Gambler's 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#

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 and 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.

How to avoid the gambler's fallacy#

1. Keep position size fixed by rule, never based on recent results. See 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.
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.

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.

Sources#

• Wikipedia, Gambler's fallacy