100, $200, $400, $800 and $1,600, a total of $3,100 lost, with the next bet needing $3,200 to recover. Five losses in a row at a 50% win rate happens about 3% of the time on any given sequence, which means it is likely to happen at some point over hundreds of trades. This doubling approach, known as the martingale, has ruined many gamblers and traders. See [Risk of Ruin](https://learn.tradelabsai.com/risk/risk-of-ruin/). ## When past results do matter Markets are not coin flips, and some past information is genuinely useful. Momentum effects, volatility clustering and mean reversion in specific conditions are real, measurable patterns. The fallacy is not using history; it is assuming a streak itself creates a force towards reversal without evidence. If a pattern exists, it should show up in testing on large samples, not just in a feeling. See [Momentum Trading](https://learn.tradelabsai.com/strategies/momentum-trading/) and [Mean Reversion](https://learn.tradelabsai.com/strategies/mean-reversion/). ## Prediction markets and the fallacy Short term up or down markets are especially prone to the fallacy. After several "up" rounds, it can feel as if "down" must come next. Unless you have evidence of real reversal tendencies at that timescale, each round should be judged on current information, such as the price relative to the round's starting price and time left, not on the previous rounds' results. See [Up or Down Markets Explained](https://learn.tradelabsai.com/prediction-markets/up-or-down-markets-explained/). ## How to avoid the gambler's fallacy 1. **Keep position size fixed by rule,** never based on recent results. See [Position Sizing](https://learn.tradelabsai.com/risk/position-sizing/). 2. **Never use martingale style doubling.** 3. **Base trades on setups and evidence,** not on streaks. 4. **Expect streaks** and know their typical length for your win rate. See [Losing and Winning Streaks](https://learn.tradelabsai.com/risk/losing-and-winning-streaks/). 5. **Test streak based ideas** on data before trading them. ## Hot hand and its opposite The opposite error, believing a streak will continue just because it has, is called the hot hand fallacy. Both errors come from the same place: the human brain's tendency to see patterns and causes in random sequences. See [Recency Bias](https://learn.tradelabsai.com/psychology/recency-bias/). ## Frequently asked questions ### What is the gambler's fallacy? The belief that past random outcomes affect the probability of future independent outcomes, such as thinking a win is due after several losses. ### Does a losing streak mean a win is coming? No. If trades are independent, the chance of winning the next trade is the same as always, regardless of recent results. ### Is martingale a good trading strategy? No. Doubling after losses can produce huge losses during long streaks, which eventually occur, and has ruined many traders and gamblers. Next, learn why past losses should not affect future decisions: [Sunk Cost Fallacy](https://learn.tradelabsai.com/psychology/sunk-cost-fallacy/). ## Sources - Wikipedia, [Gambler's fallacy](https://en.wikipedia.org/wiki/Gambler%27s_fallacy) ## Continue learning - Next lesson: [Sunk Cost Fallacy](https://learn.tradelabsai.com/psychology/sunk-cost-fallacy/) - Previous lesson: [Overconfidence](https://learn.tradelabsai.com/psychology/overconfidence/) - Related: [Overconfidence](https://learn.tradelabsai.com/psychology/overconfidence/): Overconfidence makes traders overestimate their skill, which leads to oversizing and overtrading. Learn the research, warning signs and how to stay grounded. - Related: [Losing and Winning Streaks](https://learn.tradelabsai.com/risk/losing-and-winning-streaks/): Losing streaks are a normal part of any strategy. See how long streaks get at different win rates, why they happen and how to handle them without breaking rules. - Related: [Probability for Traders](https://learn.tradelabsai.com/math/probability-for-traders/): Probability is the language of uncertainty in trading. Learn the core rules, independent vs dependent events, odds, and how probability shapes sizing and edge. - Related: [Recency Bias](https://learn.tradelabsai.com/psychology/recency-bias/): Recency bias makes recent events feel more important than they are. Learn how it distorts strategy judgement, risk and market views, and how to counter it. - Related: [Revenge Trading](https://learn.tradelabsai.com/psychology/revenge-trading/): Revenge trading is trying to win back losses fast with bigger or unplanned trades. Learn the warning signs, why the brain does it and rules that stop the spiral. - Related: [Conditional Probability](https://learn.tradelabsai.com/math/conditional-probability/): Conditional probability is the chance of an event given that another has happened. Learn the formula, trading examples, base rates and how to use filters properly.